Crystallization Mechanisms
in Convective Particle
Assembly
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Abstract
Colloidal particles are continuously assembled into crystalline
particle coatings using convective fluid flows. Assembly takes place
inside a meniscus on a wetting reservoir. The shape of the meniscus
defines the profile of the convective flow and the motion of the particles.
We use optical interference microscopy, particle image velocimetry,
and particle tracking to analyze the particles’ trajectory
from the liquid reservoir to the film growth front and inside the
deposited film as a function of temperature. Our results indicate
a transition from assembly at a static film growth front at high deposition
temperatures to assembly in a precursor film with high particle mobility
at low deposition temperatures. A simple model that compares the convective
drag on the particles to the thermal agitation explains this behavior.
Convective assembly mechanisms exhibit a pronounced temperature dependency
and require a temperature that provides sufficient evaporation. Capillary
mechanisms are nearly temperature independent and govern assembly
at lower temperatures. The model fits the experimental data with temperature
and particle size as variable parameters and allows prediction of
the transition temperatures. While the two mechanisms are markedly
different, dried particle films from both assembly regimes exhibit
hexagonal particle packings. We show that films assembled by convective
mechanisms exhibit greater regularity than those assembled by capillary
mechanisms