Rolling and ageing in T-ramp soft adhesion

摘要

Immediately before adsorption to a horizontal substrate, sinkingpolymer-coated colloids can undergo a complex sequence of landing, jumping,crawling and rolling events. Using video tracking we studied the soft adhesionto a horizontal flat plate of micron-size colloids coated by a controlled molarfraction $f$ of the polymer PLL-g-PNIPAM which is temperature sensitive. Weramp the temperature from below to above $T_c=32\pm 1^{\circ}$C, at which thePNIPAM polymer undergoes a transition triggering attractive interaction betweenmicroparticles and surface. The adsorption rate, the effective in-plane ($x-y$)diffusion constant and the average residence time distribution over $z$ wereextracted from the Brownian motion records during last seconds beforeimmobilisation. Experimental data are understood within a rate-equations basedmodel that includes ageing effects and includes three populations: theuntethered, the rolling and the arrested colloids. We show that pre-adsorptiondynamics casts analyze a characteristic scaling function $\alpha (f)$proportional to the number of available PNIPAM patches met by soft contactduring Brownian rolling. In particular, the increase of in-plane diffusivitywith increasing $f$ is understood: the stickiest particles have the shortestrolling regime prior to arrest, so that their motion is dominated by untetheredphase.