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Jul 2009

Volume 21, Issue 7, Articles (07xxxx)

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Phys. Fluids 21, 072107 (2009); http://dx.doi.org/10.1063/1.3177339 (10 pages)

François Blanchette, Laura Messio, and John W. M. Bush
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back to top Micro- and Nanofluid Mechanics

Droplet breakup in microfluidic T-junctions at small capillary numbers

M.-C. Jullien, M.-J. Tsang Mui Ching, C. Cohen, L. Menetrier, and P. Tabeling

Phys. Fluids 21, 072001 (2009); http://dx.doi.org/10.1063/1.3170983 (6 pages) | Cited 15 times

Online Publication Date: 17 July 2009

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We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10−4 and 2×10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009) ]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.
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47.55.df Breakup and coalescence
47.55.dr Interactions with surfaces
47.20.Ib Instability of boundary layers; separation
47.60.Dx Flows in ducts and channels

Nonlinear alternating electric field dipolophoresis of spherical nanoparticles

Touvia Miloh

Phys. Fluids 21, 072002 (2009); http://dx.doi.org/10.1063/1.3184535 (11 pages) | Cited 8 times

Online Publication Date: 23 July 2009

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We consider the nonlinear electrokinetic problem of a freely suspended conducting (infinitely polarized) spherical micro- or nanosize particle surrounded by an unbounded electrolyte solution. The uncharged particle is exposed to an alternating (ac), nonuniform, and axisymmetric ambient electric field. As a result, the particle acquires a dipolophoretic (DIP) mobility of magnitude, which is quadratic in the amplitude of the applied electric field. The resulting phoretic velocity is driven by two independent nonlinear mechanisms. One is the common dielectrophoretic effect, whereby the nonuniform field exerts an electrostatic force on the image multipole singularity system within the particle. The other is the so-called “induced-charge electrophoresis” resulting from the action of the electric field on the excess charge around the particle induced in the diffused layer by the field itself. Both effects are quadratic in the amplitudes of the electric field and depend on the forcing frequency and on the dimensionless Debye screening length scale. It is demonstrated in the sequel that the two generally act in opposite directions which may result in mutual cancellation. Under the assumptions of a “weak” electric field and the neglect of surface conductance, we present a concise analysis of the resulting nonlinear streaming (dc) velocity (averaged over a period) for a spherical metalic particle that is exposed to a time-harmonic oscillating (ac) electric field. The analysis of this fundamental nonlinear DIP problem is provided for arbitrary forcing frequencies and for any Debye thickness. Numerical simulations are given for the case of a “two-mode” interaction consisting of a uniform-gradient electric field combined with a uniform field, where the two modes are either “in” or “out” of phase.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.57.jd Electrokinetic effects
47.55.Kf Particle-laden flows
47.11.-j Computational methods in fluid dynamics
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