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SUMMARY:Fingers and Fractures: Instabilities in Viscoplastic Fluid Films -
  Thomasina Ball\, U. Warwick
DTSTART:20211108T130000Z
DTEND:20211108T140000Z
UID:TALK163882@talks.cam.ac.uk
CONTACT:Prof. Jerome Neufeld
DESCRIPTION:The study of gravity currents has long been of interest due to
  their prevalence in industry and in nature\, one such example being the s
 preading of viscoplastic (yield-stress) fluid films. When a viscoplastic f
 luid is extruded onto a flat plate\, the resulting gravity current expands
  axisymmetrically when the surface is dry and rough. For such a case\, the
  flow above the non-slip surface is dominated by shear stresses. In this t
 alk\, I will discuss two instabilities that arise when the two conditions 
 of a dry and rough surface are relaxed.\n\n1) Firstly\, when the underlyin
 g surface is replaced by a free-slip surface\, the fluid film spreads with
  little traction\, the main resistance to flow now stemming from the exten
 sional stresses of the material. Experiments with radially spreading slidi
 ng viscous films have suggested these flows are stable towards non-axisymm
 etric disturbances. However\, recent analysis of the stability of radially
  expanding two-dimensional flow of a shear-thinning power-law fluid sugges
 ts the presence of an extensional flow instability. We extend this analysi
 s to three-dimensional spreading films with a viscoplastic rheology in the
  thin-film limit\, confirming the existence of instability if the fluid ha
 s a yield stress or is sufficiently shear-thinning.\n\n2) Second\, when ex
 periments are performed with a viscoplastic fluid (Carbopol) extruded onto
  a flat plate wet by a thin coating of water\, fractures appear to create 
 a distinctive flower-like pattern. However\, by performing a number of var
 iations on the experiments\, we demonstrate that the shear-thinning extens
 ional-flow instability above is not responsible. Instead\, further investi
 gation suggests that these patterns arise from the solid-mechanical fractu
 ring of the complex fluid\, exacerbated by the presence at the surface of 
 the solvent making up the fluid (i.e. water) which reduces the fracture to
 ughness.
LOCATION:MR5\, CMS
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