Dynamic Forces Between Two Deformable Interfaces

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Original entry by Hyerim Hwang, AP 226, Spring 2012.

Reference

Raymond R. Dagastine, Rogerio Manica, Steven L. Carnie, D. Y. C. Chan, Geoffrey W. Stevens and Franz Grieserl, "Dynamic Deformable Oil Droplets in Water", Science 2006 313, 210-213

Keywords

Dynamic forces, Colloidal suspensions, Interfacial deformation, Static surface forces

Introduction

Dynamic interactions in soft-matter systems have been studied, which are key forces to manipulate and control soft-matter systems. This paper shows the dynamic interactios between two deformable oil droplets in water. It is applicable to most soft-matter systems and important especially in collision forces from Brownian motion. Dynamic interactions between two deformable interaces are difficult to measure, therefore many research groups have researched on direct force measurements of interactions between droplets. This paper have focused on a model for interactions between two droplets for intermediate size droplets where deformation, hydrodynamic drainage, and interaction forces are all important.

Figure 1. (A) The experiment between two oil droplets - one immobilized on the catilever and the other immobilized on the substrate of an AFM. (B),(C),(D) The dynamic interaction force versus piezo drive motion between two oil droplets in aqueous solution.

Results

Two decane droplets of different sizes in a surfactant solution, were immobilized on an AFM cantilever and substrate. Interaction force between the droplets was measured as a function of piezo drive motion of the substrate. The velocity range spans the likely velocities of an emulsion droplet of comparable size when undergoing Brownian motion. The results show that hydrodynamic interactions between droplets are important even when describing emulsion stability where equilibrium forces are dominate.

Discussion

When the pressure is on the order of the Laplace pressure of the droplet, the droplets flatten. It is regardless of the contribution to the pressure from either equilibrium surface forces or hydrodynamic drainage. The pressure changes as a function of radii. The radial pressure is positive for small radii and it is negative for larger radii. This is because of the combination of pressure with different length scales from a positive equilibrium surface force at the interfacial separations and the negative hydrodynamic drainage pressure. This paper demonstrates how much the interactions for dynamic droplets are complicated compared to the interactions of static droplets of the same size.