1,721,126 research outputs found
How electrostatic fields change contact angle in electrowetting
No full textThe electrowetting phenomenon is investigated in the viewpoint of the classical electrostatics. Special attention is devoted to excavating the influence of excess charge induced at the edge region of a droplet. For this, the electrostatic field around an infinite wedge is analyzed, assuming the droplet as a perfect conductor. It is shown that the Maxwell stress is concentrated on the small region comparable to the thickness of the thin dielectric film beneath the liquid droplet. On the basis of the macroscopic balance condition of the horizontal-force components at the three-phase contact line, the conventional electrowetting equation is derived. This result suggests that macroscopic changes of contact angle originated from the electrostatic force, rather than from the change of the interfacial tension at the droplet-dielectric interface. Moreover, as the apparent contact angle becomes small, the vertical component of the electrostatic force, which would oppose the reduction of contact angle, increases significantly. This can be another possible cause of the limited validity of the conventional electrowetting equation and subsequent occurrence of contact-angle saturation phenomenon.X11198199sciescopu
Force acting on a dielectric particle in a concentration gradient by ionic concentration polarization under an externally applied DC electric field
No full textThere is a concentration-polarization (CP) force acting on a particle submerged in an electrolyte solution with a concentration (conductivity) gradient under an externally applied DC electric field. This force originates from the two mechanisms: (i) gradient of electrohydrodynamic pressure around the particle developed by the Coulombic force acting on induced free charges by the concentration polarization, and (ii) dielectric force due to nonuniform electric field induced by the conductivity gradient. A perturbation analysis is performed for the electric field, the concentration field, and the hydrodynamic field, under the assumptions of creeping flow and small concentration gradient. The leading order component of this force acting on a dielectric spherical particle is obtained by integrating the Maxwell and the hydrodynamic stress tensors. The analytical results are validated by comparing the surface pressure and the skin friction to those of a numerical analysis. The CP force is proportional to square of the applied electric field, effective for electrically neutral particles, and always directs towards the region of higher ionic concentration. The magnitude of the CP force is compared to that of the electrophoretic and the conventional dielectrophoretic forces. (c) 2005 Elsevier Inc. All rights reserved.X111011sciescopu
Prediction of oil boom performance in currents and waves
No full textThe threshold velocity of surface currents causing the entrainment failure for oil booms is investigated and an empirical formula for the threshold velocity is proposed. A theoretical prediction of the deformation of the skirt of an oil boom due to surface currents is made, and the results of the prediction are verified by laboratory experiments. The skirt deformation results in reduction of the effective boom draft which in turn degrades the effectiveness of the boom. The motion of oil booms excited by the ocean waves are predicted and verified by experiments. The degradation in the boom effectiveness due to the wave motion is discussed. The effectiveness of tandem booms in trapping the leaked oil is investigated and a method of predicting an optimum separation distance between the two booms is described. (C) 1998 Elsevier Science Ltd. All rights reserved.X1124sciescopu
Conditions for similitude and the effect of finite Debye length in electroosmotic flows
No full textUnder certain conditions, the velocity field is similar to the electric field for electroosmotic flow (EOF) inside a channel. There was a disagreement between investigators on the necessity of the infinitesimal-Reynolds-number condition for the similarity when the Helmholtz-Smoluchowski relation is applied throughout the boundaries. What is puzzling is a recent numerical result that showed, contrary to the conventional belief, an evident Reynolds number dependence of the EOF. We show here that the notion that the infinitesimal-Reynolds-number condition is required originates from the misunderstanding that the EOF is the Stokes flow. We point out that the EOF becomes the potential flow when the Helmholtz-Smoluchowski relation is applied at the boundaries. We carry out a numerical simulation to investigate the effect of finiteness of the Debye length and the vorticity layer inherently existing at the channel wall. We show that the Reynolds number dependence of the previous numerical simulation resulted from the finiteness of the Debye length and subsequent convective transport of vorticity toward the bulk flow. We discuss in detail how the convection of vorticity occurs and what factors are involved in the transport process, after carrying out the simulation for different Reynolds numbers, Debye lengths, corner radii, and geometries. (c) 2007 Elsevier Inc. All rights reserved.X114sciescopu
