  Controlled crystallization, melting and vitrification are important
fundamental processes in nature and technology. However, the microscopic
details of these fundamental phenomena still lack understanding, in particular
how the cooling rate and presence of a wall influence the crystal nucleation
and glass formation. Thermoresponsive microgels provide the possibility to
study phase transitions on a single-particle level, owing to the ability to
tune the particle size with temperature. In this study, we employ composite
microgels consisting of a hard core and a crosslinked poly(N-isopropyl
acrylamide-co-methacrylic acid) shell to study the crystallization of dense
suspensions of soft colloids near a wall using confocal microscopy. We
investigate the effect of cooling rate on the fluid-to-solid transition close
to the sample wall. The structures formed during cooling range from glassy in
case of a rapid temperature quench, to crystalline when a slow cooling rate is
used. Detailed analysis of the final structure reveals that the cooling rate
also sets the degree of alignment of the crystal domains with the wall as a
result of a balance between homogeneous and heterogenous crystal nucleation.
The results presented here yield valuable insight into the microscopic details
of temperature-controlled crystallization near a wall. This understanding will
help pave the way towards optimal crystallization conditions for microgel
applications.
