Colloidal Particles Bind Slowly to Liquid Interfaces

How does a colloidal particle bind to a liquid interface?  The phenomenon is important for many industrial and materials applications.  There are decades-old static models that show that surface tension drives the binding, so that for spherical particles the equilibrium configuration obeys Young's equation.  But there are few experiments that measure how long it takes for the particle to reach equilibrium.  We use holographic microscopy to image a micron-sized 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 thermally-activated hopping of the contact line over surface defects.  The results make us question whether equilibrium models are relevant for many particle-interface systems.