An Innovative Approach for Assessing Foam Stability Based on Electrical Conductivity Measurements of Liquid Films

Abstract

Foam stability plays a critical role in a wide range of industrial and scientific applications. In this study, an innovative method is presented for assessing foam stability through electrical conductivity measurements of liquid films formed within a controlled experimental setup. A modified horizontal glass capillary system with vertically aligned copper electrodes was developed, allowing the continuous monitoring of film drainage and rupture behavior under precise humidity (92% RH) and temperature (30 °C). Experiments were conducted using various concentrations of sodium dodecyl sulfate and Ethylan 1005, with and without NaCl addition. The results demonstrate that film stability increases with higher surfactant concentrations up to a point, beyond which the addition of salt can have either stabilizing or destabilizing effects depending on whether concentration levels are below or above the Critical Micelle Concentration (CMC). At sub-CMC levels, NaCl enhanced film stability by promoting surfactant adsorption and reducing electrostatic repulsion. Conversely, in super-CMC conditions, NaCl led to film destabilization, likely due to changes in interfacial structure and micellar behavior. This approach provides a simple, sensitive, and reproducible technique to quantitatively characterize foam film stability, offering key mechanistic insights and practical guidance for the formulation and optimization of foaming systems across diverse applications.

1. Introduction

Foams can be defined as dispersion systems in which a gas phase is distributed within a continuous liquid phase. These systems have found applications across a wide range of products and industrial processes, including pharmaceuticals, cosmetology, as well as in the production and use of detergents and personal care products [1,2]. The use of foams has also been integrated into protocols related to enhanced oil recovery, as well as into mineral extraction processes through flotation techniques. Additionally, foams play a crucial role in the formulation and operational deployment of firefighting agents [3]. They are also crucial for both the production processes and final products of the food and beverage industry with representative examples including wine, beer, and cider. Furthermore, foams are present in confectionery products such as whipped cream and ice cream [4,5]. In all of the aforementioned applications, the presence of stable foam is of critical importance, as it ensures the consistent operation of the production process and contributes added value to the final product [6]. However, there are several industrial sectors in which the presence of stable foam is considered undesirable. In particular, stable foam formation can cause significant issues in lubrication systems using mineral oils, textile dyeing processes, paper manufacturing, and fermentation operations, whether aerobic or anaerobic, such as those carried out in biogas plants and wastewater treatment facilities [7].

In both cases, where foam stability is either desired or needs to be minimized, the typical strategies to control foam behavior involve adjusting surfactant concentration, formulating surfactant mixtures, and incorporating additives such as salts or antifoaming agents [8]. However, these interventions often have a direct and typically negative impact on production costs, thereby affecting the overall competitiveness and sustainability of the industry itself. Furthermore, the excessive use of surfactants increases the volume of waste generated by such processes, heightening the risk of environmental pollution and further raising waste management costs [9].

Based on the above, understanding the stability or instability of the foam produced by a specific surfactant solution is almost imperative. This knowledge enables a comparative evaluation of the effectiveness of different surfactant agents, as well as their mixtures, and assists in determining the optimal concentration that delivers the desired performance with the lowest possible cost and minimal environmental impact.

To effectively assess the suitability of a surfactant solution or mixture and its impact on a product or industrial process, it is essential to understand the forces and phenomena that contribute to foam stabilization or collapse. In particular, foams, being dynamic systems, begin to destabilize immediately after their formation through various mechanisms. Evaporation, which mainly occurs at the foam’s surface, is one of the primary factors leading to foam breakdown [10]. Another equally important mechanism is gas diffusion from smaller to larger bubbles, known as Ostwald ripening [11]. However, the most significant mechanism responsible for foam destabilization is liquid drainage and the subsequent coalescence of adjacent bubbles. Specifically, the liquid within the foam flows downward through the Plateau borders, driven by gravitational forces and capillary phenomena [12]. As a result, the thickness of the liquid films between the bubbles progressively decreases leading to rupture. The elasticity, and consequently the stability, of these thin liquid films or more precisely, the gas/liquid interfaces, determines the overall foam stability and its lifespan [13].

The most commonly applied methods, both in the laboratory and at industrial scale, for evaluating the performance of surfactant solutions, or their mixtures with other surfactants or additives such as salts with regard to foam stability, are primarily macroscopic in nature. These methods focus on generating a relatively large volume of foam within specific containers and subsequently recording the time required for the foam volume to decrease to a predetermined height. This approach offers a practical assessment of the foam’s lifetime and is widely used due to its simplicity and relevance to real-world applications [10]. The considerable volume of surfactant solution required for these measurements is a major drawback, as it results in the generation of substantial waste, which increases both the risk of environmental contamination and the associated management costs. Additionally, the time required to complete the measurement process and obtain the final results is relatively long [14]. Another drawback of these methods is the difficulty in accurately determining the onset of foam collapse, as foams begin to destabilize immediately after their formation [15]. Alternative methods for evaluating the performance of surfactant solutions involve determining the size distribution and number of bubbles within a foam. In such cases, since the size of bubbles increases and their number decreases during foam collapse, the evolution of the entire bubble population is examined as it changes due to coalescence resulting from the rupture of the thin liquid films that separate them. However, this presents a highly complex problem that requires specialized equipment, and only a few attempts have been documented in the literature [16].

