Detaching droplets in immiscible fluids from a solid substrate with the help of electrowetting

Magnetically Driven Manipulation of Nonmagnetic Liquid Marbles: Billiards with Liquid Marbles

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively.

Droplet dynamics driven by electrowetting

Even though electrowetting-on-dielectric (EWOD) is a useful strategy in a wide array of biological and engineering processes with numerous droplet-manipulation applications, there is still a lack of complete theoretical interpretation on the dynamics of electrowetting. In this paper we present an effective theoretical model and use the Onsager variational principle to successfully derive general dynamic shape equations for electrowetting droplets in both the overdamped and underdamped regimes. It is found that the spreading and retraction dynamics of a droplet on EWOD substrates can be fairly well captured by our model, which agrees with previous experimental results very well in the overdamped regime. We also confirm that the transient dynamics of EW can be characterized by a timescale independent of liquid viscosity, droplet size, and applied voltage. Our model provides a complete fundamental explanation of EW-driven spreading dynamics, which is important for a wide range of applications, from self-cleaning to novel optical and digital microfluidic devices.

Compact Three-Dimensional Digital Microfluidic Platforms with Programmable Contact Charge Electrophoresis Actuation

Digital microfluidics (DMF) has garnered considerable interest as a straightforward, rapid, and programmable technique for controlling microdroplets in various biological, chemical, and medicinal research disciplines. This study details the construction of compact and low-cost 3D DMF platforms with programmable contact charge electrophoresis (CCEP) actuations by employing electrode arrays composed of a small commercial pin socket and a 3D-printed housing. We demonstrate basic 3D droplet manipulation on the platform, including horizontal and vertical transport via lifting and climbing techniques, and droplet merging. Furthermore, phenolphthalein reaction and precipitation process are evaluated using the proposed 3D DMF manipulations as a proof of concept for chemical reaction-based analysis and synthesis. The threshold voltage (or electrical field) and maximum vertical transport velocity are quantified as a function of applied voltage and electrode distance to determine the CCEP actuation conditions for 3D droplet manipulations. The ease of manufacturing and flexibility of the proposed 3D DMF platform may provide an effective technique for programmable 3D manipulation of droplets in biochemical and medical applications, such as biochemical analysis and medical diagnostics.

Trampolining of Droplets on Hydrophobic Surfaces Using Electrowetting

Droplet detachment from solid surfaces is an essential part of many industrial processes. Electrowetting is a versatile tool for handling droplets in digital microfluidics, not only on plain surface but also in 3-D manner. Here, we report for the first time droplet trampolining using electrowetting. With the information collected by the real-time capacitor sensing system, we are able to synchronize the actuation signal with the spreading of the droplet upon impacting. Since electrowetting is applied each time the droplet impacts the substrate and switched off during recoiling of the droplet, the droplet gains additional momentum upon each impact and is able to jump higher during successive detachment. We have modelled the droplet trampolining behavior with a periodically driven harmonic oscillator, and the experiments showed sound agreement with theoretical predictions. The findings from this study will offer valuable insights to applications that demands vertical transportation of the droplets between chips arranged in parallel, or detachment of droplets from solid surfaces.

Manipulation of a Nonconductive Droplet in an Aqueous Fluid with AC Electric Fields: Droplet Dewetting, Oscillation, and Detachment

Electrowetting (EW) is an effective method for droplet manipulation in microfluidics. In traditional EW, a conductive droplet is actuated, which spreads on a solid substrate. Recently, we considered an opposite phenomenon of droplet actuation in EW: inducing nonconductive droplet dewetting and detaching from the substrate. An oil/water system is used in which the oil droplet (nonconductive) is actuated on a flat substrate in surrounding water (conductive) by EW. In this work, alternating current (AC) electric fields are applied to EW, and the transient dynamics of droplet dewetting, oscillation, and detachment with the AC signals are investigated. The droplet is not in contact with electrodes, and it dances freely on the substrate. Experiments are performed in a wide range of voltages and AC frequencies. To demonstrate the droplet dynamics, we divide the full process of droplet manipulation into three distinguishable periods, that is, an initiating period, a steady oscillation period, and a detaching condition. Transient droplet dewetting is considered in the initiating period, and we obtain the distribution of the contact line friction factor. In steady oscillation, the oscillation resonance is verified from the oscillating amplitude of the contact line. Different periodical features are found for the droplet dancing at the resonance frequencies and departure from resonance. The droplet is detached at high voltages, and we provide a map for the detachable and nondetachable zones. The voltage is the dominant factor determining the droplet detachment; however, the AC frequency has notable influences on the critical voltage. The detachment is promoted when the AC frequency is within the region of the oscillation resonance (e.g., 20 < f < 75 Hz). In this region, the detaching process is not monotonic but instead, the droplet rebounds by several times before it is completely detached.

