Translational diffusion of tracer molecules in confined water.*

Understanding the structure and flow of water confined between surfaces is vital in lubrication, protein folding, nano-fluidics, etc. Current methods and experiments to measure nano-confined water's dissipative properties are not conclusive enough to tell whether it deviates from its bulk properties[1,2]. This work will introduce an independent optical method. Our newly developed instrument and Monte-Carlo based analyzing technique allows us to study tracer diffusion in nano-confined water with controlled confinement gaps[3]. We can measure the translational diffusion-time-scales of a fluorescent-tracer molecule and the population-fraction for each time-scale. Our preliminary experiments indicate that even in 1nm gap, the bulk-like dynamics prevail. An additional coexisting slow-moving tracer population is observed depending on the electrostatic interactions of confining interfaces with tracers. The gap dependence studies showed such slow dynamics are localized near to the confining interfaces. More control experiments are being done to understand these results and single out the effect of confinement on water's dissipative properties.
[1] Phys. Rev. Lett. 100(2008), 106102
[2] Nature 413(2001), 51
[3] Rev. Sci. Instrum. 91(2020), 013702