Title: A particle walks into an interface...

Author (Invited): Vinothan Manoharan, SEAS and Dept of Physics, Harvard University

Abstract:

Small solid particles have a strong tendency to stick to liquid interfaces, a consequence of surface tension. The phenomenon is important in a wide range of processes ranging from wastewater purification to the making of nanostructured materials. Our understanding of these processes is largely based on models that assume the particles have reached an equilibrium contact angle with the interface, but there are few experiments that measure how long a particle takes to reach equilibrium. We use holographic microscopy to image a micron-sized solid particle as it approaches and binds to a liquid interface. Sometimes we see particles stick to but not penetrate the interface. When they do penetrate, they take a long time to relax -- 100 seconds or more. Furthermore, the relaxation of the particles is logarithmic in time, suggesting that they may take weeks or even months to reach equilibrium. This behavior can be understood in terms of a dynamic wetting mechanism involving thermally-activated hopping of the contact line over surface defects. The results call into question the assumption of equilibrium and may shed some light on the mysterious interactions observed between small particles adsorbed at interfaces. Experimental work was performed by David M. Kaz and Ryan McGorty (Harvard, Manoharan lab), and theoretical work by Madhav Mani (KITP) and Michael P. Brenner (Harvard). This work was funded by NSF grant no. CBET-0747625.