Ali Borhan

Thermocapillary motion of deformable drops at finite Reynolds and Marangoni numbers

We present the results of numerical simulations of the three-dimensional thermocapillary motion of deformable viscous drops under the influence of a constant temperature gradient within a second liquid medium. In particular, we examine the effects of shape deformations and convective transport of momentum and energy on the migration velocity of the drop. A numerical method based on a continuum model for the fluid–fluid interface is used to account for finite drop deformations. An oct-tree adaptive grid refinement scheme is integrated into the numerical method in order to track the interface without the need for interface reconstruction. Interface deformations arising from the convection of energy at small Reynolds numbers are found to be negligible. On the other hand, deformations of the drop shape due to inertial effects, though small in magnitude, are found to retard the motion of the drop. The steady drop shapes are found to resemble oblate or prolate spheroids without fore and aft symmetry, with the d...

Effect of surfactants on the motion of drops through circular tubes

The effect of surfactants on the motion and deformation of liquid drops in Poiseuille flow through circular tubes at low Reynolds numbers is examined. Assuming no bulk transport of surfactant, the boundary integral method is used in conjunction with a convective–diffusion equation to determine the distribution of surfactant on the deformed surface of the drop. The velocity and shape of the drop as well as the extra pressure loss due to the presence of the drop are calculated. Increasing the surface Peclet number is found to produce large variations in surfactant concentration across the surface of the drop. The resulting interfacial tension gradients lead to tangential (Marangoni) stresses that oppose surface convection and retard the motion of the drop as a whole. For large Peclet numbers, Marangoni stresses immobilize the surface of the drop, leading to a significant increase in the extra pressure loss required to move the drop through the tube. The accumulation of surfactant near the trailing end of the drop partially lowers the interfacial tension on that side, thereby requiring larger deformations to satisfy the normal stress balance. At the same time, the increase in interfacial area associated with drop deformation causes an overall dilution of the surfactant, which, in turn, counteracts the effect of convective transport of surfactant at large Peclet numbers. The effects of these coupled responses are studied over a wide range of the dimensionless parameters.

Microencapsulation of Chemotherapeutics into Monodisperse and Tunable Biodegradable Polymers via Electrified Liquid Jets: Control of Size, Shape, and Drug Release

This paper describes microencapsulation of antitumor agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU, Carmustine) into biodegradable polymer poly(lactic-co-glycolic) acid (PLGA) using an electrojetting technique. The resulting BCNU-loaded PLGA microcapsules have significantly higher drug encapsulation efficiency, more tunable drug loading capacity, and (3) narrower size distribution than those generated using other encapsulation methods.

Effects of Hierarchical Surface Roughness on Droplet Contact Angle

Superhydrophobic surfaces often incorporate roughness on both micron and nanometer length scales, although a satisfactory understanding of the role of this hierarchical roughness in causing superhydrophobicity remains elusive. We present a two-dimensional thermodynamic model to describe wetting on hierarchically grooved surfaces by droplets for which the influence of gravity is negligible. By creating wetting phase diagrams for droplets on surfaces with both single-scale and hierarchical roughness, we find that hierarchical roughness leads to greatly expanded superhydrophobic domains in phase space over those for a single scale of roughness. Our results indicate that an important role of the nanoscale roughness is to increase the effective Youngs angle of the microscale features, leading to smaller required aspect ratios (height to width) for the surface structures. We then show how this idea may be used to design a hierarchically rough surface with optimally high contact angles.

A Theory for the Morphological Dependence of Wetting on a Physically Patterned Solid Surface

We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using free-energy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

Effect of inertia on the thermocapillary velocity of a drop

Abstract We consider the effects of inertia on the thermocapillary migration velocity of a small liquid droplet in a microgravity environment. The externally imposed temperature gradient is assumed to be constant, and the fluid surrounding the droplet is taken to be unbounded and otherwise quiescent. With the convective transfer of heat neglected, droplets with densities higher/lower than the outside liquid deform to prolate/oblate spheroidal shapes, at small values of the capillary and Reynolds numbers. The corrections to the temperature field and the migration velocity of the droplet, resulting from this deformation, are obtained using the so-called Lorentz reciprocal theorem. It is found that the migration velocity could increase, decrease, or remain unchanged depending on the value of certain controlling parameters. The results are presented in vector-invariant form.

Creeping flow through sinusoidally constricted capillaries

The steady pressure‐driven flow of an incompressible Newtonian fluid through an axisymmetric capillary whose diameter varies sinusoidally in the axial direction is studied numerically under conditions of negligible inertial effects. The boundary integral formulation is used to obtain detailed velocity and pressure distributions within the capillary, and to determine the effects of amplitude and wavelength of corrugation on the kinematic structure of the flow. In particular, the critical values of the geometric parameters leading to flow reversal within the expansion region of the capillary are established. The dependence of the pressure drop through the capillary on the geometric parameters is also discussed with reference to the predictions of various effective pore models.

Wetting on physically patterned solid surfaces: the relevance of molecular dynamics simulations to macroscopic systems.

We used molecular dynamics (MD) simulations to study the wetting of Lennard-Jones cylindrical droplets on surfaces patterned with grooves. By scaling the surface topography parameters with the droplet size, we find that the preferred wetting modes and contact angles become independent of the droplet size. This result is in agreement with a mathematical model for the droplet free energy at small Bond numbers for which the effects of gravity are negligible. The MD contact angles for various wetting modes are in good agreement with those predicted by the mathematical model. We construct phase diagrams of the dependence of the wetting modes observed in the MD simulations on the topography of the surface. Depending on the topographical parameters characterizing the surface, multiple wetting modes can be observed, as is also seen experimentally. Thus, our studies indicate that MD simulations can yield insight into the large-length-scale behavior of droplets on patterned surfaces.

Breakup of drops and bubbles translating through cylindrical capillaries

We examine the shape deformation and breakup of air bubbles and viscous drops moving through vertical cylindrical capillaries under the action of pressure and/or buoyancy forces. Experimental observations of fluid particle shape are reported over a wide range of particle sizes and capillary numbers in a variety of two-phase systems. Four different modes of breakup are identified, and the critical conditions for the onset of various modes are examined. It is found that buoyancy forces can have a stabilizing effect on the breakup mechanism observed by Olbricht and Kung [Phys. Fluids 4, 134, (1992)] for low viscosity-ratio drops, wherein a growing indentation at the trailing end of the drop develops into a penetrating jet of outer phase fluid.