Going with the light: all-optical measurements of thermomechanical properties of single polymeric nanospheres

Dramatic changes in the rheological response of polymers can arise from temperature variations, thus significantly affecting their use in applications involving deformation and flow. Furthermore, polymers can exhibit unusual rheological properties in confinement. Here, we present an all-optical methodology to study the deformation and flow behavior of single polystyrene nanospheres. In our approach, a high-power density focused laser beam is utilized to controllably apply a thermocapillary shear on a single nanosphere, while another low-power density wide-field imaging beam is employed for in-situ monitoring of the nanosphere’s response. Deformation and flow of polystyrene nanospheres were observed in response to the thermocapillary shear and quantified for nanospheres at different temperatures and of different molecular weights. The technique opens an avenue to determine the thermomechanical properties of single polymeric nanospheres and motivates a simple strategy to control nanosphere motion on solid substrates.