Optically Guided Pyroelectric Manipulation of Water Droplet on a Superhydrophobic Surface

Controlled droplet manipulation by light has tremendous technological potential. We report here a method based on photothermally induced pyroelectric effects that enables manipulation and maneuvering of a water droplet on a superhydrophobic surface fabricated on lithium tantalite (LiTaO₃). In particular, we demonstrate that the pyroelectric charge distribution has an essential role in this process. Evenly distributed charges promote a rapid hydrophobic to hydrophilic transition featuring a very large water contact angle (WCA) change of ∼76.5° in air. This process becomes fully reversible in silicone oil. In contrast, the localized charge distribution induced by guided laser illumination leads to very different and versatile functionalities, including droplet shape control and motion manipulation. The influence of a saline solution is also investigated and compared to the deionized water droplet. The focusing effect of the water droplet, a phenomenon that widely exists in nature, is particularly of interest. Simple tuning of the laser incident angle results in droplet deformation, jetting, splitting, and guided motion. Potential applications, such as droplet pinning and transfer, are presented. This approach offers a wide range of versatile functionalities and ready controllability, including contactless, electrodeless, and precise spatial and fast temporal control, with tremendous potential for applications requiring remote droplet control.

Actuation of a Nonconductive Droplet in an Aqueous Fluid by Reversed Electrowetting Effect

Manipulation of a conductive droplet by electrowetting has been a popular topic in microfluidics whereby wettability of the droplet on a solid surface is increased by applying a voltage between the conductive droplet and the insulated surface. However, the opposite phenomenon, e.g., decreasing the wettability of a nonconductive droplet and increasing its contact angle (CA) by the reversed electrowetting (REW) effect, has been scarcely reported. Such a process involves not only the transient dynamics of droplet dewetting but also a critical condition for droplet detachment from the adhesive surface. In this work, actuation of a nonconductive droplet in an aqueous surrounding fluid by REW is studied experimentally. Silicone oil is used for the actuated droplet, and filtered water is used as the surrounding fluid. The solid substrate is made of a glass substrate coated with an indium tin oxide (ITO) film and then deposited by a dielectric layer of Parylene C. Potential difference is applied between the substrate and the surrounding fluid, eliminating the disturbance from the top needle on the motion of the droplet. Three different regimes are identified in the full range of operation. An underactuated regime occurs at low applied voltages, in which the CA of the droplet shows a monotonic increase with the increase of voltage (V). The friction coefficient of the contact line decreases with V before the CA saturation (V_s) but shows little change when V > V_s. At high voltages, the contact line of the sessile droplet is contracted excessively by REW. The droplet shows oscillation, and it refers to the overactuated regime. A combined time scale is proposed, and it verifies that the viscous dissipation of the contact line and liquid inertia show comparable contributions in the droplet dynamics. At sufficiently high voltages, the droplet is rejected completely from the surface. A critical equation for the threshold voltage of droplet detachment is built, and its validity is confirmed by experimental results.

Coplanar Electrowetting-Induced Droplet Detachment from Radially Symmetric Electrodes

This work demonstrates electrowetting-induced droplet detachment in air from coplanar electrodes using a single voltage pulse. It also presents two models to predict when this detachment will occur. Previous works approximated the minimum energy for detachment based on (i) adhesion work at the solid-liquid interface and (ii) interfacial energy changes along all three interfaces in the system. This investigation updates those models to include changes in gravitational potential energy during detachment and provides validation by testing predicted detachment thresholds against experimental observations. Droplets of varying volume were ejected from electrowetting devices with (i) radially symmetric four-part coplanar electrodes and (ii) single electrodes with a ground wire inserted directly into the droplet. All experiments were performed in air. Incorporation of gravitational potential energy improves predictions for critical electrowetting number and captures the observed increase in applied voltage required with increased droplet volume. These new models will be of particular benefit in three-dimensional digital microfluidics applications that manipulate droplets in air.

