# Limited coalescence

Original entry: Sujit S. Datta, APPHY 225, Fall 2009.

## Reference

S. Arditty, C. P. Whitby, B. P. Binks, V. Schmitt, F. Leal-Calderon, *Eur. Phys. J. E* **11,** 273 (2003).

## Keywords

emulsion, Pickering emulsion, interface, coalescence

## Key Points

Unlike "conventional" surfactant-stabilized emulsions (dispersions of droplets of one immiscible fluid in another), "Pickering" emulsion droplets are stabilized by solid particles (typically much smaller than the droplet size) adsorbed at the droplet interface. The particle wetting properties determine the bulk properties of the emulsion in two crucial ways:

- The continuous phase of the emulsion tends to be the phase that preferentially wets the particles. This can be understood in analogy to surfactants - a particle at the interface between the two fluid phases will sit deeper in the wetting phase, and the effective "packing shape" (Israelachvili, page 381) will be similar to a cone, with tapered end inside the non-wetting phase. That is to say, the majority of the particle volume prefers to be immersed in the wetting phase.

- Unless particles very significantly prefer one phase over the other, they will tend to be irreversibly adsorbed at the interface between the two fluids. This potential energy well is due to the reduced bare surface area in contact between the two fluids, and is given by <math>\pi R^{2} \gamma_{12}(1+cos\theta)^{2}</math>, where R is the particle radius, <math>\gamma</math> is the interfacial tension between the two fluids, and <math>\theta</math> is the contact angle at the particle surface. For contact angles between ~20-160 degrees, this is many times larger than kT.