Droplet transportation using EWOD-induced wettability gradient

Numerical estimation of droplet motion on linear wettability gradient surface in microgravity environment

This review reports the recent progress and future prospects of wettability gradient surfaces (WGSs), particularly focusing on the governing principles, fabrication methods, classification, characterization, and applications. While transforming the inherent wettability into artificial wettability via bioinspiration, topographic micro/nanostructures are produced with changed surface energy, resulting in new droplet wetting regimes and droplet dynamic regimes. WGSs have been mainly classified in dry and wet surfaces, depending on the apparent surface states. Wettability gradient has long been documented as a surface phenomenon inducing the droplet mobility in the direction of decreasing wettability. However, it is herein critically emphasized that the wettability gradient does not always result in droplet mobility. Indeed, the sticky and slippery dynamic regimes exist in WGSs, prohibiting or allowing the droplet mobility, respectively. Lastly, the stringent bottlenecks encountered by WGSs are highlighted along with solution-oriented recommendations, and furthermore, phase change materials are strongly anticipated as a new class in WGSs. In all, WGSs intend to open up new technological insights for applications, encompassing water harvesting, droplet and bubble manipulation, controllable microfluidic systems, and condensation heat transfer, among others.

Application of voltage across a liquid-dielectric interface resulting in the change of fluid contact angle is known as electrowetting on dielectric (EWOD) effect [1]. EWOD actuation is one of the most preferred techniques to move liquid at microscale due to relatively low voltages, low currents and power consumption requirements, and the absence of electrolysis. Our recent work has shown that nanofluids containing semiconductor nanoparticles exhibit a very strong EWOD effect [2, 3]. In particular, in the tested voltage range of 0 to 60 V, nanofluids loaded with molecularly-capped bismuth telluride nanoparticles show enhanced stability and increased actuation range of contact angle change. Developing a fundamental understanding of the role of nanostructure inclusions in controlling and potentially enhancing the EWOD effect would pave the way for the efficient use of EWOD with nanofluids in a wide range or microfluidic applications. Here we extend the studies on the EWOD behavior of nanofluids, to a system containing noble metal nano particles. In this abstract we report the preliminary results of the electro-wetting studies on nanofluids containing silver nano particles.

This paper details the first reported integration of two advanced digital microfluidic technologies where 100 µm silicon cubes are transported with ElectroWetting On Dielectric (EWOD) and the droplet then held with EWOD while the silicon cubes are mixed with another liquid using a Surface Acoustic Wave (SAW). Together these two technologies provide a comprehensive lab-on-a-chip combination with well developed functionalities. These include droplet generation, splitting and transportation offered by EWOD with transportation, mixing and biosensing being potentially available with SAW. The fabrication of both EWOD and SAW structures on LiNbO 3 substrates used low temperature Ta/Ta 2 O 5 /CYTOP layer deposition and patterning technologies, which enabled efficient transportation and mixing functions to be demonstrated.

The gradient wettability surface is processed on the surface of Si3N4 ceramics using a 1030 nm femtosecond laser. Both the microdimple morphology, surface contact angle, and surface chemical structure are determined using optical microscopy, optical profilometry, contact angle measuring instrument, and X‐ray photoelectron spectroscopy (XPS). The effects of average laser power, scanning passes, and microdimple distribution density on both surface morphology and contact angle are also studied. Accordingly, the wettability surfaces of different gradients are processed by changing the microdimple distribution density, and the flow state of the cutting fluid on the gradient wettability surface is observed using a high‐speed camera. The results show that both the diameter and depth of the microdimple increase with the increase in the average laser power and scanning passes. As the average laser power increases, the surface contact angle gradually reduces and then increases. Moreover, because of the increase in scanning passes, the surface contact angle gradually decreases. The microdimple morphology is an important factor affecting the wettability of the sample surface. Reason construction of the surface microdimple distribution density can obtain different gradients of wettability surface. Furthermore, with a larger gradient, the wetting rate of the cutting fluid will be faster.

