Rolling and ageing in T-ramp soft adhesion

Abstract

Immediately before adsorption to a horizontal substrate, sinking polymer-coated colloids can undergo a complex sequence of landing, jumping, crawling and rolling events. Using video tracking we studied the soft adhesion to a horizontal flat plate of micron-size colloids coated by a controlled molar fraction $f$ of the polymer PLL-g-PNIPAM which is temperature sensitive. We ramp the temperature from below to above $T_c=32\pm 1^{\circ}$C, at which the PNIPAM polymer undergoes a transition triggering attractive interaction between microparticles and surface. The adsorption rate, the effective in-plane ($x-y$) diffusion constant and the average residence time distribution over $z$ were extracted from the Brownian motion records during last seconds before immobilisation. Experimental data are understood within a rate-equations based model that includes ageing effects and includes three populations: the untethered, the rolling and the arrested colloids. We show that pre-adsorption dynamics casts analyze a characteristic scaling function $\alpha (f)$ proportional to the number of available PNIPAM patches met by soft contact during Brownian rolling. In particular, the increase of in-plane diffusivity with increasing $f$ is understood: the stickiest particles have the shortest rolling regime prior to arrest, so that their motion is dominated by untethered phase.