Aerosol surface tension and viscosity
Understanding the kinetics of transport of species across the interfacial boundary and within the bulk of a particle is dependent on knowing the surface composition and the bulk viscosity. The surface composition, as inferred from the surface tension, also plays an important role in the equilibrium size of small aerosol droplets (<150 nm diameter) and the activation of cloud condensation nuclei to form cloud droplets. The bulk viscosity governs the timescale for a colaesced particle to relax to a sphere.
In this project, we are measuring the timescale for a coalescing pair of particles to relax to form a single sphere. For low viscosity particles, the relaxation can occur extremely fast on a timescale of less than 100 microseconds. For such coalescing pairs, the surface tension of the particle can be determined. For more viscosus particles, the relaxation timescale can take many hours or even days, and the time can then be used to infer the viscosity of the coalescing pair. Viscosities can be measured by this technique over a range of 12 orders of magnitude.
For examples, see:
R.M. Power and J.P. Reid, 'Probing the Micro-Rheological Properties of Aerosol Particles using Optical Tweezers', Reports on Progress in Physics 77 (2014) 074601 - pdf
R. M. Power, S. H. Simpson, J. P. Reid and A. J. Hudson, 'The Transition from Liquid to Solid-Like Behaviour in Ultrahigh Viscosity Aerosol Particles', Chemical Science 4(6) (2013) 2597 - 2604 - pdf