Polymer nanopillar arrays with continuously varying diameters were fabricated by nanoimprinting using anodic porous alumina molds. The obtained polymer nanopillar array exhibited wettability gradient properties, and when a water droplet formed on its surface, it moved spontaneously. A wettability gradient surface was fabricated using a nanopillar array structure, and its inverted structure, nanohole array, and the spontaneous transport distance of water droplets were compared. The spontaneous transport distance of water droplets on the nanopillar array surface was longer than that on the nanohole array surface. To confirm this, the relationships between surface porosity and contact angle were determined in both structures. Although no significant difference in statistical contact angle was found between the two structures, a significant difference was observed in their dynamic contact angles. The contact angle hysteresis of the nanopillar array structure was smaller than that of the nanohole array, and the adhesive force of the water droplets was smaller. This indicates that surfaces with the small contact angle hysteresis and low surface adhesion of water droplets effectively increase the distance over which water droplets move spontaneously, owing to the wettability gradient structure. In addition, a polymer nanopillar array with a partially gradient structure could be formed by nanoimprinting using anodic porous alumina molds. On surfaces with a patterned wettability gradient structure, moving water droplets in any direction is possible. The polymer nanopillar arrays obtained by this process can be used in various applications such as microfluidic devices, water harvesting, and heat transfer enhancement.

Interaction of Different Types of Cells on Polymer Surfaces with Wettability Gradient

This paper describes effect of characteristics of dielectric layer on performances of droplet transport in EWOD (Electro wetting on dielectric) microfluidic devices. EWOD is a technology for manipulating droplets discretely by controlling the surface wettability electrically. Films of Parylene C, PDMS, and SU-8 were prepared for the dielectric layer and effects of deposition method, thickness and permittivity of these films were investigated experimentally. The results show that water droplets can be easily transported at low voltage when the dielectric film with high permittivity is deposited thinly and uniformly.

We report on experiments of electrowetting on dielectric (EWOD) on functional fibers in air. Firstly, we investigate the evolution of drop contact angle θ with the applied voltage Vrms. θ(Vrms) relationship is compared to the theoretical Young–Dupre model and shows a good agreement at low voltage. However it appears that the saturation effect is different when compared to the EWOD response on a planar surface. We then study reversibility of the phenomenon to show that the drop recovers its initial shape when the voltage is switched off. Secondly, EWOD is used to modulate the maximal volume ΩM of a hanging drop. Results are compared to numerical simulations performed with Surface Evolver software. A good agreement is observed and it appears that ΩM increases linearly with Vrms. Finally, we present a proof of concept of a drop manipulation tool on fiber networks. This consists of an EWOD driven valve that is capable of regulating the drop traffic at a cross-junction between two fibers.

We have designed a novel microfluidic dispensing device using automatic droplet transportation on a wettability gradient surface that is sequentially followed by electrowetting-based liquid handling. While a droplet moves along the wettability gradient in a main channel, a portion of the droplet is drawn into a side channel and stored in a reservoir temporarily. Next, the stored liquid is dispensed into a mixing channel when a voltage is applied between two electrodes. The fabricated device succeeded in dispensing two liquids of different nanoscale volumes after extracting them from a microscale volume droplet.

Tuning the spontaneous motion of pure nanodroplets on programmed surfaces has progressed notably. However, impurities exist in droplets more often than not. We present molecular dynamics simulations of the dynamic behaviors of a mixture of water droplets and particles at the nanoscale, as they are placed on a wettability gradient substrate. We uncover three different styles of motion, namely, sweeping, separating, and pinning, as interaction strengths among the substances vary. By investigating the potential energies between the droplet and the nanoparticle/substrate, we explain the competition of the driving force and the impedance due to an imbalance of van der Waals force, which is responsible for differentiating the styles. We also find in some specific cases the phenomenon of tailing, where one side of the droplet is pinned by the nanoparticle as the other side moves forward to generate an extremely large difference of the contact angles. The results provide a fundamental understanding of the dynamic behaviors at the nanoscale and may inspire potential applications in a broad range, such as drug delivery, dust cleaning of nanomachines, liquid purification, etc.

ABSTRACTPhysical and chemical modifications were made on the surface of the aluminum sheet to change the surface properties and superhydrophobic–hydrophilic wettability gradient surface was made on the perspex surface by using microstructure-pattering technique and self-assembled-monolayer method. By using high-speed video camera system and optical tensiometer, this paper discusses the influence of special surfaces with different wettability on spreading and motion of water, oil, and W/O emulsion droplets both experimentally and theoretically. In addition, the paper also discusses the influence of the superhydrophobic–hydrophilic wettability gradient on fluidity of W/O emulsion droplets and the coalescence process of droplets. The results showed that the contact angle of W/O emulsion droplets on the modified surfaces was related to the water and oil distribution at the three-phase line. On the wettability gradient surface, the droplet moved spontaneously when the droplet was located at the junction of the gradient. A quasi-steady theoretical model was used to analyze the driving and resistant forces acting on a droplet to improve the understanding of the self-transport behavior of the droplets.