Prediction of drift in a free surface
No full textAn analytical model including the boundary-layer effect is developed to find the steady drift induced by the non-linear effect of the surface-wave motion. The analytical result is compared with the experimental results obtained from a wave tank. Results show a qualitative agreement. A semi-empirical formula which predicts drift velocity of a contaminant layer on the free surface is introduced and its validity is investigated.X115sciescopu
Modal properties of beams and plates on resilient supports with rotational and translational complex stiffness
No full textFor beam and plate vibrations, energy dissipation may be mostly attributed to internal damping inside the structure and external damping at the supports. Surface damping treatment is one of the most effective tools to control vibrations of beam and plate in general. Sometimes it is not always desirable, however, to apply this damping treatment, for practical reasons. In such cases, one must rely on the damping treatment at the supports. In this presentation, the effects of damped compliant boundary conditions on modal parameters are investigated for two types of continuous systems; an elastic beam and a circular plate. The impedances at the boundaries to translational and rotational motions are given by springs with complex stiffness. The governing equations are solved numerically to obtain the natural frequencies and modal loss factors. Numerical results are represented in dimensionless terms, based on which a procedure of selecting the support parameters is illustrated to obtain desirable modal properties. (C) 1996 Academic Press Limite
Validity of the Derjaguin approximation on electrostatic effect in the Frumkin-Derjaguin approach
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Shape oscillation of a drop in ac electrowetting
No full textA sessile drop oscillates when an ac voltage is applied in electrowetting. The oscillation results from the timevarying electrical force concentrated on the three-phase contact line. Little is known about the feature of drop oscillation in electrowetting. In the present work, the drop oscillations are observed systematically, and a theoretical model is developed to analyze the oscillation. It is revealed that resonance occurs at certain frequencies and the oscillation pattern is significantly dependent on the applied ac frequencies. The domain perturbation method is used to derive the shape-mode equations under the assumptions of a weak viscous effect and small drop deformation. The electrical force concentrated on the three-phase contact line is approximated as a delta function, which is decomposed and substituted into each shape-mode equation as a forcing term. The theoretical results for the shape and frequency responses are compared with experimental results, which shows qualitative agreement.X119194Nsciescopu
A synthetic jet produced by electrowetting-driven bubble oscillations in aqueous solution
No full textWe developed a method to actuate oscillation of a sessile bubble in a fluid to produce steady streaming within the fluid. This method is based on time-periodic control of the wettability of the bubble by electrowetting. Jet velocity is proportional to oscillation amplitude and is greatest at natural oscillation frequencies. Analytical and numerical analyses indicate that the jet is produced by steady streaming in the Stokes layer.open112533sciescopu
Trapping of leaked oil with tandem oil fences with Lagrangian analysis of oil droplet motion
No full textThe effectiveness of two oil fences deployed in tandem to maximize the containment of oil is investigated To assess the effectiveness of the tandem fences, the viscous flow field around the fences in tandem are analyzed numerically. Then, the trajectories of oil droplets which escaped beneath the fore fence are computed applying the Lagrangian particle-tracking method, to check under what conditions the droplet can be contained between the tandem fences. The validity of the calculated trajectories is checked experimentally by using spherical beads made of paraffin and droplets of kerosene, and the model fence of draft of 4 cm. The numerically predicted trajectories of the droplets show fairly good agreement with the experimental results. The method is applied to predict the motion of the weathered oil. It is shown, numerically, that most of the leaked oil can be trapped between tandem fences, when the distance between the fences is about IO times the draft of the fore fence.X117sciescopu
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