Crystallization kinetics of polydisperse colloidal hard spheres: Experimental evidence for local fractionation

Abstract

We present the crystallization kinetics for two polydisperse hard-sphere particle stocks with differing particle size distributions. One of the latexes had a relatively symmetrical distribution, the other had a more polydisperse distribution, which was highly skewed to smaller sizes. The emerging Bragg reflections from the crystallizing samples were measured using a technique that provides improved statistical averaging over our previous methods. It was observed that, for the more polydisperse particles, the onset of nucleation was delayed by up to an order of magnitude in reduced time, and displayed qualitatively different growth behavior compared to the particles with the more symmetric size distribution. Based on these measurements and time lapse photographs, we propose a growth mechanism whereby crystallization occurs in conjunction with a local fractionation process near the crystal-fluid interface, which significantly alters the kinetics of crystallite nucleation and growth. This fractionation effect becomes more significant as polydispersity or skewness increases.

Hiding in Plain View: Colloidal Self-Assembly from Polydisperse Populations

“…Experimentally, the best prior evidence of colloidal fractionation is the work of van Megen and collaborators [10,55,56], who invoke it to explain the nucleation processes of colloidal crystals near a terminal polydispersity. The coexistence of multiple solid phases is also known in cases of low-dimensional systems such as platelets [57] or particles confined to a plane [58].…”

Measuring local volume fraction, long-wavelength correlations, and fractionation in a phase-separating polydisperse fluid

“…Fractionation of polydispersity may be very important where crystals nucleate from a metastable liquid 41 ; depending on the shape of the parent distribution and the interaction potential, fractionation can reduce or increase the polydispersity of the liquid, thus promoting or suppressing subsequent crystal nucleation. Where crystals are involved, even small changes in polydispersity can have strong effects 18,25 . The results show emphatically that complex fractionation is involved right from the beginning of polydisperse phase separation, local composition relaxing alongside local density rather than long after it.…”

“…The kinetics by which polydisperse systems approach equilibrium are, however, just as complex and far less well understood 2,5,[16][17][18][19][20][21] . Fractionation (partitioning between phases) of the polydisperse property is a key aspect of polydisperse phase separation and has typically been measured either in equilibrium simulations 13 or in experiment after long equilibration time 9,22,23 .…”

Nucleation and growth of micellar polycrystals under time-dependent volume fraction conditions

“…in protein crystallization [1]), and as a means to understand crystallization in atomic and molecular systems, for which colloids are often regarded as a model system [2] whose characteristic time and length scales are more readily accessible. Thanks to advanced light scattering techniques and scanning confocal microscopy, experiments have provided unprecedented information on the nucleation and growth of crystals, including the volume fraction dependent nucleation rate or the structure of the nuclei [3][4][5][6][7][8][9][10][11]. Most experiments have focussed on hard-sphere or charged colloids, although more recently more complex systems, such as thermosensitive colloidal microgels or mixtures of colloids and polymers have emerged [12][13][14][15][16].…”