Sliding behavior of water droplets on flat polymer surface

Flat films of methyl methacrylate-fluoroalkyl methacrylate copolymers were prepared, and their hydrophobicity was investigated. It was revealed that the F concentration directly affects the static hydrophobicity on the flat polymer surface in a systematic manner. Furthermore, the sliding behavior of a water droplet on these surfaces depends on the static hydrophobicity; the sliding motion changes from constant velocity to constant acceleration with an increase in the water contact angle.     

Dynamic behavior of water droplets on solid surfaces with pillar-type nanostructures

In the present study, we investigated the static and dynamic behavior of water droplets on solid surfaces featuring pillar-type nanostructures by using molecular dynamics simulations. We carried out the computation in two stages. As a result of the first computational stage, an initial water cube reached an equilibrium state at which the water droplet showed different shapes depending on the height and the lateral and gap dimensions of the pillars. In the second computational stage, we applied a constant body force to the static water droplet obtained from the first computational stage and evaluated the dynamic behavior of the water droplet as it slid along the pillar-type surface. The dynamic behavior of the water droplet, which could be classified into three different groups, depended on the static state of the water droplet, the pillar characteristics (e.g., height and the lateral and gap dimensions of the pillars), and the magnitude of the applied body force. We obtained the advancing and receding contact angles and the corresponding contact angle hysteresis of the water droplets, which helped classify the water droplets into the three different groups.     

Interaction of water molecules with the electric field of an ionic-crystal surface

A theoretical investigation of the coulombic interaction of water molecules with an ideal surface of an ionic crystal is conducted. The calculation is based on the Fourier transform of the long-range part of an coulombic potential with subsequent summation of its Fourier images and tabulation of the results. The method is numerically tested when solving the problem concerning the interaction with the surface of a silver iodide crystal. A comparison of the electric field near an ideal crystalline surface and its finite fragment is performed. The data obtained point to a strong dependence of electrochemical properties of the surface on the presence of crystalline defects on it.     

Role of electric field on surface wetting of polystyrene surface

The role of surface charge in fluid flow in micro/nanofluidics systems as well as the role of electric field to create switchable hydrophobic surfaces is of interest. In this work, the contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water were measured with applied direct current (DC) and alternating current (AC) electric fields. The droplet was deposited on a polystyrene (PS) surface, commonly used in various nanotechnology applications, coated on a doped silicon (Si) wafer. With the DC field, CA decreased with an increase in voltage. Because of the presence of a silicon oxide layer and a space charge layer, the change of the CA was found to be lower than with a metal substrate. The CAH had no obvious change with a DC field. An AC field with a positive value was applied to the droplet to study its effect on CA and CAH. At low frequency (lower than 10 Hz), the droplet was visibly oscillating. The CA was found to increase when the frequency of the applied AC field increased from 1 Hz to 10 kHz. On the other hand, the CA decreased with an increasing peak-peak voltage at or lower than 10 kHz. The CAH in the AC field was found to be lower than in the DC field and had a similar trend to static CA with increasing frequency. A model is presented to explain the data.     

Surfactant and electric field strength effects on surface tension at liquid/liquid/solid interfaces

We performed a series of experiments designed to elucidate the effects of the presence of sodium dodecyl sulfate (SDS) surfactant and an applied electrical field on the wetting behavior in a system containing a sessile droplet of phenylmethyl polysiloxane (PMPS) oil on a polished stainless steel surface submersed in aqueous solution. The voltage difference ranged from -3 to +3 V, which is at least 3 orders of magnitude smaller than from comparable recent work. We report the measured equilibrium contact angle of the droplet as a function of surfactant concentration and field strength. We then modeled the system. We solved the Laplace equation to obtain the 3D field within our system. We expanded the three surface tensions (oil droplet-aqueous solution (oa), oil droplet-metal surface (os), and aqueous solution-metal surface (as)) in a Taylor series with respect to surfactant concentration and local field strength. We use these three surface tensions in Young's equation to obtain the theoretical contact angle of the organic droplet. We demonstrate that the large changes in contact angle due to the simultaneous presence of small concentrations of surfactant and small voltage differences can be accounted for by changes in the oa and as surface tensions.     

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.     

