Probing the critical behavior of colloidal interfaces by gravity

Abstract

We present a study of the interface between fluid–fluid phase separated colloid–polymer mixtures of identical composition but with varying suspension height. The significance of the sedimentation gradient present in the suspension is controlled by the ratio between the suspension height and the gravitational length of the colloids. We demonstrate that increasing the suspension height, and thus the importance of gravity leads to a systematic roughening of the gas–liquid interface as if one approaches the critical point. By carefully tuning the system height, the suspension can be brought arbitrarily close to criticality, irrespective of the overall composition of colloid and polymer. Our findings are based on measurements of the interfacial tension and capillary wave properties and supported by predictions from a simple density functional theory.