Session A29: Drops: Electric Field Effects

Mechanism of splitting in Electrospinning

Electrospinning involves the vertical stretching of the liquid jet after ejection due to the electric field and the oblique stretching of the jet caused by the Coulomb force.  The liquid jet emerging with excess ions along the surface ultimately leads to electrically driven instabilities leading to whipping and branching. The controlled branching and splitting can lead to the formation of polymer nanofibers with diameters ranging from tens of nanometers to microns.  We demonstrate the mechanism of jet splitting generated by electrospinning of a PEO (400,000 g/mol) in a water-ethanol drop. To scrutinize the branching event and the intricate splitting, the experiments were conducted with stereo imaging from two high-speed video cameras, at up to 200,000 fps using high magnification.  For strong applied electric field, the primary jet thins into a ribbon with secondary jets emanating from the highly curved sides.  The filament aspect ratio depends on the strength of the applied electric field.  Above a certain threshold voltage a hole nucleates at the base of the secondary jet, which expands rapidly and then splits the jet in two.    We investigate how the hole nucleation and dynamics also depend on the fluid properties.

    

A spectral boundary integral method for simulating electrohydrodynamic flows in liquid droplets

A weakly conducting liquid droplet immersed in another leaky dielectric liquid can exhibit rich dynamical behaviors under the effect of an applied electric field. Depending on material properties and field strength, the nonlinear coupling of interfacial charge transport and fluid flow can give rise to various electrohydrodynamic instabilities leading to shape deformations and complex dynamics. Here, we present a spectral boundary integral method for the simulation of droplet electrohydrodynamics in uniform applied fields. All physical variables, such as drop shape and interfacial charge density, are represented using spherical harmonic expansions. In addition to its excellent accuracy, the spectral representation affords a nondissipative de-aliasing method required for numerical stability. We also use a reparametrization technique, which we find required to explore regimes where the drop undergoes significant deformations. Our simulations closely match existing experimental data and past analytical predictions in both the axisymmetric Taylor and Quincke electrorotation regimes. Moreover, we demonstrate that the dynamics of low-viscosity drops are strongly affected by charge convection by the flow, which results in the emergence of steep gradients in the interfacial charge density.   

Electric field induced ion evaporation from charged nano-droplets undergoing Rayleigh fission

It is well understood that droplets charged beyond a certain charge (referred to as the Rayleigh limit) are unstable and undergo fission, where the parent droplet sheds a significant fraction of its charge in the form of smaller progeny droplets. Alternatively, charged droplets in the nano-metric size regime can support electric field large enough to directly extract ions from the surface of the droplet (referred to as the ion emission process). The two competing mechanisms are of fundamental practical importance in the electrospray community such as electrospray ionization mass spectrometry (ESI-MS) and electrospray propulsion. However, studying the fate of ion emitting charged droplets has been challenging due to the nano-metric length scale along with the fast transient nature of these processes. We developed a continuum phase field numerical model to study the effects of ion emission and Rayleigh fission processes on charged droplets.  The minimum electric field required to trigger the emission of ions along with the amount of charge the parent droplets shed in the form of ions and smaller progeny droplets is numerically quantified. We show that for droplets with size <50nm, ion emission completely suppresses the Rayleigh fission process, thereby having a stabilizing effect. However, ion emission is not sufficient to suppress Rayleigh fission for relatively larger droplets (50-100nm). For these larger droplets, both ion emission and the Rayleigh fission processes are at play with the droplet overall shedding 23% of its original charge as ions and 15% as smaller progeny droplets.

    

Effect of viscoelasticity on drop deformation in a uniform electric field

We study the effect of an external electric field on the deformation of a viscoelastic drop suspended in another viscoelastic medium. Leaky dielectric drops suspended in another leaky dielectric medium, when subjected to a uniform electric field, undergo deformation. We use the Oldroyd-B constitutive relation to model viscoelasticity and equations for leaky dielectric materials to solve for the electric field. We perform numerical simulations of the Navier-Stokes equations using a volume-of-fluid method to simulate the two-phase electrohydrodynamic flow. By performing an asymptotic analysis, we solve for the deformation of a droplet in the limit of small capillary number (ratio of viscous to capillary forces) and Deborah number (ratio of relaxation time constant and flow time scale). A viscoelastic drop suspended in Newtonian medium results in a smaller deformation compared to a Newtonian drop suspended in a Newtonian medium. The effect of viscoelasticity is more prominent when a Newtonian drop suspended in a viscoelastic medium is subjected to an external electric field. We also study effect of permittivity and conductivity ratios on the extent of deformation. Results from the volume of fluid based numerical simulations are in good agreement with the asymptotic analysis results.   