In order to overcome such difficulties and to gain a deeper understanding of the complex mechanisms involved in foam stabilization and destabilization and, by extension, the effectiveness of the surfactants themselves, numerous studies have been conducted aiming to establish principles and evaluation criteria based on the analysis of individual thin liquid films [17]. More specifically, the thin liquid films of foams, which are formed between bubbles under laboratory conditions, can be represented by the creation of liquid bridges. These liquid bridges are formed between two solids, usually with a vertical and sometimes with a horizontal orientation and are surrounded by atmospheric air [18]. The visual observation and recording of liquid bridges require precise knowledge of their shape, which changes continuously over time due to evaporation, and more significantly, due to drainage from the moment of their formation until rupture. The determination of their shape is achieved through the application of the Young–Laplace equation. Given the lack of an analytical solution, the literature has introduced approximate methods to address the problem, as exemplified by Chen and collaborators [19]. As advancement in the process for a more comprehensive study of liquid bridges, the measurement of their electrical conductivity has been proposed. To make this feasible, liquid bridges are formed between two metallic rods, which act as electrodes and are traversed by an electric current. A significant advantage of this method is that the behavior, shape, and overall stability of the liquid bridge can be monitored from its initial thickness to the formation of a thin film while simultaneously allowing for an accurate determination of the time required until rupture occurs [20].

The present study introduces an original method for evaluating the effect of surfactant substances by investigating the stability of thin liquid bridges formed by their solutions. This approach builds upon and further develops previous studies in the field by Kostoglou and Karapantsios [20,21,22,23]. Specifically, the formation of an air bubble within a liquid bridge created between two vertical, rigid copper rods and surrounded by atmospheric air constitutes a unique experimental configuration that closely resembles the case of two adjacent bubbles within a foam structure. This setup can be employed as a model system for studying the dynamic evolution of foams, from their initial formation to their eventual collapse, by correlating foam behavior with the stability of liquid films within the liquid bridges. In particular, the controlled removal of liquid from the bridge at a specified flow rate induces drainage conditions, leading to a gradual thinning of the liquid film and ultimately to its rupture. This thinning process is monitored through measurements of the electrical conductivity of the liquid bridge, allowing for a comparative assessment of the stability of liquid bridges formed by various surfactant solutions.

Furthermore, in the present study which, as noted at the beginning of the paragraph, constitutes a continuation of previous work, significant improvements were made to the electrical conductivity measurement system through the integration of an additional, newly developed device, thereby enhancing the accuracy of the measurements. In addition, a new experimental setup was designed to maintain constant humidity within the chamber, ensuring that the experimental results were not affected by the evaporation of the liquid bridge or the condensation of vapor on its surface. In the following sections, the experimental setup and the experimental procedure are described in detail. Then typical results for specific systems are demonstrated and discussed extensively.

2. Materials and Methods

2.1. Experimental Setup and Environmental Control

The experimental setup used in the present study is illustrated in Figure 1. Specifically, a liquid bridge is formed between the ends of two vertically aligned copper rods of equal diameter, which serve as electrodes and are subjected to an alternating electric current at a fixed frequency of 25 kHz. The electrodes are cylindrical, with a radius of R = 3.2 mm, and each is perforated with a central hole. Through the hole in the upper electrode, air is supplied in order to generate a single air bubble within the liquid bridge. Similarly, the hole in the lower electrode allows for the controlled withdrawal of liquid from the bridge at a specified flow rate, mimicking the drainage process observed in foam structures. This procedure is implemented to reduce the volume of the liquid bridge and promote the formation of thin liquid films (gas/liquid interfaces), thereby enabling a more thorough investigation of their behavior and stability. These gas/liquid interfaces are formed between the bubbles enclosed within the liquid bridge and the surrounding atmospheric air. The entire experimental apparatus is housed within a sealed chamber constructed from Plexiglas, aluminum, and glass. The chamber has a rectangular parallelepiped shape, and its metallic components are designed to not come into contact with the copper electrodes, in order to prevent electrical leakage. The chamber dimensions are 15 cm × 10 cm × 10 cm, significantly smaller than those of the setup described in a previous study [21], to which the present work constitutes a continuation.

The reduction in the size of the chamber allows for the effective maintenance of the experimental conditions, namely relative humidity and temperature, throughout the entire duration of the liquid bridge drainage. Additionally, a small cylindrical reservoir with a total capacity of 30 mL is placed inside the chamber, near the lower electrode, and is filled with water. A portion of the water evaporates due to the heating of the chamber, resulting in a relative humidity level of up to 92 ± 2%, which remains constant during the experimental procedure. Under these humidity conditions, neither evaporation of the liquid in the bridge nor condensation phenomena (i.e., increases in the liquid volume) are observed. The chamber is heated by conduction, utilizing two 100 W incandescent lamps positioned externally. This heating configuration ensures that the temperature within the chamber remains stable at 30 °C throughout the duration of the experiment. The temperature of the experimental setup, particularly that of the liquid bridge itself, is not measured directly. Instead, an indirect method is employed, whereby the temperature of the lower electrode is monitored. For this purpose, a fine thermocouple with a diameter of only 0.1 mm is attached to the electrode surface, enabling precise and minimally invasive temperature readings that are representative of the local thermal conditions in close proximity to the liquid bridge.

