The motion of two drops in a uniform electric field is considered using the leaky dielectric model. The drops are assumed to have no native charge and a dielectrophoretic effect favours translation of the drops toward one another. However, circulatory flows that stem from electrohydrodynamic stresses may either act with or against this dielectrophoretic effect. Consequently, both prolate and oblate drop deformations may be generated and significant deformation occurs near drop contact owing to enhancement of the local electric field. For sufficiently widely spaced drops, electrohydrodynamic flows dominate direct electrical interactions so drops may be pushed apart, though closely spaced drops almost always move together as a result of the electrical interaction or deformation.

The onset of electrohydrodynamic motion associated with the imposition of an electric field across a thin layer of liquid has been investigated for the case in which the electrical conductivity varies linearly over the depth of the layer. The variation of the conductivity is due to concentration gradients in the charge-carrying solutes and its spatiotemporal evolution is represented by a convective-diffusion equation. When the viscous relaxation time is long compared to the time for charge relaxation, the analysis reveals that the neutral stability curves for the layer can be characterized by three dimensionless parameters: Rae≡dεE02Δσ/μKeffσ0, an electrical Rayleigh number; Δσ/σ0, the relative conductivity increment; and α, the transverse wave number of the disturbance. Here d is the thickness, ε is the dielectric constant, and μ is the viscosity of the layer, E0 is the applied field strength at the lower conductivity boundary, and Keff is an effective diffusivity associated with the Brownian motion of the charge-carrying solutes. With stress-free boundaries, at which the electrical conductivity and current are prescribed, the critical Rae is 1.416×104 at a critical transverse wave number of 1.90 when Δσ/σ0 is 8. As Δσ/σ0 increases, the critical Rae increases and shifts to slightly shorter wavelength disturbances; the critical imposed field strength, however, passes through a minimum because the lower-conductivity boundary exerts a considerable stabilizing influence in the presence of steep conductivity gradients. For Δσ/σ0≲8, the critical Rayleigh number increases as Δσ/σ0 decreases and the layer is only sensitive to long wavelength disturbances (α<0.1) for Δσ/σ0 below 4. Similar trends were obtained for liquid layers with other boundary conditions; e.g., rigid boundaries and constant potential boundaries.

The forced convection of a monodisperse, monoclonal suspension of bacteria through a uniform, saturated porous medium has been investigated. Bench-scale column studies were carried out to measure the removal of microorganisms from suspension due to attachment to the surfaces of the solid phase. The columns were packed with 40-μm borosilicate glass beads, and bacterial sorption was measured as a function of depth in the column using a leucine radiolabel assay. The strains A1264 and CD1 were examined separately. Colloid filtration theory was used to interpret the data, and the average, or effective, affinity of the bacteria for the glass beads was found to decrease with distance traveled through the column. It is postulated that, under these circumstances, the cell/collector affinity (that is, the collision efficiency α) varied due to intrapopulational differences in bacterial surface characteristics. A simple bimodal probability density function, consisting of two Dirac delta functions, was found to satisfactorily represent the α distribution in the original bacterial population. This form of the distribution function was supported by capillary electrophoresis measurements on the bacteria, which showed intrapopulational differences in the surface charge density under the conditions of the transport experiments. These variations in surface charge density are significant inasmuch as they give rise to substantial differences in the colloidal interaction potentials and, presumably, large differences in cell affinity for negatively charged collectors such as glass beads or quartz.

W e have examined the electrophoresis of drops and bubbles, computing t h e electrophoretic mobility as a function of t h e [-potential and several other parameters. Our treatment differs from previous work in that we incorporate a more representative picture of t h e interface. W e have found that drops and bubbles are electrophoretically distinct from particles; perhaps the most striking result obtained was that, when the diffuse layers are thin, conducting drops do not always migrate in t h e direction that would be anticipated from t h e sign of their surface charge. Thus, the [-potential alone is not sufficient to characterize the surface. The analysis shows t h e sense of t h e migration is dictated by t h e net electrochemical stress acting along the interface. For similar reasons, large inviscid spheres tend to remain stationary at modest [-potentials and, in contrast to rigid particles, their mobility is actually enhanced by polarization of t h e double layer. Further, we have uncovered conditions for which t h e mobility of non-conducting drops is insensitive to the interior viscosity. This 'solidification effect' stems in part from interfacial tension gradients associated with specific adsorption of the ionic solutes, as well a s from polarization and, moreover, need not involve t h e presence of surface-active i rn pu ri ties.

Articles you may be interested inDynamic and dielectric response of charged colloids in electrolyte solutions to external electric fields Sound speed dispersion measurements and their interpretation in the presence of a shallow buried layer.The standard description of electrokinetic phenomena deals with a particle whose charge is uniformly smeared over its surface and considers ion transport only within a Gouy-Chapman diffuse layer. Experimental studies with colloidal dispersions have shown that this model is not applicable to many systems. To encompass a wider class of behavior, the standard model was extended to include ion migration within the Stern layer, the region between the shear envelope and the rigid particle. Computations show that Stern layer transport increases the conductivity and dielectric response of suspensions as well as the magnitude of the {; potential inferred from mobility measurements. Model predictions are compared with experimental measurements on two well-defined systemscolloidal silica and a polymer latex. The inclusion of surface transport processes markedly improves agreement between theory and the experimental data. For example, in situations where the standard theory underpredicts the measured dielectric increments by factors of 2 or 3, the dynamic Stern layer model yields results within 5% to 20% of the experimental data at frequencies in the kHz range.cess,

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Attachment, deformation and detachment of N-cadherin expressing prostate and breast cancer cell lines in a functionalized microchannel under hydrodynamic loading have been studied. N-cadherin antibodies are immobilized on the microchannel surface to capture the target cancer cells, PC3N and MDA-MB-231-N, from a homogeneous cell suspension. Although difficult, a significant fraction of moving cells can be captured under a low flow rate. More than 90% of the target cells are captured after a certain incubation time under no flow condition. The mechanical response of a captured cancer cell to hydrodynamic flow field is investigated and, in particular, the effect of flow acceleration is examined. The observed cell deformation is dramatic under low acceleration, but is negligible under high acceleration. Consequently, the detachment of captured cells depends on both flow rate and flow acceleration. The flow rate required for cell detachment is a random variable that can be described by a log-normal distribution. Two flow acceleration limits have been identified for proper scaling of the flow rate required to detach captured cells. A time constant for the mechanical response of a captured cell, on the order of 1 min, has been identified for scaling the flow acceleration. Based on these acceleration limits and time constant, an exponential-like empirical model is proposed to predict the flow rate required for cell detachment as a function of flow acceleration.

The growth of icicles is considered as a free-boundary problem. A synthesis of atmospheric heat transfer, geometrical considerations, and thin-film fluid dynamics leads to a nonlinear ordinary differential equation for the shape of a uniformly advancing icicle, the solution to which defines a parameter-free shape which compares very favorably with that of natural icicles. Away from the tip, the solution has a power-law form identical to that recently found for the growth of stalactites by precipitation of calcium carbonate. This analysis thereby explains why stalactites and icicles are so similar in form despite the vastly different physics and chemistry of their formation. In addition, a curious link is noted between the shape so calculated and that found through consideration of only the thin coating water layer.