Biomedical subjects

O K Matar

Publications and source records attributed to O K Matar.

10 recordsLinked to original sources

PubMed2006

Drop manipulation and surgery using electric fields.

We study the dynamics of a slender drop sandwiched between two electrodes using lubrication theory. A coupled system of evolution equations for the film thickness and interfacial charge density is derived and simplified for the case of a highly conducting fluid. The contact line singularity is relieved by postulating the existence of a wetting precursor film, which is stabilised by intermolecular forces. We examine the motion of the drop as a function of system parameters: the electrode separation, beta, an electric capillary number, C, and a spatio-temporally varying bottom electrode potential. The possibility of drop manipulation and surgery, which include drop spreading, translation, splitting and recombination, is demonstrated using appropriate tuning of the properties of the bottom potential; these results could have potential implications for drop manipulation schemes in various microfluidic applications. For relatively small beta and/or large C values, the drop assumes cone-like structures as it approaches the top electrode; the latter stages of this approach are found to be self-similar and a power-law exponent has been extracted for this case.

Electrodes↗

PubMed2006

Collapse of a bubble in an electric field.

A low-Mach-number analysis is presented of the collapse of a bubble in an electric field, which is assumed to be homogeneous, but may be unsteady. Ellipsoidal shape deformations are accounted for in the analysis, but are assumed to be small. It is shown that the presence of an electric field leads to additional terms in a modified Rayleigh-Plesset equation. This differential equation for the bubble radius and a corresponding equation for ellipsoidal shape deformations have been integrated numerically. The results indicate that a bubble can be made to collapse by instantaneously switching on an electric field. Also, nonharmonic volumetric oscillations are observed for time-dependent electric fields of sufficiently large amplitude. It is shown that the rate of a collapse driven by external pressure variations due, for instance, to acoustic forcing can be accelerated.

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PubMed2006

On the dynamics of liquid lenses.

The spreading of a lens of one liquid on the surface of another liquid is examined. Lubrication theory is used to derive a coupled system of equations for the air-liquid and liquid-liquid interfaces. In the case of highly viscous lenses, extensional stresses are promoted and an additional equation for the lens velocity is derived. The potential singularity at the three-phase line is relieved by a microscopic precursor layer of the spreading fluid assumed to be present ahead of the macroscopic lens. This layer is stabilised via the inclusion of disjoining pressure effects in the lens. The results of our full parametric study show that, for weak gravitational forces, the shape of the lens at equilibrium depends solely on the surface tension ratio for sufficiently deep substrate thicknesses. For thin substrates, the underlying liquid film deforms severely near the point of deposition exhibiting flattening and dimpling.

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PubMed2006

Collisions of liquid coated solid spherical particles in a viscous fluid.

An analytical description is presented for the head-on collision of two spherical rigid particles that are coated with a thin layer of one liquid and immersed in another. Lubrication theory is used to resolve the spatio-temporal evolution of the coating surfaces, in conjunction with the fluid flow in the gap region between the particles. The analysis is carried out up to the point where the gap region has almost completely been drained; intermolecular forces are neglected. The effects of particle inertia, the ratio of particle radii, surface tension, and the viscosity ratio of the coating and carrier fluids are studied; these are parameterised by St, beta, Ca and m, respectively. The results of the present work elucidate the effect of the above-mentioned factors on the conditions under which particles rebound (assumed to occur if the distance between the particles becomes very short while the relative velocity does not vanish) or stick. In particular, summarizing flowmaps show that the likelihood of particles rebounding increases with increasing St and decreasing beta, Ca and m. On the other hand, it is shown that the force on approaching particles depends on all of these parameters in a non-monotonic manner.

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PubMed2005

A note on the coating of an inclined plane in the presence of soluble surfactant.

