Session LH: Drops VI

Conditions for Destabilizing Pickering emulsions using external electric fields

Fine particles are readily adsorbed at fluid-fluid interfaces, and can be used as stabilizers in emulsion technology by preventing adjacent drops from coalescing with each other. We investigate a new technique to destabilize such emulsions, or Pickering emulsions, by applying an external electric field. Experiments show that the latter has two effects: (i) the drops elongate in the direction of the electric field, (ii) the local particle density varies on the drop surface due to the dielectrophoretic (DEP) force acting on the particles. It is shown that the latter is the dominant factor in the destabilization process. Particularly, the success of the method depends on the values of certain dimensionless parameters; specifically, the ratio of the work done by the dielectrophoretic force must be larger than the work done by the buoyant force. Moreover, drops do not coalesce through the regions where the particles locally cluster, whether those are gathered at the poles or at the equator of the drops. As particles move, particle-free openings form on the drop's surface, which allow for adjacent drops to merge. This process takes place even if the particles are fully packed on the drops' surfaces as particles get ejected from the clustering areas due to a buckling phenomenon.   

Deformation and Rotation of a Drop in a Uniform Electric Field

Drop deformation in uniform electric fields is a classic problem. The pioneering work of G.I.Taylor demonstrated that for weakly conducting media, the drop fluid undergoes a toroidal flow and the drop adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. However, recent studies have revealed a nonaxisymmetric rotational mode for drops of lower conductivity than the surrounding medium, similar to the rotation of solid dielectric particles observed by Quincke in the 19th century. We will present an experimental and theoretical study of this phenomenon in DC fields. The critical electric field, drop inclination angle, and rate of rotation are measured. For small, high viscosity drops, the threshold field strength is well approximated by the Quincke rotation criterion. Reducing the viscosity ratio shifts the onset for rotation to stronger fields. The drop inclination angle increases with field strength. The rotation rate is approximately given by the inverse Maxwell-Wagner polarization time. We also observe a hysteresis in the tilt angle for low-viscosity drops. The effects of AC fields and surfactants are also explored.   

Dynamics of rotating high viscous droplet by using electrostatic levitator

The electrostatic levitation is one of the containerless processing techniques. Thermo-physical properties of extreme high temperature molten metals have been measured with an oscillating drop method based on linear approximations. For example, a viscosity has been estimated from the damping constant of oscillation after the deformation imposed. However, this method is limited to the low viscosity fluid because the oscillation is not excited on viscous liquid drop. Thus, the objective is to develop a new method to measure the wide range of viscosity liquid. In the present study, several different droplets with different viscosity were levitated by the electrostatic force. Drop deformation was imposed applying rotation. Dynamics of rotating droplet was investigated. The time dependence of drop midpoint radius for drop breakup when the rotation speed increased was experimentally measured. The effect of viscosity on the deformation was made clear.   

Co-flowing liquids in the presence of electric fields

We apply electric fields to a liquid that is extruded through a capillary tip in the presence of a surrounding co-flowing liquid and induce formation of a jet that ultimately breaks into drops. We will present some results related to this experiment, which can lead to a better and extended control of the drop sizes.   

Analysis of Electrowetting Dynamics with Level Set Method

Electrowetting is a versatile tool to handle tiny droplets and forms a backbone of digital microfluidics. Numerical analysis is necessary to fully understand the dynamics of electrowetting, especially in designing electrowetting-based liquid lenses and reflective displays. We developed a numerical method to analyze the general contact-line problems, incorporating dynamic contact angle models. The method was applied to the analysis of spreading process of a sessile droplet for step input voltages in electrowetting. The result was compared with experimental data and analytical result which is based on the spectral method. It is shown that contact line friction significantly affects the contact line motion and the oscillation amplitude. The pinning process of contact line was well represented by including the hysteresis effect in the contact angle models.   

