Electrically induced drop detachment and ejection

A deformed droplet may leap from a solid substrate, impelled to detach through the conversion of surface energy into kinetic energy that arises as it relaxes to a sphere. Electrowetting provides a means of preparing a droplet on a substrate for lift-off. When a voltage is applied between a water droplet and a dielectric-coated electrode, the wettability of the substrate increases in a controlled way, leading to the spreading of the droplet. Once the voltage is released, the droplet recoils, due to a sudden excess in surface energy, and droplet detachment may follow. The process of drop detachment and lift-off, prevalent in both biology and micro-engineering, has to date been considered primarily in terms of qualitative scaling arguments for idealized superhydrophobic substrates. We here consider the eletrically-induced ejection of droplets from substrates of finite wettability and analyze the process quantitatively. We compare experiments to numerical simulations and analyze how the energy conversion efficiency is affected by the applied voltage and the intrinsic contact angle of the droplet on the substrate. Our results indicate that the finite wettability of the substrate significantly affects the detachment dynamics, and so provide new rationale for the previously reported large critical radius for drop ejection from micro-textured substrates

Electrical control and enhancement of dropwise condensation

Condensation of vapor typically occurs via the formation of condensate films on condensing surfaces; however, the liquid film imposes a substantial thermal resistance to heat transfer. Filmwise condensation heat transfer can be enhanced by 5-7X by condensing vapor as droplets, which roll-off the surface, thereby preventing buildup of a liquid film. Dropwise condensation heat transfer can be enhanced by the use of electrowetting (EW) to enhance coalescence, growth and shedding of condensed droplets. This dissertation includes several fundamental studies on EW-enhanced dropwise condensation. Experiments, analytical modeling and statistical modeling are used to gain a deeper understanding of droplet growth, coalescence and shedding under EW. Chapter 1 details the motivation for this study and the objectives of this dissertation. Chapter 2 includes a literature review of condensation, electrowetting and data science- based statistical methods. Chapter 3 presents a detailed experimental study of dropwise condensation of humid air under the influence of electrowetting fields. An analytical heat transfer model, which accounts for the presence of non-condensable gases, is used to predict the heat transfer benefits associated with electrowetting-assisted condensation. Chapter 4 presents a detailed analysis of electrowetting-induced coalescence dynamics of a distribution of water droplets. Statistical modeling-based algorithms are used to identify key electrowetting-related parameters that influence droplet coalescence; the influence of these parameters on coalescence is quantified. Chapter 5 studies droplet shedding dynamics under electrowetting and shows that an intermittent electric field can significantly increase condensation rates (as compared to a continuous electric field). A key finding is the almost complete removal of water from surfaces in very short durations (< 1 sec) is observed. It is also found that the extent and rate of water removal depends on the applied voltage and frequency of the AC EW waveform, respectively. Chapter 6 presents a novel approach and an experimentally validated model to analyze the oscillations of water droplets under the influence of AC electrowetting. Chapter 7 summarizes key conclusions and outlines suggestions for future work. Overall, the research reported in this dissertation has led to fundamental contributions in the areas of condensation and microfluidics. This multidisciplinary work has involved experiments, analytical modeling and statistical modeling. Results show that electrowetting fields influence all the phenomena important in dropwise condensation (growth, coalescence, shedding of droplets). Electrowetting is therefore a powerful tool to control and enhance condensation heat transfer. This research impacts applications in energy (steam condensation, refrigeration), water (atmospheric water harvesting, desalination) and infrastructure (self-cleaning).Mechanical Engineerin

Electric-field induced droplet vertical vibration and horizontal motion: Experiments and simulations

