How PNIPAM Microgel Architecture Controls Pickering Foam Formation

The generation of foams using particle-like objects has gained momentum in the past years as such Pickering foams are relevant in many industrial applications. In this context, we investigate the foamability of microgels, which are soft polymeric particles which architecture can be finely tailored during synthesis. Herein, foams were produced by continuously bubbling air into dispersions of poly(N-isopropylacrylamide) (PNIPAM) microgels. The foaming ability of the microgel dispersions has been assessed by combining observations at both the macroscopic and the local scales. Increased microgel concentration and more pronounced core-shell structure lead to a higher foamability, smaller bubbles, and wetter foams. In contrast, microgel size variation has a small effect on the foam properties. These results are correlated to the adsorption kinetics assessed by pendant drop tensiometry. Indeed, faster adsorption kinetics are expected to promote larger surface coverage during bubble formation, which increases its stability against coalescence and the subsequent ability of the microgel dispersion to generate large volumes of foam. Such a hypothesis is confirmed by salt addition, which accelerates microgel adsorption and thus enhances foamability.