An AFM study of the deformation and nanorheology of cross-linked PDMS droplets

Interaction forces between a spherical silica probe and a cross-linked poly(dimethylsiloxane) (PDMS) colloidal droplet have been determined by atomic force microscopy (AFM), On approach of the surfaces, as a result of deformation of the PDMS, the repulsive force increases much less rapidly than expected for electrical double layer interaction of rigid particles. The amount of deformation is dependent on the loading force and the drive velocity and reflects the nanorheological response of the PDMS. On retraction of the surfaces, force curve hysteresis is observed and is dependent on the rate of approach/retraction. Hysteresis is due to the viscoelastic response of the PDMS droplet and can be described by theory (Attard, P. Phys. Rev. E 2001. 63, 061604). The moduli and the characteristic relaxation time of the PDMS colloid have been determined from the force measurements and offer a novel description of the nanorheological properties.