Tunable and switchable soft adsorption of polymer-coated microparticles on a flat substrate
Résumé
Soft adhesion to a horizontal flat substrate of micron-sized colloids coated by acontrolled molar fraction f of PLL-g-PNIPAM is tuned and triggered by controllingthe temperature of the sample. This mechanism is possible becausePNIPAM is a thermo-responsive polymer, which undergoes a phase transitionat a LCST (lower critical solution temperature) Tc = 32 +- 1°C. In order tocapture the very final events before the immobilization of colloids the T-rampprotocol 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, andkept constant until the end of the acquisition. Attraction between beads andthe flat substrate is thereby triggered when crossing the critical temperatureTc. Ascending and descending ramp experiments are performed in order to accessboth adsorption and desorption kinetics. 3-D motion of beads is real-timetracked using slightly defocused microscopy in parallel illumination. We usetrack records to have access to pre-adsorption dffusion coeffcients and to characterizeadsorption and desorption dynamics. Present results corroborate thatadsorption is controlled by PNIPAM doping, and indeed dominated by rollingand memory (aging) effect on the contact domain. On the contrary, the desorption kinetics is independent of doping. Moreover, the sharpness (in T) ofthe PNIPAM transition is quantified in a set of experiments at different rampspeeds. We show that the transition smearing can reach up to +-1°C for higherPNIPAM coverage, while it is not detectable for the lowest ones. The combinationof all these observations paves the way to practical applications. Asan example, we will discuss a soft adhesion-based method to sort and separatecolloids in microfluidic channels.
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