Microfluidics relies on the increasingly dominant role of phase interfaces at smaller scales, that allow tiny volumes of fluid to be manipulated. However, at smaller, nanofluidic scales new physics appears that simultaneously creates remarkable new counter-intuitive flow behaviour and also renders the classical Navier-Stokes-Fourier paradigm impotent.
Thermal fluctuations (which create Brownian motion) in fluids become prevalent at nanofluidic scales and can drive thermal capillary waves on phase interfaces that make the free surface appear ‘rough’. These waves can radically alter the generation of instabilities and the modify classical singularities in drop dynamics.
Our approach has been to use molecular dynamics (MD) as a nanoscale ‘experiment’ and then develop theory based on fluctuating hydrodynamics, which introduces thermal noise into the Navier-Stokes equations, to describe the observed behaviours. With this theory we are able to describe scales where MD is computational intractable, e.g. >10nm or so.
This work has thus far enabled us to show how thermal fluctuations:
- modify the classic Rayleigh-Plateau theory of jet breakup
- create new regimes of drop coalescence
- drive the breakup of thin liquid films