Once the desired experimental conditions of relative humidity (RH: 92 ± 2%) and temperature (T = 30 °C) were established, the liquid bridge was formed using a 10 μL glass syringe (Hamilton, Bonaduz, Switzerland) positioned between two vertically aligned copper tubes. The final total volume of the bridge was 20 μL. In order to accurately determine the precise amount of liquid used in the formation of the bridge, the Hamilton syringe was weighed before and after dispensing the solution onto the electrodes. The distance between the electrodes was maintained constant at 3.12 mm throughout the duration of the experiment. This specific combination of inter-electrode distance and liquid volume ensured that the shape of the liquid bridge remained essentially unaffected by gravitational forces. Subsequently, an air bubble was introduced into the liquid bridge by supplying air through the upper electrode using a second Hamilton syringe with a total volume of 10 μL.

2.2. Electrical Measurements and Instrumentation

The technology described in patent EP 3 005 942 A1 [24] and used in several two-phase applications [25,26,27], was employed for the measurement of the electrical resistance and, by extension, the electrical conductance of the liquid bridge. This particular technology offers significant advantages over conventional methods, as it exhibits exceptionally high sensitivity (0.001%) and reduced electrical noise in both the generated and recorded signals. These features are especially beneficial in applications where the accurate detection of low electrical resistance values is of primary importance. The aforementioned technique was applied in combination with other electrical systems, similar to those previously employed by other researchers [22], which are described in detail in the following sections. Τhe operating principle of the electrical technique employed in this study is based on the variation in the measured electrical resistance, and thus the apparent electrical conductivity, of the liquid bridge as drainage progresses. This variation is primarily attributed to the continuous reduction in the cross-sectional area of the liquid bridge, which directly affects the conductive path between the electrodes.

The electrical equivalent of the present experimental setup is illustrated in Figure 1. A function generator (V_Gen) is connected to the electrical circuit and serves as a voltage source, delivering an input signal of 2 V_pp at a frequency of 25 kHz. This signal corresponds to the input voltage V_1R, which is used to electrically excite the liquid bridge. The input voltage V_1R is applied to the upper electrode (input electrode), thereby inducing an electric current that flows through the bridge toward the lower electrode (output electrode). Within the bridge, the liquid phase functions as the conductive medium, while the encapsulated air bubble acts as the non-conductive component of the system. Within this electric current, a voltage drop is observed due to the variation in the resistance, R_Bridge, of the liquid bridge and consequently its conductance variation caused by the drainage process. Specifically, as liquid is gradually removed through the orifice of the lower electrode, the volume of the liquid bridge decreases continuously, leading to an increase in resistance. The electric current then exits through the lower electrode (output electrode) at a different potential, denoted as V_2R. The output electrode is connected to one terminal of a final resistor R_t, the other terminal of which is grounded. The mathematical expression describing this electrical circuit is as follows:

$$V_{1R}={\displaystyle \frac{R_t\ast V_{Gen}}{R_t+R_{Bridge}}}={\displaystyle \frac{R_t\ast V_{2R}}{R_t+R_{Bridge}}}$$

(1)

As previously described, the parameters V_1R and V_2R are recorded in real time during the experimental procedure, while the terminating resistance is measured at the conclusion of each experiment using a digital multimeter. Once the values of these parameters are known, the resistance of the liquid bridge, R_Bridge, can be determined through appropriate calculations. To express the electrical behavior of the bridge in terms of conductance rather than resistance, the following relationship can be employed:

$$K_{app}={\displaystyle \frac{1}{R_{Bridge}}}$$

(2)

The electronic instrumentation employed for the execution of the measurements in this study consists of the following devices: A function/arbitrary waveform generator (20 MHz, 33,220 A, Agilent Technologies, Walbronn, Germany) was used to produce a sinusoidal alternating input signal with a frequency of 25 kHz and an amplitude of 2 V_pp. A sound card (E-MU 1616 m, 192 kHz/24 bit, Creative Labs, Dublin, Ireland) was utilized for the simultaneous acquisition of the voltage signals V_1R and V_2R, which were recorded on a laptop computer (Intel Core 2 Duo, 1.80 GHz, 1 GB RAM, HP Deutschland GmbH, Böblingen, Germany) using a commercial software (SpectraLab v.4.32.17). Prior to conducting the experimental measurements, instrument calibration was performed using a 5 ½-digit digital multimeter (34,405 A, Agilent Technologies, Walbronn, Germany). Double-shielded silver cables were used for the interconnection of the electronic devices, ensuring signal integrity and minimizing electromagnetic interference. Additionally, a power strip equipped with an EMI/RFI filter was employed to provide a clean power supply to all instruments. A digital video camera (EOS 70D, 20 MP, Canon Europa N.V., Amstelveen, The Netherlands), fitted with appropriate macro photography lenses (EF 100 mm, f/2.8, Canon Europa N.V., Amstelveen, The Netherlands), and a set of CANON extension tubes, with a total length of 130 mm, were used to record the evolving shape of the liquid bridge during the drainage process.

