BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Freezing colloidal suspensions: Periodic ice-banding - Anthony And
 erson (Damtp)
DTSTART:20121005T150000Z
DTEND:20121005T160000Z
UID:TALK39808@talks.cam.ac.uk
CONTACT:Dr Ed Brambley
DESCRIPTION:Colloidal suspensions do not freeze uniformly --- rather\, the
  frozen phase (ice) becomes segregated\, trapping bulk regions of colloid 
 within. 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. Distinct modes of periodic ice-banding were observed depending on the f
 reezing rate.  For each mode of banding\, the distance between successive 
 layers of ice decreases with increasing freezing rate. At slow freezing ra
 tes\, the rejected particles are found to be irreversibly aggregated by cr
 yosuction forces to form a close-packed cohesive layer which is visibly da
 rker than the surrounding suspension. The temperature in this aggregated l
 ayer is depressed below the bulk freezing point by more than 2 degrees Cel
 sius before the ice lenses are encountered\; moreover\, this undercooled r
 egion appears as a light-colored layer. The magnitude of the undercooling 
 and the color change in this region both suggest the presence of pore ice 
 and the formation of a frozen fringe. As well as resolving discrepancies i
 n our experimental observations\, the frozen-fringe hypothesis also leads 
 to a theoretical framework for periodic ice-lensing. I shall describe a mo
 del for ice lensing in qualitative agreement with our experimental observa
 tions.
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberforce Road\, Cambr
 idge
END:VEVENT
END:VCALENDAR