Non-equilibrium diffusion in complex environments – NoDiCE

Diffusion is at the root of a huge number of microscopic structural and transport processes. Furthermore, many nano-technological and macromolecular-scale biological contexts involve energy injection at interfaces, notably, under dissipative shear flows and chemical reactivity. During the last century, the Einstein and Boltzmann theories were successful in describing equilibrated systems, but they are not applicable under continuous energy input. These microscopic, non-equilibrium situations are far from exceptional, but difficult to observe with precision and describe with statistical theory. NoDiCE’s purpose is thus to design model experiments placing functional colloidal particles in complex, non-equilibrium envi- ronments with quantitative assessment based on delicate and simultaneous observations of particles’ advective and diffusive motions, and their precise spatial organization. Our observations exploit evanescent-wave microscopy to observe fluorescent nanoparticles driven out-of-equilibrium in microfluidic devices, providing 3D nanometric precision at kHz temporal scales. Fluorescence correlation and atomic force spectroscopies complement these measurements, providing DC to MHz bandwidth and similar spatial precision. We will demonstrate how non-trivial interactions of hydrodynamic origin with a boundary reveal novel nanoscale self- organization strategies, requiring a hydrodynamic component in the statistical theory. We then enter a recent scientific debate about the extent to which Brownian motion can be impacted by reactive environments and by catalysis. Lastly, we combine these elements with particles that present specific binding to surfaces and are driven out of equilibrium with an externally controlled flow. The insights provided by our model experiments open a path for the developments of breakthrough self-assembly strategies, reaction-pathway analysis based on particle dynamics, and possible next-generation diagnostic tools.