2.3. Sample Preparation and Experimental Procedure

The experiments were conducted using four concentrations of the anionic surfactant SDS (sodium dodecyl sulfate-SDS, purity > 98%, Fluka) at 500, 1000, 2000, and 3000 ppm (CMC: 2.5 g/L at 25 °C) and three concentrations of the non-ionic surfactant Ethylan 1005 (Nouryon, Amsterdam, The Netherlands) at 125, 250, and 500 ppm (CMC: 2 g/L at 25 °C), all prepared in ultrapure deionized water (specific conductivity: 5.4 μS/cm at 25 °C). Both SDS and Ethylan are widely used surfactants in cleaning products, cosmetics, and the food industry, which justifies their selection for the present study. These specific concentration ranges were chosen deliberately: at lower concentrations, the liquid film fails to form, and the system undergoes neck rupture instead; at higher concentrations, the duration of the experiment becomes excessively long, delaying data acquisition and, more importantly, introducing complications due to water evaporation from the liquid bridge. Such evaporation phenomena can interfere with the measurements and compromise the reliability of the experimental results.

In a selected number of experiments (as detailed in the Section 3: Results and Discussion), a small amount of sodium chloride (NaCl, concentration 4 × 10⁻³ M, purity ≥ 99.5%, Merck KGaA, Darmstadt, Germany) was added to the surfactant solutions, which had been prepared using deionized water. Prior to use, the NaCl was calcined at 550 °C in order to eliminate any residual organic impurities. The purpose of this addition was to adjust the ionic strength of the solutions to levels comparable to those typically encountered in practical applications involving foams generated from tap or natural water sources. This approach also helps to avoid clogging issues associated with other salts such as calcium or magnesium, which can precipitate and obstruct the orifices of the electrodes. Furthermore, the incorporation of NaCl enables the investigation of its specific effect on the surfactant solutions using the present experimental setup, thereby enhancing the relevance of the findings to real-world foam systems.

Once the liquid bridge is formed between the two vertically aligned electrodes and the air bubble is introduced into the bridge by injecting air through the orifice of the upper electrode, the experimental procedure is initiated. Specifically, using a precision syringe pump (Model 100 Series, KD Scientific Inc., Holliston, MA, USA), the liquid within the bridge is withdrawn at a constant and reproducible rate through the central orifice of the lower electrode. This process effectively simulates the drainage conditions of a liquid film in a real foam system. The drainage rates applied across all experiments were 10 and 20 μL/h. Throughout the drainage process, the evolution of the electrical signal, particularly changes in electrical conductance resulting from the gradual reduction in liquid volume, is continuously monitored and recorded. Simultaneously, the morphological changes in the shape of the liquid bridge are captured using the corresponding imaging and data acquisition systems.

3. Results and Discussion

The electrical signals, namely electrical resistance and, by extension, electrical conductance recorded during the drainage of the liquid bridge, are presented in Figure 2. Specifically, from point 1 to point 2, the drainage rate reaches 630 μL/h, which is significantly higher than the typical rates of 10 and 20 μL/h applied during the main experimental procedure. This elevated drainage rate is attributed to two main reasons: (a) the reduction in total experimental duration, allowing measurements to be completed within a shorter timeframe and thus enabling faster data collection and (b) the prevention of water evaporation from the liquid bridge. Such evaporation is an undesirable phenomenon that could occur if the experiment were prolonged, potentially introducing errors and complications into the measurement process. As part of the quantitative evaluation conducted in this work,

Table 1. Lifetime of liquid films (in seconds) under two drainage rates (20 μL/h and 10 μL/h), with corresponding standard deviations (SD), for various surfactant solutions.

Solution	Lifetime (20 μL/h) (s)	SD (20 μL/h)	Lifetime (10 μL/h) (s)	SD (10 μL/h)
SDS 3000 ppm + 4 mM NaCl	166	±14	280	±8
SDS 2000 ppm + 4 mM NaCl	145	±17	195	±9
SDS 1000 ppm + 4 mM NaCl	85	±2	131	±16
SDS 500 ppm + 4 mM NaCl	0	±5	0	±7
SDS 3000 ppm	200	±5	296	±15
SDS 2000 ppm	91	±11	129	±13
SDS 1000 ppm	58	±10	80	±14
SDS 500 ppm	0	±4	0	±2
Ethylan 500 ppm + NaCl	131	±7	180	±18
Ethylan 250 ppm + NaCl	54	±15	97	±12
Ethylan 125 ppm + NaCl	0	±12	0	±17

presents the mean values and corresponding standard deviations of the time interval between points 3 and 4 (Figure 2), namely the lifetime of the liquid film for all solutions and mixtures investigated. The standard deviation presented in Table 1 results from six independent repetitions conducted for each experimental condition.

The zero time in Figure 2, Figure 3, Figure 4 and Figure 5 does not correspond to the initial condition of the bridge, but it is arbitrarily taken to be close to the moment of film rupture. In the sections that follow, particular emphasis is placed on analyzing the influence of key parameters on film stability. Specifically, the effects of (a) the drainage rate, (b) the surfactant concentration, and (c) the presence of salt on the duration of the liquid film are systematically examined and discussed. Moreover, in addition to the electrical measurements, optical observations were also carried out to capture the complementary aspects of film behavior. These visual assessments are further analyzed in the final part of this section.

Ιt is also worth noting that the liquid bridge model employed in this study offers a controlled microscale platform that captures key mechanisms governing foam film dynamics, including drainage, thinning, and rupture. Although it simplifies the geometry and multi-bubble interactions of macroscopic foams, previous studies have shown that its behavior closely mimics real foam systems, particularly regarding the influence of surfactant concentration and salt addition. Kostoglou et al. [22] demonstrated a strong agreement between the liquid bridge data and traditional foam column experiments for SDS, validating the model’s relevance. Our current results further support this applicability, extending the approach to non-ionic systems. Future work should aim to quantitatively correlate metrics such as film lifetime and drainage rate across both systems to strengthen the model’s predictive value in practical foam applications.

