Tunable and switchable soft adsorption of polymer-coated microparticles on a flat substrate

Abstract

Soft adhesion to a horizontal flat substrate of micron-sized colloids coated by a controlled molar fraction f of PLL-g-PNIPAM is tuned and triggered by controlling the temperature of the sample. This mechanism is possible because PNIPAM is a thermo-responsive polymer, which undergoes a phase transition at a LCST (lower critical solution temperature) Tc=32±1°C. In order to capture the very final events before the immobilization of colloids the T-ramp protocol is designed: the particles suspension is injected in cell at room temperature, the temperature is increased at a constant rate up to 38°C>Tc, and kept constant until the end of the acquisition. Attraction between beads and the flat substrate is thereby triggered when crossing the critical temperature Tc. Ascending and descending ramp experiments are performed in order to access both adsorption and desorption kinetics. 3-D motion of beads is real-time tracked using slightly defocused microscopy in parallel illumination. We use track records to have access to pre-adsorption diffusion coefficients and to characterize adsorption and desorption dynamics. Present results corroborate that adsorption is controlled by PNIPAM doping, and indeed dominated by rolling and memory (aging) effect on the contact domain. On the contrary, the desorption kinetics is independent of doping. Moreover, the sharpness (in T) of the PNIPAM transition is quantified in a set of experiments at different ramp speeds. We show that the transition smearing can reach up to ±1°C for higher PNIPAM coverage, while it is not detectable for the lowest ones. The combination of all these observations paves the way to practical applications. As an example, we discuss a soft adhesion-based method to sort and separate colloids in microfluidic channels.