Soft electrowetting

Frequency-Dependent Dewetting of Thin Liquid Films Using External ac Electric Field

The stability of thin liquid films on a surface can be controlled by using external stimuli, such as an electric field, temperature, or light, by manipulating the total excess free energy of the system. It has been previously shown that thin lubricating films on slippery surfaces can be destabilized via the spinodal mechanism using an external electric field, which returns to the original stable configuration upon the electric field. However, the role of the frequency of the applied ac electric field is not clear, which is the main topic of study in this report. When an ac electric field of fixed voltage and varying frequency is applied across thin lubricating films of slippery surfaces, a different dewetting behavior is observed. Characteristic length and time scales of dewetting depend strongly on the frequency of the applied voltage, which is primarily due to the change in the dielectric behavior of the lubricating fluid. In addition, the interplay of various time scales involved in the dewetting process also depends on the frequency.

Probing Liquid Drop Induced Deformation on Soft Solids Using Dual-Wavelength Reflection Interference Contrast Microscopy

A liquid drop resting on a soft solid deforms the surface at the three-phase contact line. The surface deformation, also called "wetting ridge", varies in size from nanoscales to microscales, depending on the elasticity and thickness of the soft layer. In this work, we probe how surface elasticity and coating thickness influences normal and tangential surface deformation profiles induced by a sessile liquid drop using dual-wavelength reflection interference contrast microscopy. Furthermore, we experimentally verify the appropriate characteristic length scale, which closely describes the ridge profiles on both thick and thin soft layers for two different surface elasticities.

Functional liquid droplets for analyte sensing and energy harvesting

Over the past century, rapid miniaturization of technologies has helped in the development of efficient, flexible, portable, robust, and compact applications with minimal wastage of materials. In this direction, of late, the usage of mesoscale liquid droplets has emerged as an alternative platform because of the following advantages: (i) a droplet is incompressible and at the same time deformable, (ii) interfacial area of a spherical droplet is minimum for a given amount of mass; and (iii) a droplet interface allows facile mass, momentum, and energy transfer. Subsequently, such attributes have aided towards the design of diverse droplet-based microfluidic technologies. For example, the microdroplets have been utilized as micro-reactors, colorimetric or electrochemical (EC) sensors, drug-delivery vehicles, and energy harvesters. Further, a number of recently reported lab-on-a-chip technologies exploit the motility, storage, and mixing capacities of the microdroplets. In view of this background, the review initiates discussion by highlighting the different attributes of the microdroplets such as size, shape, surface to volume ratio, wettability, and contact line. Thereafter, the effects of the surface or body forces on the properties of the droplets have been elaborated. Finally, the different aspects of such liquid droplet systems towards technological adaptations in health care, sensing, and energy harvesting have been presented. The review concludes with a tight summary on the potential avenues for further developments.

Impact of substrate elasticity on contact angle saturation in electrowetting

The electrostatically assisted wettability enhancement of dielectric solid surfaces, commonly termed as electrowetting-on-dielectric (EWOD), facilitates many microfluidic applications due to simplicity and energy efficiency. The application of a voltage difference between a conductive droplet and an insulated electrode substrate, where the droplet sits, is enough for realizing a considerable contact angle change. The contact angle modification is fast and almost reversible; however it is limited by the well-known saturation phenomenon which sets in at sufficiently high voltages. In this work, we experimentally show and computationally support the effect of elasticity and thickness of the dielectric on the onset of contact angle saturation. We found that the effect of elasticity is important especially for dielectric thickness smaller than 10 μm and becomes negligible for thickness above 20 μm. We attribute our findings on the effect of the dielectric thickness on the electric field, as well as on the induced electric stresses distribution, in the vicinity of the three phase contact line. Electric field and electric stresses distribution are numerically computed and support our findings which are of significant importance for the design of soft materials based microfluidic devices.

The role of entropy in wetting of polymer brushes

The wetting of polymer brushes exhibits a much richer phenomenology than wetting of normal solid substrates. These brushes allow for three wetting states, which are partial wetting, complete wetting and mixing. Here, we study the transitions between these wetting states for brushes in contact with polymer melts and compare them to predictions using enthalpic arguments based on brush and melt interactions. We show that the transitions are shifted compared to the enthalpic predictions and that the shifts can be positive or negative depending on the length of the melt polymer and the brush grafting density. The reason for this is that these brush and melt parameters can have a positive or negative effect on the entropic contribution to the free energy of the system. Our results highlight the relevance of entropy in predicting the exact wetting transitions, which is important for the design of brush-based coating applications.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.

A thin-film model for droplet spreading on soft solid substrates

The spreading of droplets on soft solid substrates is relevant to applications such as tumor biophysics and controlled droplet condensation and evaporation. In this paper, we apply lubrication theory to advance fundamental understanding of the important limiting case of spreading of a planar droplet on a linear viscoelastic solid. The contact-line region is described by a disjoining-pressure/precursor-film approach, and nonlinear evolution equations describing how the liquid-air and liquid-solid interfaces evolve in space and time are derived and solved numerically. Parametric studies are conducted to investigate the effects of solid thickness, viscosity, shear modulus, and wettability on droplet spreading. Softer substrates are found to speed up spreading for perfectly wetting droplets but slow down spreading for partially wetting droplets. For perfectly wetting droplets, faster spreading is a result of more liquid being pumped toward the contact line due to a larger liquid-film thickness there arising from the repulsive component of the disjoining pressure. In contrast, slower spreading of partially wetting droplets is a result of less liquid being pumped toward the contact line due to a smaller liquid-film thickness there arising from the attractive component of the disjoining pressure. The model predictions for partially wetting droplets are qualitatively consistent with experimental observations, and allow us to disentangle the effects of substrate deformability and wettability on droplet spreading. Due to its systematic formulation, our model can readily be extended to more complex situations involving multiple droplets, substrate inclination, and droplet phase changes.