We consider the flow of a thin liquid film coating an inclined plane in the presence of a soluble surfactant. A two-dimensional three-equation model is derived using lubrication theory in the rapid diffusion limit and then used to investigate the stability of the fluid height and the surfactant surface and bulk concentrations. We present solutions for an insoluble surfactant system, which are then contrasted with those obtained for a system containing a soluble surfactant; both transient growth and fully nonlinear two-dimensional simulation results are discussed. Our results indicate that the characteristics of the fingering phenomena which accompany the flow are altered by the effects of solubility. In particular, we find that these effects de-stabilise the system further over an intermediate range of surfactant solubility.

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PubMed2005

Coating of an inclined plane in the presence of insoluble surfactant.

We consider the flow of a thin liquid film coating an inclined plane in the presence of an insoluble surfactant. A fully non-linear two-dimensional system of governing equations is formulated using lubrication theory to describe the dynamics. Numerical simulations of this system highlight a fingering instability present at the main fluid front and elucidate the role of surfactant in the destabilizing mechanism. A full parametric study is undertaken which reveals the dependence of the fingering characteristics on system parameters. Numerical solutions at low angles of inclination are also obtained in order to illustrate the connection between gravitationally driven fingering and the instability induced by surfactant on a flat substrate. The similarities and differences between the destabilizing mechanisms in each case are discussed.

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PubMed2004

Simultaneous thermal and surfactant-induced Marangoni effects in thin liquid films.

The deformation of a thin liquid film in the presence of a surfactant monolayer, varying temperature distributions, and limited mass flux is considered. Use of lubrication theory yields a coupled pair of partial differential equations for the film height and surfactant surface monolayer concentration. The long-wave stability of the isothermal film is examined over a wide range of parameter values. It is shown that droplet patterns are obtained under certain thermal conditions for both an isothermal and nonisothermal underlying substrate. For the case of a localized thermal gradient initially imposed at the air-liquid interface, severe film thinning beneath the heat source was observed, which was not accompanied by droplet formation; pseudo steady states are observed in this case. In all situations the surfactant is found to rigidify the air-liquid interface, retarding thermally driven flow, while evaporation (condensation) acts to destabilize (stabilize) the film.

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PubMed2003

Pattern formation in thin liquid films with charged surfactants.

The dynamic evolution of positively charged surfactants at the interface of a thin film resting atop a negatively charged base is investigated. The lubrication approximation is used to develop coupled equations governing the dynamic evolution of such a system in the presence of charge effects coupled with van der Waals forces. The equations are investigated numerically and analytically, and, for certain parameter ranges, pattern formation is observed reminiscent of that accompanying thermocapillary-driven thin films. Spatial nonuniformities in the charge of the underlying substrate are also studied as a possible tool for film rupture wavelength selection.

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PubMed2003

Surface patterning via evaporation of ultrathin films containing nanoparticles.

The dewetting dynamics of ultrathin films containing potentially surface-active nanoparticles is considered in the presence of evaporation. Evolution equations for the film height and particle surface and bulk concentration are derived using a lubrication model coupled by a constitutive relation for the dependence of the viscosity on local particle concentration. A linear stability analysis and numerical simulations are used to determine how particle mass distribution depends on the various physical parameters such as equilibrium film separation distance, initial packing concentration, rate of evaporation, and particle surface activity. Our results show that when starting from an initially uniform distribution the particles become aligned into distinct "bands" in rectilinear geometry, or "rings" in cylindrical geometry. The functional dependence of the pattern spacing on relevant system parameters is studied and detailed herein.

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PubMed2001

Couette flow of two immiscible liquids between two concentric cylinders: the formation of toroidal drops and liquid sheaths.

We describe the direct observation of deforming water drops in oil undergoing shear flow in a horizontal annular Couette cell. The drops assume a wide variety of highly reproducible structures depending on drop size, rotation speed, and flow history. These structures include toroidal rings of water around the rotating shaft and water sheaths, which, depending on experimental conditions, can either expand to press against the inner walls of the outer stationary cylinder or contract to hug the outside of the rotating shaft.

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