3.1. Effect of Drainage Rate on Liquid Film Lifetime

Foam drainage, that is, the reduction in the liquid content within the foam due to capillary forces, but primarily gravitational forces, leads to the formation of extremely thin liquid films between adjacent bubbles. During the drainage process, the thicknesses of these films gradually decrease until rupture occurs, resulting in the coalescence of the bubbles that were previously separated by the film. The lifetime of the foam is directly related to its drainage rate, which in turn governs the rate at which the liquid films break [28].

In the present experimental setup (Figure 1), the rate at which liquid drains from the bridge, representing a microscale model of a foam liquid film formed between two copper electrodes and consisting of a solution of specific surfactants or their mixtures with other substances such as NaCl, has been shown to directly affect the lifetime of the resulting film. To experimentally confirm this effect, a series of measurements was conducted in the present study, as depicted in Figure 3 and presented in Table 1. Figure 3 illustrates the ratio K_app/K₀, where K₀ is the initial value of K_app, i.e., the apparent conductance measured during the experiment, plotted as a function of time t (s) for specific concentrations of surfactant solutions and their mixtures with NaCl. The combination of these data with the results summarized in Table 1 leads to the conclusion that there is a clear and direct influence of the drainage rate on the lifetime of the liquid film.

More specifically, for each of the following solutions (a) SDS at a concentration of 1000 ppm, (b) mixture of SDS at 1000 ppm with NaCl at a concentration of 4 × 10⁻³ M, and (c) Ethylan solution at 250 ppm, two curves are presented corresponding to the lifetime of the liquid bridge, and more precisely, the liquid film formed under the two drainage rates applied during the experiment (10 μL/h and 20 μL/h) (Figure 3). The observation of these curves reveals that the drainage rate is inversely related to the lifetime of the liquid film. An increase in the drainage rate leads to a greater volume of the liquid being removed from the bridge, which in turn accelerates the formation of the thin film and ultimately results in a shorter film lifetime. This inverse relationship is clearly illustrated in Figure 3. Specifically, SDS at 1000 ppm exhibits a longer film lifetime under the lower drainage rate (10 μL/h), while the film ruptures more quickly at 20 μL/h (Figure 3a). The same trend is observed in Figure 3b, where the mixture of SDS and NaCl solution shows enhanced film stability at 10 μL/h, reinforcing the idea that slower drainage promotes longer-lived films, even when stabilizing agents are present. Figure 3c further confirms this behavior for the non-ionic surfactant, where the lower drainage rate yields a more prolonged film lifetime. Across all systems tested, these results confirm that a reduced drainage rate contributes to the delayed thinning and rupture of the liquid bridge, thereby extending the lifetime of the liquid film. This observation is consistent with the findings of Kostoglou et al. (2011) [22], who investigated the drainage of liquid bridges in SDS solutions at concentrations of 300, 600, 1000, and 2000 ppm, in the presence of sodium chloride at a concentration similar to that used in the present study. The authors recorded and confirmed a direct correlation between the drainage rate and the lifetime of the liquid film. The present study, as previously mentioned, differs from that of Kostoglou et al. (apart from the much more sensitive electrical technique used), as additional experiments were conducted not only with SDS in the absence of salt, but also with the non-ionic surfactant Ethylan.

3.2. Effect of Surfactant Concentration on the Lifetime of the Liquid Film

Using the electrical technique applied in the present study, the effect of surfactant concentration on the lifetime of liquid films formed from surfactant solutions between two vertical electrodes was recorded and analyzed. Specifically, as shown in Table 1 and Figure 4, at lower concentrations, such as 500 ppm for SDS and 125 ppm for Ethylan, no liquid film formation was observed, regardless of the presence or absence of NaCl in the solution. These observations align with broader insights from the literature, which indicate that solutions lacking surfactants, or containing them at relatively low concentrations, fail to generate stable foams or liquid films, largely due to the absence of key interfacial phenomena such as the Gibbs–Marangoni effect, which refers to mass transfer along an interface between two fluids, namely liquid and gas. The term “relatively low”, as used in the previous sentence regarding surfactant concentrations that do not lead to foam formation, does not correspond to a fixed value, but rather varies depending on the type of surfactant (anionic, non-ionic, etc.) [29]. Therefore, in such cases, whether due to the absence or insufficient concentration of surfactants, the formation of liquid bridges becomes practically infeasible. Even in cases where such foams are formed, they tend to be highly unstable, with no formation of a thin liquid film, and their lifetime is very short.

