Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

The Surface Evolver is an interactive program for studying the shapes of liquid surfaces. Recently added features permit the calculation of the Hessian matrix of second derivatives of the energy. The Hessian can be used for fast convergence to an equilibrium, and eigenvalue analysis of the stability of that equilibrium. This paper describes the use of the Hessian by the Surface Evolver, presents some sample stability analyses, and gives some numerical results on the accuracy and convergence of the methods. It is also shown how one can evolve unstable surfaces.

The Surface Evolver and the Stability of Liquid Surfaces

Open droplet microfluidic platforms offer attractive alternatives to closed microchannel devices, including lower fabrication cost and complexity, significantly smaller sample and reagent volumes, reduced surface contact and adsorption, as well as drop scalability, reconfigurability, and individual addressability. For these platforms to be effective, however, they require efficient schemes for planar drop transport and manipulation. While there are many methods that have been reported for drop transport, it is far more difficult to carry out other drop operations such as dispensing, merging and splitting. In this work, we introduce a novel alternative to merge and, more crucially, split drops using laterally-offset modulated surface acoustic waves (SAWs). The energy delivery into the drop is divided into two components: a small modulation amplitude excitation to initiate weak rotational flow within the drop followed by a short burst in energy to induce it to stretch. Upon removal of the SAW energy, capillary forces at the center of the elongated drop cause the liquid in this capillary bridge region to drain towards both ends of the drop, resulting in its collapse and therefore the splitting of the drop. This however occurs only below a critical Ohnesorge number, which is a balance between the viscous forces that retard the drainage and the sufficiently large capillary forces that cause the liquid bridge to pinch. We show the possibility of reliably splitting drops into two equal sized droplets with an average deviation in their volumes of only around 4% and no greater than 10%, which is comparable to the 7% and below splitting deviation obtained with electrowetting drop splitting techniques. In addition, we also show that it is possible to split the drop asymmetrically to controllably and reliably produce droplets of different volumes. Such potential as well as the flexibility in tuning the device to operate on drops of different sizes without requiring electrode reconfiguration, i.e., the use of different devices, as is required in electrowetting—therefore makes the present method an attractive alternative to electrowetting schemes.

https://users.monash.edu/~lyeo/Dr_Leslie_Yeo/Publications_files/authorreprints.pdf

Planar Microfluidic Drop Splitting and Merging

The mobility of drops on surfaces is important in many biological and industrial processes, but the phenomena governing their adhesion, which is dictated by the morphology of the three-phase contact line, remain unclear. Here we describe a technique for measuring the dynamic behaviour of the three-phase contact line at micron length scales using environmental scanning electron microscopy. We examine a superhydrophobic surface on which a drop's adhesion is governed by capillary bridges at the receding contact line. We measure the microscale receding contact angle of each bridge and show that the Gibbs criterion is satisfied at the microscale. We reveal a hitherto unknown self-similar depinning mechanism that shows how some hierarchical textures such as lotus leaves lead to reduced pinning, and counter-intuitively, how some lead to increased pinning. We develop a model to predict adhesion force and experimentally verify the model's broad applicability on both synthetic and natural textured surfaces.

Self-similarity of contact line depinning from textured surfaces

Flow boiling is an important heat dissipation method for cooling high heat flux surfaces in many industrial applications The heat transfer can be further enhanced by using porous media surfaces due to their high specific surface areas However, although flow boiling in channels is well understood, the phase-change behavior with the additional capillary effect induced by the porous structures is not well understood, and the design of the porous structures is difficult to avoid dryout and over-temperature accidents A pore-scale lab-on-a-chip method was used here to investigate the flow boiling heat transfer characteristics inside micro-porous structures The flow patterns, captured in the two-phase region with a uniform pore-throat size of 30 μm, showed that liquid was trapped in the pore-throat structures as both dispersed liquid bridges and liquid films Moreover, the liquid film was shown to be moving on the wet solid surface by laser-induced fluorescence and particle tracking A theoretical analysis showed that the capillary pressure difference between adjacent liquid bridges could drive the liquid film flows, which helped maintain the coolant supply in the two-phase region The pore-throat parameters could be designed to enhance the capillary pressure difference with multiple throat sizes of 10 – 90 μm which would enhance the heat transfer 5% – 10% with a 5% – 23% pressure drop reduction This research provides another method for improving the flow boiling heat transfer through the porous structure design besides changing the surface wettability

