title: Melting and crystallization of colloidal hard-sphere suspensions under shear.
abstract: Shear-induced melting and crystallization were investigated by confocal microscopy in concentrated colloidal suspensions of hard-sphere-like particles. Both silica and polymethylmethacrylate suspensions were sheared with a constant rate in either a countertranslating parallel plate shear cell or a counterrotating cone-plate shear cell. These instruments make it possible to track particles undergoing shear for extended periods of time in a plane of zero velocity. Although on large scales, the flow profile deviated from linearity, the crystal flowed in an aligned sliding layer structure at low shear rates. Higher shear rates caused the crystal to shear melt, but, contrary to expectations, the transition was not sudden. Instead, although the overall order decreased with shear rate, this was due to an increase in the nucleation of localized domains that temporarily lost and regained their ordered structure. Even at shear rates that were considered to have melted the crystal as a whole, ordered regions kept showing up at times, giving rise to very large fluctuations in 2D bond-orientational order parameters. Low shear rates induced initially disordered suspensions to crystallize. This time, the order parameter increased gradually in time without large fluctuations, indicating that shear-induced crystallization of hard spheres does not proceed via a nucleation and growth mechanism. We conclude that the dynamics of melting and crystallization under shear differ dramatically from their counterparts in quiescent suspensions.
authors.affiliation: Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CC Utrecht, The Netherlands.
authors.forename: Yu Ling
authors.initials: YL
authors.lastname: Wu
authors.forename: Didi
authors.initials: D
authors.lastname: Derks
authors.forename: Alfons
authors.initials: A
authors.lastname: van Blaaderen
authors.forename: Arnout
authors.initials: A
authors.lastname: Imhof
mesh_terms: D003102:Colloids / Q000737:chemistry; D003460:Crystallization; D005615:Freezing; D007700:Kinetics; D018613:Microscopy, Confocal / Q000379:methods*; D010316:Particle Size; D044367:Phase Transition; D019904:Polymethyl Methacrylate / Q000737:chemistry*; D033081:Shear Strength; D012822:Silicon Dioxide / Q000737:chemistry*; D013314:Stress, Mechanical; D013535:Suspensions / Q000737:chemistry*; D013816:Thermodynamics; D014783:Viscosity
chemical_list: D003102:Colloids; D013535:Suspensions; D012822:Silicon Dioxide; D019904:Polymethyl Methacrylate