Microrheology of a sticking transition

Abstract

The phenomenon of sticking of one object to another, which drastically reduces their relative motion, is ubiquitous in nature. We have studied the sticking process of a colloid, suspended in a fluid medium by an optical tweezer, to a rigid substrate. The evolution of the frictional coupling between the two as a function of their separation is detected by the diffusivity of the particle and also by its phase-sensitive response to an in-plane external oscillatory drive applied to the substrate. On contact, the coupling changes abruptly from viscous to elastic for a rigid silica particle, whereas it evolves slowly with time, similar to ageing in glassy systems, for a soft and deformable polystyrene particle. Depending on the relative strengths of the particle–substrate interaction, the tweezer potential and the external drive, three regimes of dynamics—stuck, ageing and non-stuck—are observed in the dynamical phase diagram. The tendency of small objects to stick together as they come into contact is a commonly observed phenomenon. Yet the interactions that govern this behaviour can be complex. A systematic study of the variation in the force between a particle and a solid surface as they are brought together finds many parallels with the characteristics of glassy and granular systems.