Capillary Bridges in Electric Fields

Klingner, Anke; Buehrle, Juergen; Mugele, Friedrich Gunther

doi:10.1021/la036058j

Cited by 28 publications

(46 citation statements)

References 19 publications

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“…Because the channel spacing is so small, this is accurately approximated by at the liquid/gas interface (8) where is the nondimensional curvature of the droplet in the -plane, is the nondimensional curvature of a cross-section of the droplet along the axis (see Fig. 6), and is the -length scale of the device.…”

Section: ) Navier-stokes Equationsmentioning

confidence: 99%

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Modeling the Fluid Dynamics of Electrowetting on Dielectric (EWOD)

Walker

Shapiro

2006

J. Microelectromech. Syst.

137

118

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Abstract-This paper discusses the modeling and simulation of a parallel-plate Electrowetting On Dielectric (EWOD) device that moves fluid droplets through surface tension effects. We model the fluid dynamics by using Hele-Shaw type equations with a focus on including the relevant boundary phenomena. Specifically, we show that contact angle saturation and hysteresis are needed to predict the correct shape and time scale of droplet motion. We demonstrate this by comparing our simulation to experimental data for a splitting droplet. Without these boundary effects, the simulation shows the droplet splitting into three pieces instead of two and the motion is over 15 times faster than the experiment. We then show how including the saturation characteristics of the device, and a simple model of contact angle hysteresis, allows the simulation to better predict the splitting experiment. The match is not perfect and suffers mainly because contact line pinning is not included. This is followed by a comparison between our simulation, whose parameters are now frozen, and a new experiment involving bulk droplet motion. Our numerical implementation uses the level set method, is fast, and is being used to design algorithms for the precise control of microdroplet motion, mixing, and splitting.[1439]

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Section: ) Navier-stokes Equationsmentioning

confidence: 99%

“…In particular, microfluidic devices often exploit surface tension forces to actuate or control liquids [1]- [3]. Electrowetting refers to using electrical fields to effectively modify surface tension effects [4]- [6] (see [7] and [8] for some fascinating experimental demonstrations).…”

Section: Introductionmentioning

confidence: 99%

Modeling the Fluid Dynamics of Electrowetting on Dielectric (EWOD)

Walker

Shapiro

2006

J. Microelectromech. Syst.

137

118

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“…2 we find q 0:9q 0 .) The subsequent gradual increase is due to a slow discharge via leakage currents [12]. In contrast, at high ac frequency and/or low conductivity (regime II) a substantially higher and perfectly reproducible value of is found within a few ms after detachment.…”

mentioning

confidence: 92%

“…Based on this criterion and on additional geometric considerations, a morphological diagram can be constructed which gives the ranges of stability for both the attached and the detached droplet morphologies as a function of d and U 0 . In analogy with the case of capillary bridges between parallel plates [11,12], we find that there is a region within the d-U 0 plane in which both morphologies are unstable and the droplet oscillates periodically between the attached and the detached state [13].…”

mentioning

confidence: 99%

Electrical Discharge in Capillary Breakup: Controlling the Charge of a Droplet

Mugele

2006

Phys. Rev. Lett.

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We studied the detachment of sessile droplets of conductive liquids from an immersed wire by reducing the contact angle using ac electrowetting. Upon detachment, the droplets acquire a certain amount of charge, which is shown to be controlled by a dimensionless parameter . describes the interplay between the diverging Ohmic resistance of the breaking capillary neck and the ac frequency. In the specific configuration of the present experiment, discharging at high frequency leads to self-excited oscillations in which the droplets periodically detach from and reattach to the wire. DOI: 10.1103/PhysRevLett.96.016106 PACS numbers: 68.03.ÿg, 68.05.ÿn, 83.60.Np The manipulation of tiny amounts of liquids has become a paradigm in various fields of applied physics such as printing and coating technology or biotechnology related microfluidics. Pressure gradients for driving fluid motion are traditionally generated using mechanical, thermomechanical, or electrical actuators. Given the intrinsically high surface to volume ratio in microfluidics, on-demand variations of interfacial energies such as thermocapillary effects [1] and electrowetting [2] have received increasing attention in recent years. The interplay of surface tension and electric fields can also be used to generate liquid jets [3] and droplets, for example in electrospray ionization [4,5] or in continuous ink-jet printing [6]. The role of the electric field in these examples is twofold: on the one hand, it drives the instability leading to the generation and detachment of microdroplets from the reservoir. On the other hand, it is responsible for the charge acquired by the detached droplets. In common configurations using dc voltage both aspects are tightly coupled. In the present Letter, we use low frequency ac electrowetting to detach liquid droplets from an electrode. We will show that the amount of charge acquired by a droplet detaching can be controlled by tuning the electrical properties of the liquid and the applied ac frequency. In this process, the breakup of the capillary neck in the late stage of the detachment process plays a crucial role. In conjunction with the finite conductivity of typical aqueous liquids, the algebraic decrease of the neck diameter [7,8] gives rise to a continuous divergence of the neck's Ohmic resistance. The speed of this divergence compared to the ac frequency determines to what extent the detaching droplet can be discharged.Droplets of a conductive liquid (volume 1 l, various mixtures of deionized water, glycerol, and NaCl) were deposited on conductive substrates that are covered by a thin insulating layer and a hydrophobic top coating (1 m thick thermally grown SiO 2 covered by a hydrophobic selfassembled monolayer of Octadecyltrichlorosilane [9]). The composition of the liquid was varied to cover a range of viscosities and conductivities from 2 mPa s to 70 mPa s and from 0.1 to 10 mS=cm, respectively. The entire system was immersed in a silicone oil bath (Fluka, viscosity 5 mPa s) in order to reduce both contact angle...

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“…Since this phenomenon involves free, rather than bound, charge it is normally described as "contactless" electrowetting. 15,16 The present study concerns the deformation due to an electric field of a drop of ionic fluid with a high conductivity. If a neutral drop is placed in a region of uniform electric field, any mobile charges arrange so that the electric field intensity is zero inside the drop.…”

Section: Introductionmentioning

confidence: 99%

Deformation of a nearly hemispherical conducting drop due to an electric field: Theory and experiment

et al. 2014

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We consider, both theoretically and experimentally, the deformation due to an electric field of a pinned nearly hemispherical static sessile drop of an ionic fluid with a high conductivity resting on the lower substrate of a parallel-plate capacitor. Using both numerical and asymptotic approaches, we find solutions to the coupled electrostatic and augmented Young–Laplace equations which agree very well with the experimental results. Our asymptotic solution for the drop interface extends previous work in two ways, namely, to drops that have zero-field contact angles that are not exactly π/2 and to higher order in the applied electric field, and provides useful predictive equations for the changes in the height, contact angle, and pressure as functions of the zero-field contact angle, drop radius, surface tension, and applied electric field. The asymptotic solution requires some numerical computations, and so a surprisingly accurate approximate analytical asymptotic solution is also obtained.

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Capillary Bridges in Electric Fields

Cited by 28 publications

References 19 publications

Modeling the Fluid Dynamics of Electrowetting on Dielectric (EWOD)

Modeling the Fluid Dynamics of Electrowetting on Dielectric (EWOD)

Electrical Discharge in Capillary Breakup: Controlling the Charge of a Droplet

Deformation of a nearly hemispherical conducting drop due to an electric field: Theory and experiment

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