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Fluctuating Free Surface Nanoflows

New research in a series of articles (image courtesy of Dr Sreehari Perumanath).

Our recent research has led to a flurry of articles aimed at improving our understanding of practically-relevant nanoflows with free surfaces.

Breakthroughs have come in discovering ‘what changes’ to canonical flows when they reach the nanoscale, where thermal fluctuations drive nanowaves on free surfaces and alter their stability properties. To understand these flows we use fluctuating hydrodynamics (think Navier-Stokes + noise) guided and validated by molecular simulations that act as virtual experiments.

The first article in this set, all from 2020, is on the dynamics of liquid nanothreads: fluctuation-driven instability and rupture and focuses on the breakup of liquid cylinders/jets, which appear different from macroscopic shapes (as first realised in a Science article in 2000). The inherently nonlinear dynamics required computations of a ‘noisy’ stochastic lubrication equation (SLE), derived from fluctuating hydrodynamics, that agrees well with MD, in contrast to its deterministic equivalent (LE, see images below). The SLE is around 5000 times faster than MD and enabled us to test similarity solutions proposed for the process and to uncover a number of open theoretical problems.

The second article concerns the behaviour, and in particular stability, of thin layers of liquids on solid surfaces, that is key to numerous technological processes (e.g. coatings). In nanoscale thin-film flows with thermal fluctuations and slip, we combined, for the first time, two key ingredients of nanoscale flows: thermal fluctuations and liquid-solid slip. SLEs were derived for both films on planar and annular films, enabling us to derive the properties of fluctuation-induced nanowaves and rationalise molecular simulations (see below).

The third piece of research is a collaboration between Warwick-Edinburgh-Bell Labs, with Bell interested in the coalescence-induced jumping of droplets from non-wettable surfaces as a method of thermal management, see [Enright et al 2014]. In particular, can nanodrops jump? In molecular physics of jumping nanodroplets we addressed this question and discovered that typically-neglected thermal fluctuations and kinetic gas effects become critical at these scales, with the former leading to stochasticity in jumping speeds (see below).

Notably, all this research has been led by PhD students who have been central to the research of our Nano-Engineered Flow Technologies EPSRC Programme Grant and have been a pleasure to work with. Namely, Yixin Zhang and Chengxi Zhao at Warwick, and Sreehari Perumanath in Edinburgh. All have now secured PDRA positions 🙂

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