D. ZANCHI – 06 Mars

Immediately before adsorption to a horizontal substrate, sinking soft colloids can undergo a complex sequence of landing, jumping, crawling and rolling events. Using video tracking we studied the soft adhesion of micron-size colloids  coated by polymer PNIPAM which is temperature sensitive. It allows for sticking of colloids to the surface when the temperature is above Tc = 32 ◦C. In order to capture the very final events  before immobilization of colloids the T-ramp protocol was designed : the particles suspension is injected in the cell at room  temperature, the temperature is increased at 10◦ C/min up to 38◦ C>Tc , and kept constant until the end of the acquisition. Attraction between beads and the flat plate is thereby triggered by crossing the critical temperature Tc = 32◦C. 3D beads motion is observed by slightly defocus microscopy in parallel illumination decorating bead image with interference rings observed with video camera. Particles are tracked in real time using a PICOTWIST apparatus and PICOUEYE software. Analysis of the tracking records indicates the Brownian rolling is the most relevant phenomenon before the immobilization. The fundamental differences between Brownian rolling and Brownian jumping are discussed.