In this work, Electrowetting on Dielectric (EWOD) and electrostatic induction (ESI) are employed to manipulate droplet on the PDMS-ITO substrate. Firstly, we report large vertical vibrations of the droplet, induced by EWOD, within a voltage range of 40 to 260 V. The droplet's transition from a vibrating state to a static equilibrium state are investigated in detail. It is indicated that the contact angle changes synchronously with voltage during the vibration. The electric signal in the circuit is measured to analyze the vibration state that varies with time. By studying the influence of driving voltage on the contact angle and the amplitude in the vibration, it is shown that the saturation voltage of both contact angle and amplitude is about 120 V. The intrinsic connection between contact angle saturation and amplitude saturation is clarified by studying the surface energy of the droplet. A theoretical model is constructed to numerically simulate the vibration morphology and amplitude of the droplet. Secondly, we realize the horizontal motion of droplets by ESI at the voltage less than 1000 V. The charge and electric force on the droplet are numerically calculated. The frictional resistance coefficients of the droplet are determined by the deceleration of the droplet. Under consideration of frictional resistance of the substrate and viscous resistance of the liquid, the motion of the droplet is calculated at 400 V and 1000 V, respectively. This work introduces a new method for manipulating various forms of droplet motion using the single apparatus

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.open111416sciescopu

Investigation of Heat Transfer in Dropwise Condensation Facilitated by a Humid Airflow

It is necessary to understand humid air condensation because of its various applications, such as water harvesting, and environment control and life support systems. Improvement of the heat transfer rate by facilitating condensate removal (shedding of droplets) from the surface can decrease operational costs. Droplets can be shed by using airflow. Parameters such as relative humidity (RH), airflow velocity, and subcooling (T_sc) play a substantial role in condensation and heat transfer rate. Therefore, understanding the effect of these parameters is essential to analyze and evaluate the performance of the systems reliant on condensation. Firstly, this work is dedicated to investigating the influence of airflow on the condensation heat transfer coefficient (HTC) of humid air on a horizontal surface. A mini closed-looped wind tunnel was used to simulate the condensation environment and control condensation parameters. Airflow velocities from 1 to 15 m/s were investigated because the condensate's shedding usually happens in this range. Also, an RH of 10–80% and T_sc of 0–10°C were used to achieve both single-phase and condensation regimes. Transient and instantaneous heat flux measurements were required to study the relationship between condensate morphology and HTC. To facilitate this, a transient inverse heat conduction method was used to characterize the time-varying surface heat flux and associated HTC. A 5-fold decrease in response time was found for the transient method compared to the steady-state method. The effect of condensation parameters on the HTC and the relationship between condensate morphology and HTC is discussed. The results show that HTC for the subcooling of T = 0°C is smaller than for other temperatures. Also, the effect of RH on condensation was investigated, and higher HTC was found for higher RHs. The results clearly show that the shedding of condensate kept the average droplet size low and doubled heat transfer performance improvement

Investigation of Heat Transfer in Dropwise Condensation Facilitated by a Humid Airflow

It is necessary to understand humid air condensation because of its various applications, such as water harvesting, and environment control and life support systems. Improvement of the heat transfer rate by facilitating condensate removal (shedding of droplets) from the surface can decrease operational costs. Droplets can be shed by using airflow. Parameters such as relative humidity (RH), airflow velocity, and subcooling (T_sc) play a substantial role in condensation and heat transfer rate. Therefore, understanding the effect of these parameters is essential to analyze and evaluate the performance of the systems reliant on condensation. Firstly, this work is dedicated to investigating the influence of airflow on the condensation heat transfer coefficient (HTC) of humid air on a horizontal surface. A mini closed-looped wind tunnel was used to simulate the condensation environment and control condensation parameters. Airflow velocities from 1 to 15 m/s were investigated because the condensate's shedding usually happens in this range. Also, an RH of 1080% and T_sc of 010C were used to achieve both single-phase and condensation regimes. Transient and instantaneous heat flux measurements were required to study the relationship between condensate morphology and HTC. To facilitate this, a transient inverse heat conduction method was used to characterize the time-varying surface heat flux and associated HTC. A 5-fold decrease in response time was found for the transient method compared to the steady-state method. The effect of condensation parameters on the HTC and the relationship between condensate morphology and HTC is discussed. The results show that HTC for the subcooling of T = 0C is smaller than for other temperatures. Also, the effect of RH on condensation was investigated, and higher HTC was found for higher RHs. The results clearly show that the shedding of condensate kept the average droplet size low and doubled heat transfer performance improvement