Prediction of Time Response of Electrowetting

It is very important to predict the time response of electrowetting-based devices, such as liquid lenses, reflective displays, and optical switches. We investigated the time response of electrowetting, based on an analytical and a numerical method, to find out characteristic scales and a scaling law for the switching time. For this, spreading process of a sessile droplet was analyzed based on the domain perturbation method. First, we considered the case of weakly viscous fluids. The analytical result for the spreading process was compared with experimental results, which showed very good agreement in overall time response. It was shown that the overall dynamics is governed by P2 shape mode. We derived characteristic scales combining the droplet volume, density, and surface tension. The overall dynamic process was scaled quite well by the scales. A scaling law was derived from the analytical solution and was verified experimentally. We also suggest a scaling law for highly viscous liquids, based on results of numerical analysis for the electrowetting-actuated spreading process.   

Electro-hydrodynamic printing of drugs onto edible substrates

While most existing drugs are manufactured as tablets using powder processing techniques, there is growing interest in printing drops containing pharmaceutical actives on edible substrates. We have developed a drop-on-demand (DOD) printing method appropriate for either replacing existing manufacturing platforms or enabling personalized medicine that overcomes the various critical challenges facing current DOD technologies. To eliminate adverse effects of electro-chemical reactions at the fluid-electrode interface, the fluid is infused into an electrically insulating nozzle to form a pendant drop that serves as a floating electrode capacitively coupled to external electrodes. A liquid bridge is formed and broken as the voltage applied at the electrode is varied in time. This gentle method for drop deposition has been demonstrated to operate with fluids spanning over three orders of magnitude in viscosity and conductivity. The proposed method has the potential for the evolving field of pharmaceutical and biomedical applications requiring the deposition of fluids at the exact locations with high volume accuracy.   

Theoretical and experimental study of meniscus behavior under AC electric field for Electrohydrodynamic (EHD) jetting

The electrohydrodynamic (EHD) spraying technique has been utilized in applications such as inkjet printing and mass spectrometry technologies. In this paper, the role of electrical potential signals in jetting and on the oscillation of the meniscus is evaluated. The jetting and meniscus oscillation behavior are experimentally investigated under ac voltage, ac voltage superimposed on dc voltage, and pulsed dc voltage. Furthermore, the analytical simulation about the oscillation of an anchored edge hemispherical meniscus located on a conductive flat plate under a uniform ac electric field is presented. The mutual interaction between the electric field and the hydrodynamics is iteratively solved. As a result, the simulation can calculate the meniscus shapes, contours of voltage outside the meniscus and the velocity profile of liquid inside the meniscus during the period of the oscillation according to the applied frequency. Based on the present theory, one can predict the oscillation mode with a certain applied frequency. The present theory can also be applied to investigate the oscillation of a free conductive drop in a uniform ac electric field.   

Electrohydodynamic ejection without using nozzle electrode

The electrohydrodynamic (EHD) ejection technique has been applied to inkjet printing technology for fabrication of printed electronics. The conventional EHD inkjet device is based on dc voltage and requires two electrodes: a nozzle electrode and an extractor electrode. This study notes several drawbacks of the conventional EHD printing device such as electrical breakdown and demonstrates stable jetting by using the extractor electrode alone without the nozzle electrode and ac voltage. The continuous ejection of droplets can be obtained only by ac voltage, showing consistent ejection at every peak of electrical signal. The suggested EHD inkjet device prevents electrical breakdown and broaden the range of material selection for nozzle design. Experiments with high speed camera also point out that the generated droplets are much smaller than the nozzle size. Using glass capillary, we show various printing patterns of lines and characters.   

Large Deformation Studies of Vesicles under Electric Fields

Phospholipids tend to assume various liquid crystalline phases in water. One such phase, the liposome, forms an excellent model to study the properties of the (phospholipid) membranes. Liposomes have been subjected to electric fields and their deformation studied extensively, by various groups. Theydeform into prolate or oblate shapes based on the frequency of the applied field(alternating fields) or they undergo poration and fusion with the adjacent vesicles. We carry out both experimental and numerical studies on liposome deformation under applied AC electric fields. The small deformation regime agrees with the literature results. We study the deformation modes using high speed imaging. The behavior of liposomes under large field is complicated and dependent upon the properties of the fluids and the lipid membrane. The flow pattern within the medium in the liposome is investigated using flow markers. The large deformations are investigated using the Boundary integral method and comparisons made with the experimental observation. The phase lag between the applied AC field and the deformation response of the membrane is investigated and possibilities of the method as an interfacial rheometer discussed.