Dielectrophoretic Stretching of Drops of Silicone Oil: Experiments and Multi-Physical Modeling

The present work explores the electrically-driven stretching of silicone oil drops without and with metal-oxide particles on a dielectric surface, which experience significant dielectrophoretic forces resulting in body forces pulling the suspension drops. The experimental evidence reveals that drops of two pure silicone oils of different viscosities on polypropylene substrate do not react to the in-plane electric field. On the other hand, pre-treatment of silicone oil in a humid atmosphere at 80% RH (relative humidity) enriches oil with water-related ions and results in a slight drop stretching under the action of the in-plane external electric field. These phenomena demonstrate that the original silicone oils do not contain a sufficient concentration of any ions and counter-ions for the appearance of any Coulomb force or Maxwell stresses, which would result in drop stretching. Nevertheless, additional experiments show a significant drop stretching on polypropylene substrate subjected to the in-plane electric field when 5 wt.% of TiO₂ particles was suspended in silicone oil. Here, the metal-oxide microparticles behave as electric dipoles and experience forces of dielectrophoretic origin when subjected to a nonlinear symmetric electric field which attracts them to both electrodes in air and oil. Furthermore, 3D simulations of the dielectrophoretically-driven evolution of silicone oil drops with TiO₂ particles also revealed a stronger drop stretching along the inter-electrode direction, in qualitative agreement with the experimental data.

The authors acknowledge the financial support from the National Science Foundation award CBET-1906497.

 

    

Electrowetting of weak polyelectrolyte solutions

Electrowetting occurs when an electric field is applied to a fluid interface, changing its equilibrium position. To date, most electrowetting studies used aqueous electrolyte solutions as the electrowetting liquid. The present work studies the phenomena of electrowetting on a hydrophobic dielectric solid (EWOD) of a semi-dilute anionic poly(acrylic acid) (PAA) aqueous solution. Changing the polyelectrolyte ionization degree by adjusting the solution pH provides insight into the wetting behavior. Droplets with strongly ionized PAA led to a greater increase in the wetting than the weakly charged PAA. Furthermore, contact angle hysteresis was higher for the droplet of completely ionized polyelectrolyte. The results demonstrate how charge-connectivity and polyelectrolyte architecture under an external electrical field can augment electrowetting gain and contact angle hysteresis. The present findings might help order and manipulate many biological polyelectrolytes, such as polypeptides, glycosaminoglycans, and DNA.

    

Electrohydrodynamics of drops with complex interfaces

The effective removal of water from oilfield emulsions is crucial in the petroleum industry. The response of emulsion drops to electric fields has been exploited to accelerate the demulsification process. While previous studies have demonstrated different profound dynamics emulsion drops can exhibit under electric fields, many of them assumed the drop interface to be clean. In more realistic situations, crude oil-water interfaces are populated with absorbed crude oil components such as asphaltenes and resins. These complex interfaces hence display interfacial rheological behaviors that may not be captured by a single value of interfacial tension. In this talk, we report our progress in examining the electrohydrodynamic response of drops with more complex interfaces.   

A Low-Cost Electrowetting on Dielectric-Driven Micropump

A high-performance and reliable microfluidic reconfigurable radio frequency (MRRF) device has been a dream for over a decade. Electrowetting on dielectric (EWOD) driven micropumps may be able to accomplish it. RF configuration can be achieved by microelectromechanical systems (MEMS) switches, MEMS capacitors, material loadings methods, varactors, PIN diodes, or ferroelectric varactors. Although these methods have an excellent speed, cost, and size, they have limitations in power handling capability, radiation efficiency, and range of frequency tunability. An alternative to address these drawbacks is using the microfluidic device to reconfigure the RF signals. However, the current microfluidic-based RF devices have been primarily based on mechanical micropumps such as diaphragm micropumps, rotary micropumps, and peristaltic micropumps, which require expensive clean-room fabrication methods. In addition, they are limited in terms of size. To overcome these issues, for the first time we introduce a low-cost EWOD-driven micro pump that can actuate an RF switch. We will present an analytical model to predict the system flow rate and pressure, and consider the performance limits in the context of MRRF devices.

    

Shedding of condensate by a shearing airflow under an electric field

Condensation is required in the majority of industrial operations involving phase change. Efficient condensate removal (shedding) improves the performance of the systems associated with heat transfer. Apart from passive approaches, an alternating (AC) electric field has been used to promote condensation heat transfer by actively controlling droplet dynamics. To our knowledge, all the existing work has not considered the effect of electric field on the shedding of condensate by airflow. We will present our findings regarding the effect of electrowetting (EW)-based surfaces on droplet shedding and enhanced heat transfer during condensation of humid air. Condensate morphology and its impact on heat transfer coefficient will be discussed. This foundation may help one to use EW to improve heat transfer and condensate shedding under shear flow.