Effect of Interfacial Permeability on Droplet Relaxation in Biopolymer-Based Water-in-Water Emulsions

Abstract

In this paper, we show that for aqueous phase-separated biopolymer mixtures (water-in-water emulsions) both interfacial tension and permeability of the interface are important for the relaxation process of deformed droplets. We give an expression for the characteristic relaxation time that contains both contributions. With this description, the interfacial tension and the permeability can be deduced from cessation-of-flow experiments. The results show that for samples that are very close to the critical point the interfacial tension, calculated without taking into account the permeability, are overestimated significantly. For samples close to the critical point, the permeability has to be taken into account in the description for the relaxation time to get a reliable estimation of the interfacial tension. Our experiments show that for these systems the effective permeability is inversely proportional to the interfacial tension, lambda(eff) proportional, variant1/gamma, and proportional to the square of the interfacial thickness, lambda(eff) proportional, variant xi(2). We find that the permeability is related to an effective diffusion coefficient as D(eff) proportional, variant lambdagamma(eff). From this relation, we find that the diffusion coefficient is equal to 0.9.10(-9) m(2)/s, which is close to the self-diffusion coefficient of water; = 2.3.10(-9) m(2)/s. This indicates that only water diffuses through the interface, and the diffusion coefficient is independent of the composition of the system for the concentration regime that is used.

Fluctuations of Permeable Interfaces in Water-in-Water Emulsions

“…and (17) is a generalization of the expression derived in [11] for the relaxation time of a membrane, and the semiempirical expression in [4] 2 . In the last term of (16) we sum over the index j x, z.…”

“…On short time scales water is usually the only component being transferred [4]. On longer time scales the transfer of the biopolymers is also affecting the behavior of the system [6].…”

“…To illustrate the effect of permeability, surface tension, and bending rigidity on the fluctuations, we will calculate the relaxation time A for a phase-separated gelatin-dextran-water system [4]. The data used for the calculation are the data for sample 2 and 6 in [4], and are given in Table I. Sample 2 is a mixture with composition (prior to separation) close to the critical point of the system, and sample 6 is a sample far away from the critical point.…”

“…In these systems water and dissolved molecules can transfer easily from one aqueous bulk phase to the other. This high permeability of the interface has a significant effect on the stressdeformation and deformation-relaxation behavior of, for example, droplets in phase-separated biopolymer solutions [4,5].That mass transfer can have an appreciable effect on the dynamic behavior of a deformed droplet was also shown in recent interfacial tension measurements of biopolymer solutions, using the spinning drop method [6]. Droplets deformed by spinning the capillary, slowly dissolved, and the rate of dissolution increased with increasing spinning rate, or increasing deformation of the droplets.…”

“…In these systems water and dissolved molecules can transfer easily from one aqueous bulk phase to the other. This high permeability of the interface has a significant effect on the stressdeformation and deformation-relaxation behavior of, for example, droplets in phase-separated biopolymer solutions [4,5].…”