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SUMMARY:Freezing Colloidal Suspensions: Periodic Ice-banding - Anthony And
 erson
DTSTART:20121011T103000Z
DTEND:20121011T113000Z
UID:TALK40442@talks.cam.ac.uk
CONTACT:Catherine Pearson
DESCRIPTION:Colloidal suspensions do not freeze uniformly -- rather\, the 
 frozen phase (ice) becomes segregated\, trapping bulk regions of colloid w
 ithin. Ice segregation in freezing colloidal suspensions is important in a
  number of applications and is a fascinating example of pattern formation\
 ; however\, the physical processes behind ice segregation in concentrated 
 suspensions are still poorly understood. To better understand these physic
 s\, controlled freezing experiments were performed with concentrated collo
 idal alumina dispersions using a directional solidification apparatus that
  provides independent control of the freezing rate and temperature gradien
 t. Two distinct steady-state modes of periodic ice-banding were observed i
 n the range of freezing rates examined. At slow freezing rates\, the ice s
 egregates from the suspension into crack-like ice lenses and there are dar
 k- and light-colored layers visible in the boundary-layer of rejected part
 icles directly ahead of the ice lenses. At fast freezing rates\, ice bands
  appear qualitatively different and there is no visible structure in the s
 uspension ahead of the segregated ice. There is a transition between these
  two modes of ice banding at intermediate freezing rates\, marked by disor
 dered of ice segregation and coinciding with the disappearance of the dark
  boundary-layer. For each mode of banding\, the bandwidth between successi
 ve layers of ice decreases with increasing freezing rate. The boundary-lay
 er structure formed by the rejected particles is described in detail\, giv
 en the apparent importance of this structure on the resultant mode of ice 
 segregation. At slow freezing rates\, the rejected particles are found to 
 be irreversibly aggregated by cryosuction forces to form a close-packed co
 hesive layer which is visibly darker than the surrounding suspension. The 
 temperature in this aggregated layer is depressed below the bulk freezing 
 point by more than 2C before the ice lenses are encountered\; moreover\, t
 his undercooled region appears as a light-colored layer. The magnitude of 
 the undercooling and the color change in this region both suggest the pres
 ence of pore ice and the formation of a frozen fringe. As well as resolvin
 g discrepancies in our experimental observations\, the frozen fringe hypot
 hesis also leads to an established theoretical framework for periodic ice-
 lensing. I'll describe a model for ice lensing in qualitative agreement wi
 th our experimental observations.\n\n
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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