Droplet evaporation and de-pinning in rectangular microchannels
Abstract: Experimental and numerical studies are presented for evaporation of micro-droplets of deionised (DI) water and toluene on lead zirconate titanate (PZT) substrates. The microchannels are fabricated with SU-8 2025 and 2075. The effects of channel width and depth on the evaporation and de-pinning rates of embedded micro-droplets are presented and compared for both fluids. The study reveals a partially hydrophobic nature of SU-8/PZT microchannel to DI water and a complete wetting when toluene is used as the droplet. The rate of evaporation of toluene is about double the rate of evaporation of DI water. Comparisons of the rates of evaporation and de-pinning show that the channel width has a larger effect on evaporation than the depth of the channel. The equivalent contact angle of the pinned film and bulk fluid compensated for the evaporation of the droplet. Surface roughness was also shown to have a significant effect on the pinned film in the rectangular microchannels.
Summary (2 min read)
- Various ways have been invented and commercialized to harness energy from water, such as hydroelectricity, tidal energy, among others.
- Heat recovery by electrical components in MEMS devices can be used to generate voltages that power individual electrical components within microchips.
- In order to predict the dynamics of the evaporation, the hydrodynamics of droplet spreading must be well understood.
- This paper investigates droplet spreading in a rectangular microchannel.
- The constant change in contact angle leads to a variation in the advancing and receding contact angles.
2. Problem Formulation
- The evaporation of a droplet in a rectangular channel has three major thermophysical processes, namely mass transfer of the bulk fluid into the gas phase, heat transfer as a result of natural convection and Marangoni convection due to temperature gradients.
- The mass transfer and natural convection are assumed to be the major transport phenomena that affect the evaporation of the droplet.
- Heat addition was not implemented in the current experimental studies, hence the temperature gradients are neglected during the evaporation process.
2.1 Droplet evaporation model
- The droplet can either have a concave or convex meniscus with respect to the substrate of the channel.
- The model in this paper is developed for the hydrophobic case, as per the experiments.
- The model can also be extended to the hydrophilic case, since the main difference is the contact angle and volume associated with the concave meniscus.
- Combining equations (1) and (6), the combined effect of diffusion and convection of the vapor from the surface will give the total effect of evaporation on the width of the droplet.
- The experimental results show that a thin layer exists between the main part of the droplet and the wall.
2.2 Droplet de-pinning model
- This separation occurs along the contact line of the main droplet.
- Unlike the evaporation process, the de-pinning process starts from the center of the channel and progresses in both directions simultaneously towards the wall.
- The phenomenon is a complex process to predict accurately.
- The volume of the pinned film can be estimated as the volume of the spherical cap of the droplet and the cuboid along the channel length.
- A value of 0.3 was used for DI-water and a value of 0.6 was used for toluene, based on a similar asymptotic analysis performed previously by Cachile et al. .
3. Experimental Apparatus and Procedure
- Fabrication steps of the MHE are illustrated in Fig. 4. The MHE is fabricated with PZT wafers.
- Three sets of wafers of different thicknesses were examined for the fabrication (100µm, 80µm and 60µm).
3.1 Fabrication of rectangular microchannel
- The PZT substrates of different thicknesses were cleaned with an RCA 1 cleaning procedure.
- It was difficult to completely dry the fragile substrate; hence before proceeding to the next step, the substrate was heated to 120oC for two minutes on the vacuum hot plate and allowed to cool.
- SU-8 Developer form Microchem was used to process the pattern after post exposure baking.
3.2 Experimental measurement and uncertainty
- A 10µl syringe was used to dispense the droplet into a microchannel with the aid of a stereo microscope in the experimental measurements.
- The images were recorded at a frequency that varied between 25 and 32 frames per second (fps).
- The evaporation and de-pinning processes of the droplet were recorded by Stream Pix III imaging software, which allows for digitization and characterisation of the images.
- The single sample uncertainty measurement allowed this set of data to be discarded during the analysis of the experimental data.
- The following section highlights results from the analytical modeling and experimental results.
4. Results and Discussion
- Experimental and analytical results of the evaporation and de-pinning of the toluene and DI-water will be reported in this section.
- Smaller channels were observed to have larger separation distances, which indicate a higher surface tension in smaller channels.
- This occurs due to the separation distance between the bulk fluid and the pinned film as shown in Fig. 6(a).
- The effects of contact angle and channel width on the evaporation rate are reported in Fig. 12.
- The rate of de-pinning is significantly increased across the width after about 9.04s (Fig. 14(c)).
- This paper examined the use of a PZT substrate in microfluidic transport of droplets.
- New experimental and analytical results have been presented for these droplet transport processes.
- Internal recirculation was observed during the evaporation process.
- Threedimensional printing and other MEMS applications could benefit from the experimental and analytical results in this study.
- The results also have relevance in biomedical applications where small particles of solute can be distributed through complex meshes.
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Cites background from "Droplet evaporation and de-pinning ..."
...This is a common effect in microfluidic systems ....
"Droplet evaporation and de-pinning ..." refers methods in this paper
...The evaporation rate of the droplet has been predicted by Deegan , Cachile et al....
...This is the technology used for inkjet printers  as the carrier liquid evaporates and patterns are deposited on the paper....
...The equation of the average velocity over the width can be expressed as 0 1( , ) ( , )o w xu w t j w t dw A tρ ∂ = − + ∂ ∫ (8) The evaporation rate of the droplet has been predicted by Deegan , Cachile et al.  and Poulard et al.  to be proportional to the change in pinning radius based on the instantaneous time and final time....
...Deegan  used this transport phenomenon to explain the pattern of satins deposited by a drop of coffee....
...The separation of these two layers of the liquid was used to transfer solid particles in a liquid droplet to a substrate in a previous study by Deegan ....
"Droplet evaporation and de-pinning ..." refers background in this paper
...6 Past studies  have conflicting information about the behaviour of water droplets on the micro/nano-scale....
...The change in contact line elastic force has an effect on the de-pinning rate ....
...Bonn  also indicated the complexity of these phenomena remained a challenge in accurately predicting the de-pinning process....
"Droplet evaporation and de-pinning ..." refers background or methods in this paper
...Past studies have generally assumed a spherical droplet [7, 9, 11, 14], but with the existence of a film, which is...
...Secondly, the other method of evaporation occurs where the contact radius decreases while the contact angle remains constant ....
...Firstly, the droplet radius remains approximately constant, while the contact angle of the liquid-substrate contact angle decreases ....
...The authors developed a method by assuming an equivalent contact angle, based on the change in volume of the droplet and proportional to an exponential function of time ....
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Frequently Asked Questions (2)
Q1. What are the future works in this paper?
Further studies would be required to determine the actual effect of the overall volume change on the de-pinning process.
Q2. What are the contributions in this paper?
Odukoya et al. this paper investigated the evaporation and de-pinning rates of micro-droplets of deionized ( DI ) water and toluene on lead zirconate titanate ( PZT ) substrates.