Lifetime of oil drops pressed by buoyancy against a planar interface: Large drops

TL;DR

This study models the lifetime of buoyant oil drops pressed against a planar interface, revealing size-dependent behaviors and the importance of deformation and hydrodynamics, with implications for emulsion stability.

Contribution

It extends previous models to larger, deformable drops using a truncated sphere approach, highlighting size-dependent dynamics and the limitations of existing equations.

Findings

Large drops experience significant deformation affecting coalescence.

Hydrodynamic tensors primarily influence the lifetime as a function of size.

Experimental scattering suggests additional factors like capillary waves may be involved.

Abstract

In a previous report [10] it was shown that emulsion stability simulations are able to reproduce the lifetime of micrometer-size drops of hexadecane pressed by buoyancy against a planar water-hexadecane interface. It was confirmed that small drops (ri<10 {\mu}m) stabilized with {\beta}-casein behave as nondeformable particles, moving with a combination of Stokes and Taylor tensors as they approach the interface. Here, a similar methodology is used to parametrize the potential of interaction of drops of soybean oil stabilized with bovine serum albumin. The potential obtained is then employed to study the lifetime of deformable drops in the range 10 \leq ri \leq 1000 {\mu}m. It is established that the average lifetime of these drops can be adequately replicated using the model of truncated spheres. However, the results depend sensibly on the expressions of the initial distance of deformation and the maximum film radius used in the calculations. The set of equations adequate for large drops is not satisfactory for medium-size drops (10 \leq ri \leq 100 {\mu}m), and vice versa. In the case of large particles, the increase in the interfacial area as a consequence of the deformation of the drops generates a very large repulsive barrier which opposes coalescence. Nevertheless, the buoyancy force prevails. As a consequence, it is the hydrodynamic tensor of the drops which determine the characteristic behavior of the lifetime as a function of the particle size. While the average values of the coalescence time of the drops can be justified by the mechanism of film thinning, the scattering of the experimental data of large drops cannot be rationalized using the methodology previously described. A possible explanation of this phenomenon required elaborate simulations which combine deformable drops, capillary waves, repulsive interaction forces, and a time-dependent surfactant adsorption.