In contrast, in solutions where the concentration of surfactants is sufficient and often gradually increased, it has been observed that the resulting foams tend to be more stable, as surfactant molecules adsorb at the gas/liquid interface, rendering the liquid film more elastic and thus more stable. This constitutes the primary factor in the stabilization of the liquid film, both in foams and in liquid bridges [30]. This was also observed in the present study (Table 1, Figure 4), at which an increase in surfactant concentration resulted in a corresponding increase in the lifetime of the liquid film, for both drainage rates applied in the experiments (10 and 20 μL/h). Specifically, in Figure 4a, a comparison between SDS at 2000 ppm and 1000 ppm demonstrates that a higher surfactant concentration results in a slower decline in normalized apparent conductivity, indicating greater film longevity and a reduced drainage rate. This is reinforced in the presence of NaCl (Figure 4b), where both concentrations (2000 and 1000 ppm) exhibit improved stability compared with their salt-free counterparts, with the higher concentration again performing better. A similar pattern was found for Ethylan (Figure 4c), where the 500 ppm solution sustained a longer film stability than the 250 ppm solution under identical conditions. In all cases, increased surfactant concentration leads to enhanced stabilization of the liquid film, which supports the hypothesis that improved molecular packing at the gas–liquid interface due to higher surfactant availability plays a critical role in foam film resistance to rupture. This trend is in agreement with previously published work and reinforces the well-established relationship between surfactant concentration and interfacial film stability. Specifically, Nikolov and colleagues (1989) demonstrated that increasing the concentration of the surfactant sodium dodecyl sulfate led to the formation of more stable liquid films [31].

In a similar context, Belhaij and Al-Mahdy (2015) [32] reported that surfactants such as the non-ionic Zonyl FSO and the anionic Hitenol H-10 exhibited a significant increase in foam stability as their concentration increased, leading to long-lasting foam structures. Specifically, Zonyl FSO demonstrated a foam stability value (R5—defined as the percentage of foam height retained five minutes after generation, relative to the initial foam height) of approximately 65% at concentrations of 200 ppm and above, whereas significantly lower stability was observed at lower concentrations (e.g., 50–100 ppm). Likewise, Hitenol H-10 showed poor stability below 50 ppm, but a sharp increase in R5 values was recorded between 50 and 150 ppm, reaching approximately 60% at concentrations above 150 ppm, with particularly stable foam observed at 1000 ppm. These findings indicate a direct correlation between surfactant concentration and foam stability, as higher concentrations facilitate the formation of more robust and persistent foam films. Both surfactants were thus classified as metastable foaming agents, exhibiting enhanced performance at elevated concentrations [32]. Similarly, Patel et al. (1996) observed that SDS solutions exhibit maximum film stability at a concentration of approximately 200 mM, where an enhanced structural integrity of the film and increased resistance to rupture were reported. In their study, SDS concentrations ranging from 50 to 300 mM were examined, specifically at 50, 100, 150, 200, 250, and 300 mM. The results demonstrated that the stability of the thin liquid films increased progressively with concentration, up to 200 mM [33]. Therefore, based on all the above, it is evident that increasing the concentration of the surfactant, up to a certain limit, is an effective strategy for improving the stability of liquid films, making them more reliable for industrial and laboratory applications.

3.3. Effect of Salt Presence in Surfactant Solutions on the Lifetime of the Liquid Film

Surfactants are essential components of various commercial products, such as personal care items, household cleaners, and petrochemicals, as well as industrial processes, for instance, drilling operations for the recovery of oil and natural gas [34]. A fundamental characteristic of these chemical substances is their ability to reduce the surface tension of a solution, thereby promoting the rapid formation of stable foams. The coexistence of surfactants with other compounds, such as polymers (e.g., xanthan gum), polyethylene glycol (PEG), or other surfactants of similar or different categories (non-ionic, cationic, anionic, amphoteric) [35] as well as, more notably, with salts, can significantly influence their physicochemical properties. Consequently, these interactions may affect both the foaming capacity and the stability of the resulting foam [36]. More specifically, the use of salts in commercial products and industrial applications, with the aim of stabilizing foams and, particularly, their liquid films, is a common practice [37]. Although some studies report that the presence of salts may adversely affect either the formation or the stability of foams, the majority of published research indicates that the addition of salts to surfactant solutions generally has a positive effect on the stability of the resulting foam. Thus, in the present study the addition of salt to the surfactant solutions was carried out in order to investigate its potential stabilizing effect on the liquid film. As shown in Figure 5, the experimental results indicate that for SDS solutions at concentrations below the CMC (CMC = 2360 ppm), the presence of NaCl contributes to the stabilization of the liquid bridge. This observation is consistent with findings reported in the literature. More specifically, at a concentration of 1000 ppm (Figure 5a), the addition of NaCl leads to a noticeably slower decrease in the normalized apparent conductivity over time, indicating an increased film lifetime and enhanced resistance to drainage and rupture. This behavior is attributed to improved surfactant adsorption at the gas–liquid interface and reduced electrostatic repulsion, as typically observed below the CMC. At 2000 ppm (Figure 5b), NaCl continues to have a stabilizing effect, with the conductivity values remaining higher over time compared with the corresponding solution without salt. However, the difference is less marked than at 1000 ppm, suggesting that as the concentration approaches the CMC, the stabilizing influence of salt begins to diminish, possibly due to partial interfacial saturation or changes in micellar behavior.

