2.1. Testing Rig
The testing rig (
Figure 1a) includes a droplet generator unit described further in
Section 2.2, a substrate plate as explained in
Section 2.3, a microcontroller unit (MCU, Arduino UNO R3, Tokyo, Japan), a light source (Veritas, Pasadena, CA, USA), two high speed cameras: a front camera (Photron Mini UX100, Tokyo, Japan) and a top camera (Baumer VLXT-17M.I, Southington, CT, USA), and a desktop PC (HP Z240 with 64 GB of RAM), mounted on a bench-testing rig frame.
The substrate plate is fixed on the substrate platform. Both the substrate platform and the front camera are installed on a substrate-camera frame. It allows changing the substrate inclination angle by mounting wedges with the required angle between the substrate-camera frame and the bench-testing rig frame (
Figure 1b). Thus, one does not need to recalibrate the front camera every time the angle changes.
The droplet generator unit is mounted above the substrate platform on a separate tower assembled from standard 2000 mm × 40 mm × 40 mm aluminum profiles with 10 mm grooves using standard fixture elements. This design provides versatility and ease of installation on the experimental stand. The drop height is adjustable in the range between 200 and 1800 mm.
The PC and the MCU serve for synchronization of the droplet generator with cameras and the light source, and record ambient air temperature and humidity. Furthermore, they ensure the required droplet size. The control parameters for adjusting the droplet size are as follows:
In the case of
n = 0, there is an electric single pulse mode of the droplet generator with the possibility to retrieve the droplet with the minimal size by one impulse (
τl); If
n > 0, this is an electric multi-pulse mode which consists of “setting the required droplet volume” stage and “dropping” stage (
Figure 2).
The parameters were tuned so that only a single droplet was produced in each test. The MCU, after receiving a command from the PC, sends a specified series of pulses to the power amplifier, turns on the lighting, and starts recording from the high-speed cameras.
We use a frame rate of 10,000 fps for the front camera at a resolution of 1280 × 480 pixels. This limits the field of view to a rectangle of size 40.5 mm × 15.2 mm. The video files from the front camera are automatically analyzed using a custom developed program in Matlab (MathWorks, Natick, MA, USA). For each video, the number of droplets formed, the diameter of each droplet, and the speed are determined.
The top camera with a frame rate of 400 fps at a resolution of 800 × 800 pixels not only studies possible drop impact outcomes, but also monitors particle motion inside impinging drops, along with the final particle distribution. The field of view at the substrate level is limited to a 65.6 mm × 65.6 mm rectangle.
2.2. Droplet Generator Unit
The droplet generator unit consists of a piezoelectric injector and hydraulic machinery, which are mounted on the droplet generator frame (
Figure 3a). The piezoelectric injector consists of a piezoelectric element and a recoil spring, see
Figure 3b. It allows rapid changing between open and closed states, with a switching time duration of several microseconds. The control system provides the shapes, durations, and sequences of electric pulses, thereby ensuring the generation of droplets at the desired size.
The hydraulic machinery consists of a hydraulic container and a test container, see
Figure 3b. They are connected by a quick-release latch. Thus, the test container can be quickly mounted or unmounted by turning the latch. This design also allows for a quick change between test containers of different sizes. At the same time, it prevents any direct contact between the test liquid and the hydraulic system above the latch. This becomes important when carrying out experiments with corrosive liquids, liquids containing solid particles, high viscosity liquids, etc. The test container ends with a Luer lock fitting for tight connection with nozzle tips of various diameters and shapes.
2.4. Suspensions
For conducting droplet–wall impact experiments, different liquid-particles combinations were used for preparing the suspensions (
Table 2).
Suspension selection was based on a one-dimensional sediment model which allowed for estimating relative particle concentration changing in the droplet hanging onto the syringe tip. The criteria were that the particle concentration changing in the hanging droplet must be less than 20% over the 20-s duration of the experiment.
Each liquid-particle type combination (C1–C11) had two suspension variants with two different particle volume fraction values: 5% and 10%. The parameters of the liquids and the particles are summarized in
Table 3 and
Table 4. The total count of suspensions was 22.
These suspensions allowed for the study of the effects of the liquid properties (such as viscosity and surface tension) and the particle properties (such as size range, particles concentration, and particle-to-liquid density ratio) on the impact of suspension droplets on the wall. The results shown in the present paper constitute a representative subset to demonstrate the feasibility of such a suspension droplet impact test using the proposed bench-testing rig.