Professor David Haviland
Department of Applied Physics, Nanostructure Physics
Royal Institute of Technology (KTH), Stockholm, Sweden

Nano mechanical Response at Soft Matter Interfaces

The nano-mechanical response of a soft matter interface can be probed with the Atomic Force Microscope (AFM), where tip-surface interaction is typically understood in terms of quasi-static elastic forces derivable from a theory of contact mechanics.    Recent advances in the field dynamic AFM have led to quantitative and well calibrated measurement of tip-surface interaction with unprecedented speed and resolution.   These dynamic methods allow one to probe the viscous nature of interfaces between materials that can flow.    We propose a rheological model of tip-surface interaction which well describes experimental data.  The talk will explain how nonlinear dynamics of an oscillating cantilever near a high Q resonance is used to extract both conservative and dissipative forces acting on the tip.  From analysis of these ‘force quadratures’ we can  understand and visualize complex dynamics that results from the independent motion of the AFM tip and the soft interface. 

Apparent viscous and elastic stiffness of the surface of a Polymer blend of PMMA and PDMS.  The stifnesses are derived from a measurement of the amplitude dependence of the force quadratures FI and FQ.  The dependence on the setpoint, S, is explained using dynamic interaction model which allows for the independent motion of the tip and surface.