Role of trapped liquid in flow boiling inside micro-porous structures: pore-scale visualization and heat transfer enhancement

The formation of a liquid bridge in non-parallel plates is very common and the stability (whether or not it can move spontaneously) of such liquid bridges has been studied a lot for industry, e.g. in printing applications. It is generally considered that the liquid bridge stability is determined by Contact Angle (CA), Contact Angle Hysteresis (CAH), the position of the liquid bridge (represented as P) and the dihedral angle (θ) between non-parallel plates. The stability equation is θ = f(CA, CAH, P). Since P is a process quantity, which is difficult to determine, so it is also difficult to obtain the critical equation for the stability of the liquid bridge. In the previous study (J. Colloid Interface Sci., 2017, 492, 207–217), based on the fitting simulation results, the critical equation about CA, CAH and θ is obtained, as θ = f(CA, CAH). However, in some special cases, the results are still biased (e.g. the weak hydrophilic situation). In this paper, unlike simulation, we get the critical equation θ = f(CA, CAH) from a theoretical point of view. For the first time, by in-depth analysis of the process of liquid bridge formation, the theoretical calculation equation of P is obtained as P = f(CA, CAH, θ). And then, combining the equations θ = f(CA, CAH, P) and P = f(CA, CAH, θ), the theoretical equation is obtained. A lot of simulations and experiments were performed to verify our theoretical equation. Furthermore, comparing our equation with the previous equation, it was found that our equation is more consistent with the experimental results (error less than 0.2°). Finally, the importance of considering the liquid bridging process (the function of P) for stability analysis is illustrated by comparing the results with those not considered (the difference is more than 20% in some cases). The outputs of this paper provide in-depth theoretical support for the analysis and application of liquid bridges.

/pdf/influence-of-liquid-bridge-formation-process-on-its-2gk8240zc9.pdf

Influence of liquid bridge formation process on its stability in nonparallel plates

Due to the superior operability and good anti-interference, the prospect of controlling microdroplets using a parallel plate structure (PPS) is very promising. However, in practical applications, droplets in such structures are often affected by various factors, resulting in deformation, evaporation, stress rupture and other phenomena, leading to equipment failure. Therefore, how to simply and effectively transfer liquid droplets to PPS to maintain the stable and efficient operation of the system has become an urgent problem to be solved. In this paper, a simple and effective ratchet-like strategy (relaxing and squeezing actions) is introduced to transfer droplets. To analyze the mechanism of the strategy and optimize the control, we conduct this study from three aspects. First, the droplet movement trend is obtained by analyzing the pressure between SPS and PPS. Second, the reasons why the droplet can achieve this inward motion are investigated. Through theoretical analysis, which is also proven by simulations and experiments, we creatively put forward that the asymmetric change of the contact angle (CA) induced by the asymmetric structure is the fundamental cause of this kind of motion. Due to the asymmetric change of the contact angle, the CA in the PPS will reach the advancing angle first in the squeezing process, and the CA in the SPS will reach the receding angle first in the relaxing process, thus causing the inward movement of the droplet. Third, to optimize this strategy, the effects of the following governing parameters are researched individually based on the corresponding simulations and experiments: the control parameters (the initial gap width of the PPS H0 and the amount of squeezing and relaxing of ΔH) and the thickness of the top plate. Subsequently, an optimized ratchet-like cycle is achieved. In summary, these findings not only provide a new method by which to realize the movement of droplets toward hydrophobic PPSs but also creatively point out the cause of the ratchet strategy, which can be applied in many microfluidics fields.

