December 2022
The interfacial layer breaker: a violation of Stokes' law in high-speed Atomic Force Microscope flows
Structured water near surfaces is important in non-classical crystallization, biomineralization, and restructuring of cellular membranes. In addition to equilibrium structures, studied by atomic force microscopy (AFM), high-speed AFM (H-S AFM) can now detect piconewton forces in microseconds. With increasing speeds and decreasing tip diameters, there is a danger that continuum water models will not hold, and molecular dynamics (MD) simulations would be needed for accurate predictions. MD simulations, however, can only evolve over tens of nanoseconds due to memory and computational efficiency/speed limitations, so new methods are needed to bridge the gap. Here we report a hybrid, multi-scale simulation method, which can bridge the size and timescale gaps to existing experiments. Structured water is studied between a moving silica AFM colloidal tip and a cleaved mica surface. The computational domain includes 1472766 atoms. To mimic the effect of long-range hydrodynamic forces occurring in water, when moving the AFM tip at speeds from to 30 m/s, a 5 × 10-7 2 hybrid multiscale method with local atomistic resolution is used, which serves as an effective open-domain boundary condition. The multiscale simulation is thus equivalent to using a macroscopically large computational domain with equilibrium boundary conditions. Quantification of the drag force shows breaking of continuum behaviour. Non-monotonic dependence on both tip speed and distance from the surface imply breaking of the hydration layer around the moving tip at timescales smaller than water cluster formation and strong water compressibility effects at the highest speeds.