To manage scaling on heat transfer surfaces, using chemical-based antifouling treatments takes a toll on the environment, and related maintenance costs are significant. A unique and environmentally friendly scale mitigation strategy is developed in this work. By manipulating the scale layer distribution using surface wettability gradients, stronger flow shear and weaker scaling adhesion are achieved, facilitating self-cleaning process. Experimental work utilizes a horizontal-tube falling-film evaporator to mimic multiple-effect distillation process. A millimetric wettability pattern – alternating hydrophilic and hydrophobic regions – is created by coating stainless steel tubes. The results show scale layers (calcium carbonate) were either thicker in hydrophilic regions and thinner in hydrophobic regions or had approximately uniform thickness in both regions but cracks in scale layers near wettability boundaries. Scale layers wash-off with flow is observed on patterned tubes, demonstrating self-cleaning process. In contrast to persistent deterioration in overall HTC (heat transfer coefficient) for baseline tube (77 % of initial HTC after 192 h), HTC on patterned tubes initially increases due to surface wetting effects, then maintains roughly constant (15 % higher than that of baseline after 192 h) along with lower scale mass. This work provides a novel scaling control method, a potential “green” alternative to current industry scale-control practices.

Self-propelled motion of sessile droplets on gradient surfaces is key to the advancement of microfluidic, nanofluidic, and surface fluidic technologies. Precise control over droplet dynamics, which often involves liquid–vapor phase transitions, is crucial for a variety of applications, including thermal management, self-cleaning surfaces, biochemical assays, and microreactors. Understanding how specific phase changes like condensation and evaporation affect droplet motion is essential for enhancing droplet manipulation and improving transport efficiency. We use the thermal Navier–Stokes–Korteweg equations to investigate the effects of condensation and evaporation on the motion and internal dynamics of droplets migrating across a surface with a linear surface energy profile. The study focuses on the early dynamics of self-propelled motion of a phase changing droplet at sub-micron scale before viscous forces are comparable with the gradient forces. Our results demonstrate that phase change significantly affects the self-propelled motion of droplets by reshaping interfacial mass flux distributions and internal flow dynamics. Condensation increases droplet volume and promotes extensive spreading toward regions of higher wettability, while evaporation reduces both volume and spreading. These changes in droplet shape and size directly affect the driving forces of motion, augmenting self-propulsion through condensation and suppressing it during evaporation. Additionally, each phase change type generates distinct internal flow patterns within the droplet, with condensation and evaporation exhibiting unique circulatory movements driven by localized phase changes near the contact lines.

The objective of this paper is to investigate the effects of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel. The test section consists of a bottom silicon substrate bonded with a top glass cover. Three consecutive parts of an inlet fluid plenum, a central microchannel and an outlet fluid plenum were etched in the silicon substrate. The central microchannel had a width of 800 µm and a depth of 30 µm. Acetone liquid was used as the working fluid. High outlet vapor qualities were dealt with here. The flow pattern consists of a fluid triangle (shrinkage of the liquid films) and a connected long liquid rivulet, which is generated in the central microchannel in the timescale of milliseconds. The peculiar flow pattern is formed due to the following reasons: (1) the liquid rivulet tends to have a large contact area with the top hydrophilic channel surface of the glass cover, but a smaller contact area with the bottom silicon hydrophobic surface. (2) The temperature gradient in the chip width direction at the top channel surface of the glass cover not only causes the shrinkage of the liquid films in the central microchannel upstream, but also attracts the liquid rivulet populated near the microchannel centerline. (3) The zigzag pattern is formed due to the competition between the evaporation momentum forces at the vapor–liquid interfaces and the force due to the Marangoni effect. The former causes the rivulet to deviate from the channel centerline and the latter draws the rivulet toward the channel centerline. (4) The temperature gradient along the flow direction in the central microchannel downstream causes the breakup of the rivulet to form isolated droplets there. (5) Liquid stripes inside the upstream fluid triangle were caused by the small capillary number of the liquid film, at which the large surface tension force relative to the viscous force tends to populate the liquid film locally on the top glass cover surface.