# Air Trapping Mechanism in Artificial Salvinia-Like Micro-Hairs Fabricated via Direct Laser Lithography

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

^{®}(R2016b, The Mathworks, Inc., Natick, MA, USA), allowing the fabrication of different designs. The preparation of the sample required several steps for ensuring the adhesion of the structures on the glass. First of all the glass substrate was rinsed with acetone, isopropyl alcohol (IPA), and deionized water. It was submerged for 10 min in a 50 mL ethanol solution with eight drops of 3-(Trimethoxysilyl) propyl methacrylate. The glass was rinsed with deionized water and dried with air. Then the photoresist was spin coated on the substrate at a speed of 1000 rpm in order to achieve a thickness of 100 µm, which is enough to contain the artificial hairs. After a pre-bake process (10 min at 65 °C followed by a second step at 95 °C for 30 min), the sample was mounted in the laser lithography system (Photonic Professional GT, Nanoscribe GmbH) and exposed to the laser beam with a center wavelength of 780 nm (Calman laser source). The writing speed was 25 μm·s

^{−1}and the laser power 7.5 mW. After a post-bake phase (1 min at 65 °C followed by a second step at 95 °C for 10 min) the sample was developed for 20 min in SU-8 Developer (MicroChem Corp., Westborough, MA, USA) and rinsed with IPA and deionized water.

^{−1}was added to the water. Exciting the sample under the confocal microscope at 401 nm (excitation wavelength of SU-8) and 561 nm (excitation wavelength of TRITC), it was possible to distinguish them. Since experiments required prolonged immersion of the structures in small volume of liquid, samples were encapsulated by means of a poly-(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning Corp., Midland, MI, USA) ring (internal diameter 8 mm; thickness 200 μm) placed on the substrate glass around the structures and sealed with an additional glass after the deposition of 10 μL of TRITC-marked water.

^{®}. Air volumes were estimated by adding the air area in each slide multiplied by the vertical distance.

^{−1}) were tested in order to verify the effect of the colorant on the wettability properties.

## 3. Microfabrication Results

## 4. Experimental Results and Discussion

_{a}) is lower or equal the above atmospheric pressure (p

_{atm}). In fact, when p

_{a}> p

_{atm}, air particles start diffusing from the bubble and reach the atmosphere.

_{w}is the water pressure at the air–water interface, σ is the surface tension, and R is the radius of the bubble), and considering that p

_{w}depends on the submersion depth h [17],

^{3}kg/m

^{3}), we obtain a limit depth h of about 50 cm which is fully satisfied in our experiments.

_{S}, the liquid surface tension γ

_{L}and the solid-water interfacial energy γ

_{SL}

^{®}program, starting from the pictures acquired with the experimental procedure already described. In Figure 4, there are the results of the dynamics of the variation of the air volumes during time. Setting the dimension of the radius of the heads and comparing the different number of filaments, it is evident what was qualitatively appreciable from Figure 3d,f,g, that is a higher number of filaments is able to trap more air (as in the example of radius 15 µm in Figure 4a). On the other hand, setting the number of filaments and varying the dimension of the heads, the bigger one can trap, quite obviously, bigger air volumes (as in the example of N = 8 in Figure 4b). The results of dynamics of the variation of the air volumes trapped in the salvinia-like structures for all the designs in Figure 1d are illustrated in Figures S3 and S4.

_{f}is the final volume of air (stable persistent value), V

_{i}is the value at the starting point and β (min

^{−1}) is a coefficient associated with the rate of the variation of the volume. The fluctuation of the volume until a stable value inside the heads is reached could be due to effects of gravity (even if it is expected to play a minor role at that scales) and pressure fluctuations, but mainly to air diffusion in the water since the sample containing the structures is completely sealed and there is no way for the air to reach the external environment (PDMS is somehow permeable to air, but, due to geometry of the sample, this effect is neglegible). This aspect is also confirmed by the fact that the Cassie–Baxter configuration remains stable up to 100 h. Moreover, it is important to underline that air diffusion in liquid is low dynamics which, however, does not alter the long-term thermodynamics [27]. In respect, the final micro Cassie–Baxter state might indeed be metastable also in an open system if the structure (as in our case) owing to the re-entrant filaments. Nevertheless, this aspect was not investigated in open configuration because evaporation limits the sample living time.

