Liquid foams are a familiar part of everyday life from beer and frothed milk to bubble baths; they also play important roles in industrial processes like enhanced oil recovery and the production of lightweight foam packaging, cushions, and insulation. Here we report the discovery of a new class of foams, obtained by frothing a suspension of colloidal particles in the presence of a small amount of an immiscible secondary liquid. A unique aspect of these new foams, which we call capillary foams, is that the suspended particles mediate spreading of the minority liquid around the gas bubbles. The resulting mixed particle/liquid coating can stabilize bubbles against coalescence even when the particles alone cannot. Moreover, the coated bubbles become immobilized in a network of excess particles in the liquid bulk that connected via bridges of the minority liquid and are held together by capillary forces, much like the water-bridged grains of wet sand in a sand castle. We demonstrate that the formation of capillary foams is a general phenomenon by exploring a diverse set of particle/liquid combinations and by rationalizing the results with a simple free energy model. In addition to their many possible applications as liquid foams, capillary foams can serve as precursors for hierarchically-structured solids with porosity on different length scales and with significant application potential in their own right. We conclude by discussing some of the functionalization options arising from the presence of the minority liquid.

Capillary foams: A new pathway toward functional porous materials

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.