Behavior of oil droplets on an electrified solid metal surface immersed in ionic surfactant solutions

The present study investigates the change in the shape of oil droplets immersed in an ionic surfactant solution when the droplets are in contact with metal surfaces to which an electrical potential is applied. The three-phase system of aqueous solution-oil-steel was subjected to low-voltage electric potentials, which resulted in sometimes dramatic changes in droplet shape and wetting. This electric potential was applied to the conductive steel surface directly, and the counter electrode was immersed in the solution. Changes in both the shape and wetting extent of hexadecane and phenylmethyl polysiloxane were observed for voltages between +/-3.0 V in both sodium dodecyl sulfate and cetyl trimethylammonium bromide solutions. The droplets' behavior was opposite to what would be expected for traditional electrowetting. In one instance, hexadecane droplets in sodium dodecyl sulfate solutions with a voltage of -3.0 V, a rapid and repeating droplet elongation and detachment was observed. Additionally, the impact of the observed phenomena on electrowetting enhanced ultrasonication is presented to demonstrate the potential improvements in industrial ultrasonic cleaning processes. The observations lead to the possibility of employing simple electrowetting techniques in the removal of oil from metal surfaces in a manner that could greatly improve the environmental and economic performance of aqueous cleaning techniques.     

Nonlinear response of aluminum surface to electric field

The static limit of the perpendicular second-harmonic response of the exposed single-crystal planes of aluminum to applied electric field is studied using the variant of stabilized-jellium model which allows to account for the anisotropy of surface potential. The self-consistently calculated linear and second-order induced charge density distributions are used to determine the normal component of the polarization vector. The surface structure is found to have a pronounced effect on the anisotropy of second-harmonic response. © 1997 John Wiley & Sons, Inc.     

Thermodynamics of water octamer in a uniform electric field

We study the water octamer in a uniform electric field using the all-exchanges parallel tempering Monte Carlo method in the canonical ensemble. The heat capacity, quenched energy configurations, and the order parameter Q(4) are employed to understand the phase changes observed as a function of temperature and the strength of the applied electric field. At a low field strength of 0.1 V A(-1) a solidlike to liquidlike "melting" transition is detected. The corresponding heat capacity peak appears around 206 K, where Q(4) shows a significant change of slope. For E> or =0.5 V A(-1) such features are absent. However, at E=0.5 V A(-1) we find a solidlike to solidlike transition between cubic and extended structures around T approximately 25 K.     

Effects of channel sizes on traffic of solid in water in oil compound droplets through a vertical channel

The purpose of this article is to investigate the effects of channel sizes on the traffic of S/W compound droplets through a vertical channel. Compared with the horizontal channel, a vertical channel can effectively inhabit the contact of compound droplets with the channel wall, thus improving the survival rate. It is also found that the effects of tube length on droplet traffic are always dependent on the oil phase flow rate. In a short tube (L?=?2.0?cm), the survival rate increases as the oil phase flow rate increases. This may be due to significant prevention of coalescence among S/W compound droplets under a high oil phase flow rate. However, in a long tube (L?=?7.5?cm), the survival rate decreases with increasing oil phase flow rate, because disturbance of a water droplet can peel off the water phase coated on the surface of the solid particles. During the traffic process, the distance between water droplets and S/W compound droplets decreases linearly with time because of the larger diameter of the compound droplets. These study results can provide a useful guide for the preparation of high-throughput S/W compound droplets in a controllable and reproducible manner.     

Influence of electric field on the hydrogen bond network of water

Understanding the inherent response of water to an external electric (E)-field is useful towards decoupling the role of E-field and surface in several practically encountered situations, such as that near an ion, near a charged surface, or within a biological nanopore. While this problem has been studied in some detail through simulations in the past, it has not been very amenable for theoretical analysis owing to the complexities presented by the hydrogen (H) bond interactions in water. It is also difficult to perform experiments with water in externally imposed, high E-fields owing to dielectric breakdown problems; it is hence all the more important that theoretical progress in this area complements the progress achieved through simulations. In an attempt to fill this lacuna, we develop a theory based on relatively simple concepts of reaction equilibria and Boltzmann distribution. The results are discussed in three parts: one pertaining to a comparison of the key features of the theory vis a vis published simulation/experimental results; second pertaining to insights into the H-bond stoichiometry and molecular orientations at different field strengths and temperatures; and the third relating to a surprising but explainable finding that H-bonds can stabilize molecules whose dipoles are oriented perpendicular to the direction of field (in addition to the E-field and H-bonds both stabilizing molecules with dipoles aligned in the direction of the field).     

