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1. Electrowetting-assisted droplet coalescence and roll-off for condensation heat trasnfer

   Wikramanayake, Enakshi D.; Bahadur, Vaibhav (December 2019, Proceedings of the International Heat and Mass Transfer Conference)

   Dropwise condensation heat transfer is significantly higher than filmwise condensation heat transfer due to the absence of the thermal resistance associated with the condensed water film. This study uses electrowetting to enhance coalescence and roll-off of condensed droplets, with the objective of enhancing the condensation rate. Coalescence enhancement is achieved by electric field-driven droplet motion such as translation of droplets, and oscillations of the three-phase line. Experiments are conducted to study early-stage droplet growth dynamics, and steady state condensation under electrowetting fields. Results show that droplet growth and roll-off increases with the voltage and frequency of the applied AC field. AC electric fields are seen to be more effective than DC electric fields. The overall condensation rate depends on the roll-off size of droplets, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. All these phenomena can be altered by the applied electric field. An analytical heat transfer model is developed which uses the measured droplet size distribution to estimate the surface heat flux. Overall, this study reports that electric fields can enhance the condensation rate by more than 30 %. 

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2. AC permittivity of confined water in the presence of static field

   https://doi.org/10.1021/scimeetings.5c10926  

   Bratko, D; Mulpuri, N (September 2024, ACS)

   Kinetics of molecular and collective reorientations at solid/water interfaces are known to depend on local electric field. Deceleration is observed near surfaces subjected to an outgoing static field (pointing from surface to liquid) as is the case when the solid carries positive charge. In an incoming field, both the reorientation rates and local permittivity in hydration layer show a nonmonotonic dependence on field strength with fastest reorientations and highest permittivity observed when the field alignment barely offsets the orienting bias at the wall. (Mulpuri and Bratko, J. Chem. Phys. 158,134716, 2023). Here, we use Molecular Dynamics simulations to explore the impact of background field (or, equivalently, surface charge density) on high frequency (GHz to THz) AC permittivity in hydration water inside a nanosized aqueous film under perpendicular DC field. Our model system mimics conditions inside a capacitor where one of the confinement walls is subject to outgoing and the other one to incoming field. In very strong static fields, the frequency corresponding to the maximal imaginary part of AC permittivity, features a blue shift with increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under ingoing field (adjacent to the negative capacitor plate) features a red shift, which is especially pronounced at the field strength corresponding to the maxima of static-permittivity and reorientation-rate. The shift reflects the variation of the inverse static dielectric constant in normal direction, (Gekle and Netz, J. Chem. Phys. 137, 104704, 2012) with marginal effect of librational motions on the local AC permittivity. Hydration water at the opposite surface (closer to the positive capacitor plate), on the other hand, features a monotonic blue shift consistent with conventional saturation. The sensitivity of imaginary peaks on the field suggests surface charge densities could be deduced from THz dielectric spectroscopy experiments in a porous material where hydration layers comprise a major fraction of water contained in the system. 

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3. Electrokinetic Propulsion of Polymer Microparticulates Along Glassy Carbon Electrode Array

   https://doi.org/10.1115/1.4046561  

   Cortez, Jennifer; Damyar, Kimia; Gao, Runtian; Zhou, Tuo; Kulinsky, Lawrence (June 2020, Journal of Micro and Nano-Manufacturing)

   Abstract Dielectrophoresis (DEP) is a force applied to microparticles in nonuniform electric field. This study discusses the fabrication of the glassy carbon interdigitated microelectrode arrays using lithography process based on lithographic patterning and subsequent pyrolysis of negative SU-8 photoresist. Resulting high-resistance electrodes would have the regions of high electric field at the ends of microarray as demonstrated by simulation. The study demonstrates that combining the alternating current (AC) applied bias with the direct current (DC) offset allows the user to separate subpopulations of microparticulates and control the propulsion of microparticles to the high field areas such as the ends of the electrode array. The direction of the movement of the particles can be switched by changing the offset. The demonstrated novel integrated DEP separation and propulsion can be applied to various fields including in vitro diagnostics as well as to microassembly technologies. 

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4. Dynamic permittivity of confined water under a static background field

   https://doi.org/10.1063/5.0233894  

   Bratko, D; Mulpuri, N (October 2024, The Journal of Chemical Physics)

   Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system. 

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5. A “hump” in the third-order dielectric response of a highly polar liquid: Now you see it, now you don’t

   https://doi.org/10.1063/5.0248963  

   Thoms, Erik; Richert, Ranko (March 2025, The Journal of Chemical Physics)

   We have measured the linear and nonlinear dielectric responses of S-methoxypropylene carbonate, a highly polar glass-former, for which it has been reported that the “hump,” which is typical of third harmonic susceptibilities, disappears across a 5 K temperature change. To understand this unusual feature, we have measured the responses to high amplitude ac and dc electric fields at the fundamental frequency. The static limits of these results are entered into a model aimed at reproducing nonlinear dielectric susceptibility spectra using the concept of a fictive electric field. This model reproduces the “hump” in the third-harmonic response and its seeming disappearance. It is revealed that the “hump” is predominantly the result of reduced time constants, a consequence of the energy the sample absorbs from the electric field. At elevated temperatures, the “hump” only appears to vanish because its reduced amplitude submerges below the extraordinarily high level of polarization saturation of this liquid. 

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