Understanding the Role of Dynamic Wettability for Condensate Microdrop Self-Propelling Based on Designed Superhydrophobic TiO2 Nanostructures

The ability to release the adhered drops on superhydrophobic surfaces is very important for self-cleaning, antifrosting/icing, microfluidic device, and heat transfer applications. In this paper, three types of in situ electrochemical anodizing TiO₂ nanostructure films are rationally designed and fabricated on titanium substrates with special superwettability, viz., TiO₂ nanotube arrays, irregular TiO₂ nanotube arrays, and hierarchical TiO₂ particle arrays (HTPA), and their corresponding behavior in condensate microdrop self-propelling (CMDSP) is investigated. Compared to the flat titanium counterpart, all three types of rough TiO₂ samples demonstrate a uniform distribution of smaller microscale droplets. Among the treated surfaces, the HTPA possesses the highest condensate density, and more than 80% of the droplets possess a diameter below 10 μm. Theoretical analysis indicates that the feature is mainly due to the morphology and structure induced extremely low droplet adhesion on super-antiwetting TiO₂ hierarchical surfaces, where the excess surface energy released from the migration leads to the self-propelling of merged microdrop. This work offers a way to rationally construct CMDSP surfaces with excellent self-cleaning, antifrosting/icing ability, and enhanced condensation heat transfer efficiency.

Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids

The dispensing of tiny droplets is a basic and crucial process in a myriad of applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles, and digital microfluidics. This work systematically demonstrates droplet dispensing into immiscible fluids through electric charge concentration (ECC) method. It exhibits three main modes (i.e., attaching, uniform, and bursting modes) as a function of flow rates, applied voltages, and gap distances between the nozzle and the oil surface. Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few μL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, including preparation of colloidosomes and fabrication of Janus microparticles with anisotropic internal structures. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide increased utility and flexibility in microfluidic applications because a charged droplet can be delivered toward the intended position by programmable electric control.

Bioinspired Interfacial Materials with Enhanced Drop Mobility: From Fundamentals to Multifunctional Applications

The development of bioinspired interfacial materials with enhanced drop mobility that mimic the innate functionalities of nature will have a significant impact on the energy, environment and global healthcare. Despite extensive progress, state of the art interfacial materials have not reached the level of maturity sufficient for industrial applications in terms of scalability, stability, and reliability. These are complicated by their operating environments and lack of facile approaches to control the local structural texture and chemical composition at multiple length scales. The recent advances in the fundamental understanding are reviewed, as well as practical applications of bioinspired interfacial materials, with an emphasis on the drop bouncing and coalescence-induced jumping behaviors. Perspectives on how to catalyze new discoveries and to foster technological adoption to move this exciting area forward are also suggested.

Three-dimensional digital microfluidic manipulation of droplets in oil medium

We here develop a three-dimensional DMF (3D DMF) platform with patterned electrodes submerged in an oil medium to provide fundamental solutions to the technical limitations of 2D DMF platforms and water–air systems. 3D droplet manipulation on patterned electrodes is demonstrated by programmably controlling electrical signals. We also demonstrate the formation of precipitates on the 3D DMF platform through the reaction of different chemical samples. A droplet containing precipitates, hanging on the top electrode, can be manipulated without adhesion of precipitates to the solid surface. This method could be a good alternative strategy to alleviate the existing problems of 2D DMF systems such as cross-contamination and solute adsorption. In addition, we ascertain the feasibility of temperature-controlled chemical reaction on the 3D DMF platform by introducing a simple heating process. To demonstrate applicability of the 3D DMF system to 3D biological process, we examine the 3D manipulation of droplets containing mouse fibroblasts in the 3D DMF platform. Finally, we show detachment of droplets wrapped by a flexible thin film by adopting the electro-elasto-capillarity (EEC). The employment of the EEC may offer a strong potential in the development of 3D DMF platforms for drug encapsulation and actuation of microelectromechanical devices.