Water vapor nucleation on a crystal surface in a strong electric field

Water vapor nucleation at 260 K in a transverse electric field has been simulated by the Monte Carlo method under conditions corresponding to an internal wall of a spatially extended microcrack in a silver iodide crystal. The bicanonical statistical ensemble method has been employed to calculate, at the molecular level, the free energy of addition and the work of formation of dense phase nuclei in fields with different strengths. In a moderate field, the film mechanism of nucleation characterized by intense distortions on the film surface remains preserved. A domain structure of a film layer in contact with the surface exhibits a high stability with respect to an external field and remains preserved until the film is completely destroyed. In a strong electric field, the nucleation mechanism is fundamentally changed; i.e., the film is destroyed to yield threadlike structures. Therewith, the area of the contact with the surface drastically decreases. The orientation of nanothreads along the electric field lines overcomes a low free energy barrier. The point of equilibrium of nanothreads with vapor depends on the presence of hydrogen bonds, while their stability is determined by longer-range dipole-dipole interactions. The observed form of existence of the condensate as polarized nanothreads seems to be analogous to the superpolarized state previously revealed for water microdroplets, the transition to which has the character of the first-order phase transition.     

An analysis of the influence of vibrations on electric field gradient tensors in solid hydrates

A method is described for calculating the influence of vibrational motion on the electric field gradient tensors at deuterons in solid hydrates using librational tensors. The method is applied to model calculations to illustrate the effect of normal modes which involve a mixture of standard wagging, twisting and rocking modes.     

Dissociative ionization of triethylamine molecules on a tungsten surface in an electric field

The surface ionization of triethylamine molecules (M) in accelerating fields of strength F from 2·10⁴ to 4·10⁶ V/cm has been studied under stationary conditions with the aid of mass spectrometry. The predominant formation of (M-H)⁺ fragment ions has been discovered. The logarithm of the relative intensity of the ion currents at T = const varies linearly with the value of F. The temperature dependence of the ion currents I(T) at F = const, which are in good agreement with the experimental dependences and make it possible to determine the kinetic parameters of the reaction, have been obtained with the aid of the theory of monomolecular dissociation of molecules in the gaseous phase and the theory of surface ionization.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 62–67, January–February, 1988.We thank N. D. Potekhina and é. Ya. Zandberg for some useful discussions.     

Phase transition of nanotube-confined water driven by electric field

The effects of electric field on the phase behaviors of water encapsulated in a thick single-walled carbon nanotube (SWCNT) (diameter = 1.2 nm) have been studied by performing extensive molecular dynamics simulations at atmospheric pressure. We found that liquid water can freeze continuously into either pentagonal or helical solidlike ice nanotube in SWCNT, depending on the strengths of the external electric field applied along the tube axis. Remarkably, the helical one is new ice phase which was not observed previously in the same size of SWCNT in the absence of electric field. Furthermore, a discontinuous solid-solid phase transition is observed between pentagonal and helical ice nanotubes as the strengths of the external electric field changes. The mechanism of electric-field-induced phase transition is discussed. The dependence of ice structures on the chiralities of SWCNTs is also investigated. Finally, we present a phase diagram of confined water in the electric field-temperature plane.     

Particle-stabilized emulsions comprised of solid droplets

We kinetically stabilize oil-in-water emulsions comprising paraffin crystals by adsorbing solid particles (silica) of colloidal size at the oil/water interface. We obtain a set of emulsions that are quiescently stable for a long period of time (months), while the same emulsions are destabilized after only a few hours in the presence of surfactant molecules alone. The emulsions are submitted to a shear stress in order to probe their stability under flow conditions. Partial coalescence and gelation occur when the shear is applied for a sufficiently long period of time. The experiments reveal the existence of a critical droplet mass fraction, phi*, that defines a sharp transition between slow and fast gelation. The process of gelation is rather slow for phi < phi*, occurring at the scale of hours, and becomes almost instantaneous above phi*.     

Energy landscapes for water clusters in a uniform electric field

The behavior of small water clusters, (H2O)n, n=2-5 and n=8, in a uniform electric field is investigated for three related rigid-body models. Changes in the properties of the low-lying potential energy minima and the rearrangement pathways between them are examined. Results for certain structural transitions are compared with recent ab initio calculations. The models are found to give qualitatively similar trends, and there is some evidence that as the applied field strength is increased the quantitative differences between the models are also reduced.     

Suspension of water droplets on individual pillars

We report results of extensive experimental and numerical studies on the suspension of water drops deposited on cylindrical pillars having circular and square cross sections and different wettabilities. In the case of circular pillars, the drop contact line is pinned to the whole edge contour until the drop collapses due to the action of gravity. In contrast, on square pillars, the drops are suspended on the four corners and spilling along the vertical walls is observed. We have also studied the ability of the two geometries to sustain drops and found that if we compare pillars with the same characteristic size, the square is more efficient in pinning large volumes, while if we normalize the volumes to pillar areas, the opposite is true.