## 5. Conclusions

## Supplementary Materials

^{−1}) on flat SU-8, Figure S3: (a–e) Results of the dynamics of the variation of the air volumes trapped in the salvinia-like structures for all the tested designs (see Figure 1d), grouped for crown-like radius heads. N represents the number of filaments composing the crown-like heads while R is the radius of the heads, Figure S4: (a–c) Results of the dynamics of the variation of the air volumes trapped in the salvinia-like structures for all the tested designs (see Figure 1d), grouped for number of crown-like heads filaments. N represents the number of filaments composing the crown-like heads while R is the radius of the heads.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**(

**a**) Pattern of ‘eggbeater’ hairs that cover the upper side of the Salvinia molesta leaves; (

**b**) Interaction of the hairs with a droplet water, showing the hydrophilic apex able to pin the droplet, while the remaining structure is hydrophobic; (

**c**) Three different designs for the crown-like heads tested in our work consisting of four, six, and eight filaments obtained by the intersection of two, three, and four circumferences rotated by 90°, 60°, and 30° respectively; (

**d**) Table with all the 15 designs studied for air retaining: r is the radius of the head, N the number of its filaments, δ is the number of parallel circumferences actually exposed to the laser beam for the fabrication of the filaments, t is their actual thickness, h is the height of the stalk, and w is its diameter.

**Figure 2.**SEM images of the results of the microfabrication of some hairs designs by means of direct laser lithography: hairs with head radius of 15 µm and four (

**a**–

**c**), six (

**d**–

**f**), and eight (

**g**–

**i**) filaments (design number 4, 5 and 6 respectively). Hairs with head radius of 25 µm and four (

**j**–

**l**), six (

**m**–

**o**) and eight (

**p**–

**r**) filaments (design number 10, 11 and 12 respectively). See Figure 1d for all the geometrical parameters of the presented designs.

**Figure 3.**(

**a**) Sketch of the sample used for the experimental procedure at the confocal microscope: the array (the square in the center) with the structures is confined by a poly-(dimethylsiloxane) (PDMS) ring (in blue), filled with water marked with tetramethylrhodamine-5-(and-6)-isothiocyanate (5(6)-TRITC) (in red) and then sealed with a thin glass slide on the top (not shown); (

**b**) Scheme of the ‘macro’ Wenzel state (the water fills all the room over the hairs and between the stalks) and the ‘micro’ Cassie–Baxter (the water cannot reach the inside of the heads), that are experienced by all the tested samples; (

**c**) Scheme (not in scale) of the interaction of water (in red), air (in white) and SU-8 (structural material, in blue), related to Equation (1). The red and violet arrows show the variation of the contact angle θ during time (from θ

_{t0}to θ

_{t1}respectively); (

**d**) Example of heads with four filaments (design 10) recorded at the confocal microscope, showing the profile of the air–water interface (in white) at the starting point of the experiment; (

**e**) Same sample reported in (

**d**) after 150 min, showing the variation of the air–water interface (in white). Similarly, in (

**f**,

**g**) examples of heads with six filaments (design 14) and in (

**h**,

**i**) of eight filaments (design 12), taken at the starting point of the experiment and after 150 min.

**Figure 4.**(

**a**) Trend of the air volume trapped inside the crown-like heads with a radius of 15 μm for different numbers of filaments; (

**b**) Variation over time of the volume of air retained inside the crown-like heads in the case of N = 8, for different values of the radius;(

**c**) Comparison, for all designs, of the coefficients β related to the rate of reduction of air inside the heads.

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**MDPI and ACS Style**

Tricinci, O.; Terencio, T.; Pugno, N.M.; Greco, F.; Mazzolai, B.; Mattoli, V. Air Trapping Mechanism in Artificial Salvinia-Like Micro-Hairs Fabricated via Direct Laser Lithography. *Micromachines* **2017**, *8*, 366.
https://doi.org/10.3390/mi8120366

**AMA Style**

Tricinci O, Terencio T, Pugno NM, Greco F, Mazzolai B, Mattoli V. Air Trapping Mechanism in Artificial Salvinia-Like Micro-Hairs Fabricated via Direct Laser Lithography. *Micromachines*. 2017; 8(12):366.
https://doi.org/10.3390/mi8120366

**Chicago/Turabian Style**

Tricinci, Omar, Tercio Terencio, Nicola M. Pugno, Francesco Greco, Barbara Mazzolai, and Virgilio Mattoli. 2017. "Air Trapping Mechanism in Artificial Salvinia-Like Micro-Hairs Fabricated via Direct Laser Lithography" *Micromachines* 8, no. 12: 366.
https://doi.org/10.3390/mi8120366