While this stabilizing effect was confirmed for NaCl in our study, the broader literature indicates that such effects are highly dependent on the specific type and concentration of both the surfactant and the salt involved [38]. Numerous salts, such as CaCl₂, MgCl₂, AlCl₃, KCl, Na₂SO₄, ZnSO₄, and CuSO₄ have been documented and studied in numerous surfactant solutions. However, the present study focused exclusively on NaCl, and its selection was based on its relevance to real-world applications and its well-documented interactions with surfactant systems. Among various salts studied in the literature, NaCl is by far the most commonly encountered, both in natural environments and industrial processes, making it a logical choice for investigating its effect on film stability in surfactant solutions. This prevalence renders its interaction with surfactants quite common, either directly or indirectly. The main reason for this emphasis, however, lies in its low cost and easy availability in large quantities. As a result, this particular salt is frequently added to soaps and to foams used in enhanced oil recovery, while there have also been reports of the use of saline (seawater) in flotation processes to stabilize froth and increase mineral recovery [39,40]. Furthermore, NaCl is among the most extensively studied salts in the international literature with regard to its effect on surfactant behavior. Notably, its most significant positive influence has been observed in solutions of anionic surfactants, especially SDS, which is the primary reason it was selected for the present study.

For instance, as reported in the study by Obisesan et al. [41], the addition of NaCl reduces surface tension and enhances the adsorption of surfactant molecules at the gas–liquid interface, thereby increasing the stability of both the foam and the interfacial films. Moreover, the article states that this salt decreases electrostatic repulsion, allowing for a denser packing of surfactant molecules, which in turn reinforces the elasticity and stability of the foam. Specifically, it is observed that at moderate NaCl concentrations (7% and 9% w/w) in the solution, the drainage rate is reduced, resulting in more stable foam systems. In continuation of the literature review, beyond the aforementioned study, a substantial number of research articles report that the presence of salts in surfactant solutions enhances foam stability by stabilizing bubbles and preventing their coalescence [42]. This inhibition of bubble coalescence is attributed to the interactions between salt ions and surfactant molecules [43]. However, it is important to note that this beneficial effect diminishes when the salt concentration exceeds a certain threshold, which is directly related to the surfactant concentration. In all cases, the specific nature of both the surfactant and the salt plays a critical role.

Specifically, Wang, Nguyen and Farrokhpay [44] observed in their study that increasing the concentration of NaCl (from 0 mM to 1 mM) initially stabilizes the foam by reducing electrostatic repulsion and enhancing the adsorption of SDS at the interface (at a bulk concentration of 0.1 mM). However, when the salt concentration (NaCl) exceeds a certain threshold (10 mM and 100 mM), foam stability decreases significantly. This is attributed to a substantial reduction in the Debye length and zeta potential, which weakens the repulsive forces between the thin liquid films, ultimately leading to increased foam instability. Therefore, it can be concluded that the beneficial effect of NaCl is lost at these higher concentrations.

The foam stability of anionic surfactants, such as SDS, is significantly influenced by the presence of inorganic salts in the aqueous phase. According to the study by Gao et al. [45], which investigated the behavior of SDS in solutions of varying salinity, maximum foam stability was observed at an SDS concentration of 0.5% w/w and NaCl concentrations of up to 7.5% w/w. Under these conditions, both the foam longevity (half-life) and mechanical strength were markedly enhanced compared with solutions of lower or higher salinity. The addition of Na⁺ ions reduces the electrostatic repulsion between the negatively charged headgroups of SDS molecules, promoting tighter molecular packing at the gas–liquid interface. This effect leads to a thicker liquid film surrounding the bubbles and decreased liquid drainage between them—key factors contributing to the overall increase in foam stability. Similar results have been reported in the study by Bera et al. [46], in which the foaming behavior of solutions containing anionic and cationic surfactants was examined under environments of varying salinity. Specifically, the authors investigated solutions of 0.5% w/w (5000 ppm) SDS, which, as previously mentioned, is a widely used anionic surfactant, as well as solutions of 0.5% w/w Cetyltrimethylammonium Bromide (CTAB), which belongs to the category of cationic surfactants. These solutions were studied in environments containing 2% and 4% w/w NaCl. The results showed that, for both surfactant systems, the presence of 4% w/w NaCl had a positive effect on the stability of the generated foam. This observation was attributed, as in previous studies, to the ability of the salt to reduce the electrostatic repulsive forces between surfactant molecules, thus promoting the formation of more stable and durable films.

Particularly noteworthy are the findings of the study by Wang and Yoon [47], which once again confirm the positive role of salts. According to their results, the addition of NaCl to SDS solutions significantly influences foam stability. The authors observed that at an SDS concentration of 1 × 10⁻⁴ M (approximately 28.8 ppm) and NaCl concentration of 1 × 10⁻³ M (approximately 58.5 ppm), the highest stability of the liquid films within the foam was recorded. In this case as well, the presence of NaCl was reported to enhance the adsorption of surfactant molecules at the gas/liquid interface and to increase the elasticity of the film, by reducing electrostatic repulsions and delaying film rupture. In contrast, at higher NaCl concentrations (e.g., 0.3 M), foam stability was found to decrease, indicating the existence of an optimal salt concentration range for effective foam stabilization. This was also the case for our data. Specifically, at SDS concentrations above the CMC (Figure 5c), the presence of salt exhibited a destabilizing effect on the liquid film. At a concentration of 3000 ppm, which is above the CMC, the addition of NaCl did not significantly improve film stability and may have even slightly reduced it. Although initial conductivity values were somewhat elevated, the decline occurred more rapidly, suggesting that overpacking at the interface or disruption of micellar equilibrium may counteract the stabilizing mechanisms observed at lower concentrations. This observation is in agreement with those reported by Petkova et al. [48]. The researchers demonstrated that the addition of NaCl to SDS solutions at concentrations above the critical micelle concentration (CMC ≈ 8.3 mM or ~2390 ppm) led to a reduction in foam stability. More specifically, for NaCl concentrations ranging from 0.01 to 0.1 M (~585–5850 ppm), a significant thinning of the liquid films and faster drainage were observed, which was attributed to the reduction in repulsive forces between the surfactant molecules. Similar results were reported by Jiang et al. [49], who observed that in SDS solutions with concentrations of 12–15 mM (~3450–4310 ppm), the presence of NaCl at levels of up to 0.2 M (~11,700 ppm) accelerated foam collapse. The authors partially attributed this phenomenon to the potential formation of crystalline structures and alterations in the interfacial behavior of SDS molecules under these conditions. These findings collectively support the hypothesis that, at supramicellar SDS concentrations, the presence of electrolytes such as NaCl exerts a destabilizing effect, impairing the properties of liquid films and consequently reducing the overall foam stability.