/pdf/motion-of-droplets-into-hydrophobic-parallel-plates-3mzlwds8uc.pdf

Motion of droplets into hydrophobic parallel plates

The potentiostat is critical in the development of electrochemical systems;
 however, its cumbersome detection and high cost considerably limit its
 large-scale application. To provide an affordable alternative to developing
 countries and resource-constrained areas, this study designs an
 electrochemical detection system based on smartphones, which uses Bluetooth
 Low Energy to convert open-source potentiostat data based on PSoC-5LP. The
 WeChat application on the smartphone provides an interface for entering
 experimental parameters and visualizing the results in real time. The
 smartphone-based electro-chemical detection system has a simple design and
 reduces the size (10?3?0.3 cm) and the cost of the hardware ($ 18). The
 system performs the most commonly used cyclic voltammetry for
 electrochemical detection, with results that are comparable to those
 obtained using a commercial potentiostat and an error rate of 1.3 %. In the
 classical teaching experiment of electrochemical determination of ascorbic
 acid in orange juice samples, the measured value of the system is 0.367 ?
 0.012 mg/mL, compared with the standard reference value of 0.37 mg/ mL,
 which is obviously a convincing value. Therefore, this system is a low-cost,
 reliable alternative to a potentiostat for research, education, or product
 integration development.

http://www.doiserbia.nb.rs/ft.aspx?id=0352-51392200018S

Design and implementation of low-cost portable potentiostat based on wechat

When a droplet is pinned between a single plate structure (SPS) and a parallel plate structure (PPS), a liquid bridge is formed, which often occurs in digital microfluidics. The understanding of this liquid bridge formation process is still undeveloped; for this reason, the function of the side surface of the up-plate is ignored. In this paper, the formation of a liquid bridge is studied. First, based on pressure analysis, the formation process of a liquid bridge can be divided into two parts. The first part is the wetting motion, which indicates that the side and bottom surfaces attract the droplets according to their wetting force to form a liquid bridge. The second part is the pressure motion, which indicates whether the droplets enter or exit the PPS unidirectionally according to the pressure difference (capillary force) between the SPS and the PPS. The influence of the contact angle hysteresis (CAH) is simulated, and the results indicate that the CAH on the bottom surface plays a more important role than the CAH on the side surface. In addition, the influence of both the material and geometric parameters on the amount of bridge motion is studied. The results show that the thinner the upper plate is, the larger the PPS gap is, and the better the droplet entering the PPS is. These conclusions can be used to obtain better performance when droplets need to be delivered to the PPS on a microfluidic chip.When a droplet is pinned between a single plate structure (SPS) and a parallel plate structure (PPS), a liquid bridge is formed, which often occurs in digital microfluidics. The understanding of this liquid bridge formation process is still undeveloped; for this reason, the function of the side surface of the up-plate is ignored. In this paper, the formation of a liquid bridge is studied. First, based on pressure analysis, the formation process of a liquid bridge can be divided into two parts. The first part is the wetting motion, which indicates that the side and bottom surfaces attract the droplets according to their wetting force to form a liquid bridge. The second part is the pressure motion, which indicates whether the droplets enter or exit the PPS unidirectionally according to the pressure difference (capillary force) between the SPS and the PPS. The influence of the contact angle hysteresis (CAH) is simulated, and the results indicate that the CAH on the bottom surface plays a more important role ...

Formation of special liquid bridges between a single plate and parallel plates

When a digital microfluidic experiment was done the process of droplet moving from single-plate structure (open part) into parallel-plate structure (covered part) was always needed. But it should be noted that it is difficult for big water drops moving into parallel-plate structure with hydrophobic surfaces. But we have recently observed an interesting phenomenon: by repeatedly up and down the upper plate, a small scaled water drop was able to run into the gap of parallel plates with hydrophobic surfaces that were approximately vertically placed. We have analyzed the subtle physical mechanism responsible for droplet transport and demonstrated experimentally. This incident effect of dynamic wetting provides a new method to move droplet and this dynamics of controlling way may be widely used in future application.

Xiongheng Bian

Papers

Influence of liquid bridge formation process on its stability in nonparallel plates

Motion of droplets into hydrophobic parallel plates

Design and implementation of low-cost portable potentiostat based on wechat

Formation of special liquid bridges between a single plate and parallel plates

Behavior of a water drop moving inside parallel plates