The dynamics of directional transport of a droplet in programmable electrowetting channel

Inspired by the light-diverging effect induced by convex mirrors, we propose a novel droplet actuator for potential application in mixing/reaction. We use a rectangle formed by an open tunnel and two convex surfaces. When a droplet contacts a convex surface, a sufficiently large thrust force is generated due to contact-induced explosive boiling, generating self-sustained droplet oscillation. The convex surface enhances the droplet motion in the width direction. A thermal insulation technique is applied to reach hotter hydrophilic convex surfaces but a colder superhydrophobic tunnel. The former generates thrust and scatters the droplet trajectories, but the latter is used for rapid droplet transportation. The collision frequency with the tunnel surface when using the convex side surface (CSS) with θ = 15.0° is three times that obtained from the flat side surface (FSS), where θ is the half-curvature angle of CSS. Holding CSS constant, the enhanced collision with the tunnel surface creates fold line trajectories for the droplet. Meanwhile, the statistical droplet velocity in the y-component when using the CSS is two times that obtained using the FSS. Incident angles and reflection angles are analyzed for collisions with the contact boiling surfaces. We conclude that the range of reflection angles becomes larger when using the CSS. Droplet rotation further increases the reflection angle. The enlarged reflection angle is the key mechanism for forming the fold line trajectory and enhancing the motion in the width direction. This work provides a new principle for creating perturbations to enhance mixing/reaction for chemical and biological applications.

Enhanced y-component droplet motion via droplet-wall-contact induced explosive boiling on a convex surface

2023

Morphology evolution and dynamics of sliding nanodroplets under external forces: A molecular dynamics study

Liu

Zhang

2023

The effects of external forces and surface wetting properties on the directional motion of nanodroplets are studied through molecular dynamics simulations. We find that when droplet motion reaches a steady state, the steady velocity shows an unusual relationship with the external force such that the profile has two nearly linear regimes. This behavior differs from previous experimental and theoretical results. The underlying mechanism of the two linear regimes is explored and is closely related to the two distinct regimes of droplet deformation. The transition between these two regimes can be roughly determined by a value of approximately 1.2 in terms of the ratio of Le*L0* or Se*S0*. In addition, we observe an interesting phenomenon, a “caterpillar-crawling-like motion,” rarely reported in experiments thus far, which involves periodical oscillations in both the droplet shape and the velocity of the center of mass of the droplet. Moreover, we construct a dynamic phase diagram to describe the correlation between the transition of dynamics and the morphological transition among various shapes (nearly round, corner-like, pear-like, zucchini-like, calabash-like, and rivulet-like).

Three-dimensional pore-scale simulations of dynamic wicking processes on micro-structured wicks

Li,

Zhang,

Cheng

et al. 2024

Capillary wicking characteristics play an important role in two-phase thermal management devices including heat pipes and vapor chambers, yet three-dimensional (3D) pore-scale simulations of the dynamic capillary wicking process on various micro-structured surfaces have been rare. In this paper, we conduct 3D pore-scale simulations of capillary wicking on three commonly used micro-structured wicks including micro-pillar array, micro-channel, and sintered particles. The micro-scale liquid propagation dynamics and the “stick-slip” behavior of the propagating liquid front are captured using a 3D pseudo-potential multiple-relaxation-time lattice Boltzmann method. Based on the Lucus–Washburn approach and a work-energy approach, we theoretically analyze wickabilities of different micro-structured wicks. Effects of wick geometry and structural parameters on the capillary wicking characteristics are discussed. We demonstrate that an optimal pillar pitch distance exists, which maximizes the wickability of the micro-pillar array. We show that when the porosity is relatively low, the wickability of the micro-channel is higher than that of the micro-pillar array and the sintered particles. When the porosity is large, however, the sintered particles exhibit higher wickability than the micro-pillar array and the micro-channel. We also demonstrate that the capillary pressure of the sintered particles is always higher than that of the micro-pillar array and the micro-channel throughout the porosity range investigated. The numerical simulation results are compared with theoretical predictions. Findings in this work provide guidelines for the designs of porous wick in various two-phase thermal management systems for high heat flux devices.