The observed dual effect of NaCl on foam film stability aligns with the mechanistic principles well established in interfacial and colloid science. At concentrations below the critical micelle concentration (CMC) of anionic surfactants such as sodium dodecyl sulfate (SDS), the presence of electrolytes like NaCl significantly enhances the adsorption of surfactant molecules at the gas–liquid interface. This occurs due to the screening of electrostatic repulsions between the negatively charged sulfate headgroups of SDS molecules, effectively lowering the surface potential and facilitating denser molecular packing at the interface. As a result, the interfacial film becomes more elastic and cohesive, increasing its resistance to drainage and rupture. This enhanced adsorption mechanism leads to thicker, more stable films, contributing to prolonged foam lifetimes. Quantitatively, it has been reported that the addition of NaCl (4 mM) can reduce the CMC of SDS from approximately 8.2 mM (~2360 ppm) in pure water to around 5.0–5.5 mM (~1450–1600 ppm) [50,51]. This shift reflects the enhanced thermodynamic favorability of micelle and interface formation under higher ionic strength conditions. However, at concentrations above the CMC, the beneficial effects of NaCl diminish or even reverse. Increased ionic strength in this regime may lead to structural rearrangements of micelles and excessive interfacial compression. This reduces the disjoining pressure and accelerates drainage, ultimately promoting film rupture and foam destabilization. These findings are in excellent agreement with prior studies showing optimal NaCl concentrations between 1 and 10 mM for maximum foam stability, particularly in SDS-based systems. In conclusion, the effect of NaCl is strongly dependent on surfactant concentration: at sub-CMC levels, it facilitates interfacial adsorption and stabilizes films, while at super-CMC conditions, it may compromise stability by perturbing micellar and interfacial structure.

In light of the reviewer’s comment, it is important to acknowledge the relevance of dynamic surface tension (DST) in interpreting the stability behavior of foam films. Although DST measurements were not performed in the current study, the existing literature provides valuable insights into how surfactant adsorption kinetics influence interfacial behavior. For instance, refs. [10,52] demonstrated that in SDS solutions near the CMC, the presence of NaCl significantly accelerated interfacial adsorption, reducing surface tension from approximately 60 mN/m (at ~0.1 s) to 45 mN/m within a few seconds. This behavior is attributed to the screening of electrostatic repulsions between the negatively charged headgroups, which facilitates denser surfactant packing and promotes Marangoni elasticity. These mechanisms are particularly relevant to our observations for SDS–NaCl systems at sub-CMC levels, where enhanced film stability was recorded. The improved resistance to film thinning and rupture may thus stem from faster interfacial saturation and increased viscoelastic response. While our electrical technique captures the macroscopic manifestation of such interfacial phenomena, the absence of DST data remains a limitation. Future studies should aim to directly correlate conductivity-based film lifetime measurements with DST profiles, particularly under varying ionic conditions, to elucidate the interplay between adsorption dynamics and foam stability.

4. Conclusions

This study introduced an innovative method for evaluating foam film stability using electrical conductance measurements of liquid bridges formed between vertically aligned electrodes. The technique allowed for the real-time monitoring of film drainage and rupture under controlled humidity and temperature conditions, providing a sensitive and reproducible model system that closely resembles the behavior of films within real foam structures. The experimental findings revealed a clear inverse relationship between drainage rate and film lifetime, with faster drainage (20 μL/h) leading to more rapid thinning and earlier rupture. Surfactant concentration was also found to be a key factor: higher levels of both SDS and Ethylan enhanced film stability by promoting more effective molecular adsorption at the gas–liquid interface, resulting in increased interfacial elasticity. The addition of NaCl exhibited a concentration-dependent influence. At sub-CMC levels, NaCl enhanced film stability by reducing electrostatic repulsion and promoting surfactant adsorption. However, at concentrations above the CMC, the presence of salt destabilized the film, likely due to interfacial overpacking and micellar restructuring that accelerated drainage. Beyond these mechanistic insights, the methodology developed here offers a valuable platform for the quantitative characterization of foam stability with minimal sample volumes and a high temporal resolution. This has significant implications for applications in cosmetics, pharmaceuticals, food technology, firefighting foams, and oil recovery, where foam control is critical. Future research should explore the extension of this technique to:

• a wider range of surfactant types (e.g., cationic, amphoteric),
• different salt species and ionic strengths, and
• complex formulations that better represent real-world systems.

Additionally, integration with optical or rheological measurements may further enhance the mechanistic understanding of foam dynamics.