Nivedita R. Gupta

The detachment of a viscous drop in a viscous solution in the presence of a soluble surfactant

When a buoyant viscous drop is injected into a viscous fluid, it evolves to form a distended shape that detaches via the rapid formation and pinching of a neck. The effects of surfactants in altering this process are studied numerically. In the absence of surfactants, surface contraction is fastest in the vicinity of the neck. Thus, when surfactants are present, they accumulate there and alter the ensuing dynamics by reducing the surface tension that drives the contraction. The surface tension is described by a nonlinear surface equation of state that accounts for the maximum packing of surfactant in a monolayer. When surfactant adsorption-desorption is very slow, interfaces dilute significantly during drop expansion, and drops form necks which are only slightly perturbed in their dynamics from the surfactant-free case. When adsorption-desorption dynamics are comparable to the rate of expansion, drops thin to form a primary neck at low surfactant coverage, to form both primary and secondary necks at moderate coverages, form only a secondary neck at higher coverages, or fail to neck at elevated coverages. When surfactant adsorption-desorption kinetics are rapid, the surface remains in equilibrium with the surrounding solution, and drops behave like surfactant-free drops with a uniform surface tension. These arguments are used to construct a phase diagram of drop neck shapes as a function of surfactant coverage. A map of neck/no-neck thresholds is also constructed as a function of surfactant coverage and sorption dynamics, suggesting that drop detachment can be used as a means of characterizing surfactant dynamics.

Inertial and surfactant effects on the steady droplet flow in cylindrical channels

The flow of neutrally buoyant droplets in circular channels at finite Reynolds numbers (0.1 ≤ Re ≤ 400) and moderate capillary numbers (0.005 ≤ Ca ≤ 0.1) is studied numerically using a front tracking method. The drops are either clean or contain surfactants which are modeled to behave according to the Langmuir equation of state. The numerical results agree well with previous studies in the Stokes flow regime for small, undeformed drops, as well as very large drops. Increasing the Reynolds number causes a non-monotonic trend in both the relative velocity of the drop and the extra pressure loss required to maintain a constant flow rate. The trends are attributed to changes in drop shape caused by increasing inertial effects. For moderate-sized drops with radii 0.5 to 0.9 times the tube radius, the velocity first decreases and then increases with Reynolds number. For larger drops with radii 1.2 to 1.5 times the tube radius, the effect of inertia is to further elongate the drop and a non-monotonic trend in velocity is not observed. At large Reynolds numbers, stable, oscillatory flows with shape changes confined to the rear of the drop are observed. For long viscous drops, the film thickness increases monotonically with the Reynolds number for all capillary and Reynolds numbers studied. In the presence of inertia, surfactant-laden drops show a maximum in the drop velocity (and a minimum in extra pressure loss) at an intermediate Biot number. In general, at large Reynolds numbers, the effects of surfactants tend to diminish as compared to previous Stokes flow simulations.

Double-Layer Thermocapillary Convection in a Differentially Heated Cavity

Abstract:  Many materials‐processing applications such as crystal growth from the melt involve thermocapillary flows that can affect the quality of the final product, particularly under microgravity conditions where the influence of buoyancy‐driven convection is minimized. When the melt contains volatile components, as in the production of III–V semiconductor crystals, it is often encapsulated in a low‐melting point amorphous molten glass phase such as boron oxide or pyrolytic boron nitride in order to prevent evaporation of the volatile components. The addition of the encapsulant layer and the melt–encapsulant interface in such cases can alter the thermocapillary flow in the melt. In this study, thermocapillary convection within a differentially heated rectangular cavity containing two immiscible liquid layers is considered in the absence of gravity. Domain mapping is used in conjunction with a finite difference scheme on a staggered grid to solve for the temperature and flow fields. The melt–encapsulant and the air–encapsulant interfaces are allowed to deform, with the contact lines pinned on the solid boundaries. The computed flow fields are compared to the corresponding results for a cavity with a rigid top surface. The presence of a free surface at the top leads to increased convection in the encapsulant phase while suppressing the thermocapillary flow in the melt phase. The flow pattern in the encapsulated layer is strongly dependent on the viscosity of the encapsulant layer. The intensity of the thermocapillary flow within the melt is significantly reduced as the viscosity of the encapsulant layer is increased. However, for a higher encapsulant viscosity, the retarding effect of the free top surface on thermocapillary convection in the melt is weakened.

Improved Water Barrier Properties of Calcium Alginate Capsules Modified by Silicone Oil

Calcium alginate films generally offer poor diffusion resistance to water. In this study, we present a technique for encapsulating aqueous drops in a modified calcium alginate membrane made from an emulsion of silicone oil and aqueous alginate solution and explore its effect on the loss of water from the capsule cores. The capsule membrane storage modulus increases as the initial concentration of oil in the emulsion is increased. The water barrier properties of the fabricated capsules were determined by observing the mass loss of capsules in a controlled environment. It was found that capsules made with emulsions containing 50 wt% silicone oil were robust while taking at least twice the time to dry completely as compared to capsules made from only an aqueous alginate solution. The size of the oil droplets in the emulsion also has an effect on the water barrier properties of the fabricated capsules. This study demonstrates a facile method of producing aqueous core alginate capsules with a modified membrane that improves the diffusion resistance to water and can have a wide range of applications.