Viscosity of suspensions and glass: turning power-law divergence into essential singularity

Abstract

Starting with an expression, due originally to Einstein, for the shear viscosity η (δΦ) of a liquid having a small fraction δΦ by volume of solid particulate matter sus-pended in it at random, an effective-medium viscosity η(Φ) for arbitrary Φ is derived, which is precisely of the Vogel–Fulcher form. An essential point of the derivation is the incorporation of the excluded-volume effect at each turn of the iteration Φn+1 = Φn + δΦ . The model is frankly mechanical, but applicable directly to soft matter like a dense suspension of microspheres in a liquid as a func-tion of the number density. Extension to a glass-forming supercooled liquid is plausible inasmuch as the latter may be modelled statistically as a mixture of rigid, solid-like regions (Φ) and floppy, liquid-like regions (1–Φ), for Φ increasing monotonically with supercooling.