122 research outputs found
Kinetic Self-Assembly of Metals on Co-polymer Templates
In this work we seek to understand some of the fundamental processes that
govern self-assembly at the nanoscale in the context of the formation of
metallic structures on patterned co-polymer templates. To this end we focus on
the experiments conducted by Lopes {\it et al} (Nature, 2001), where
morphologies resulting from the evaporation-deposition of different metals on
PS-b-PMMA phase separated templates were studied. We show that the different
morphologies obtained can be understood in terms of the relative importance of
the energetics and kinetics. We then focus on a particular morphology: micron
long wire-like states obtained by the evaporation-deposition of silver on the
template. We show the existence of ``non-trivial'' correlations between
adjacent wires that can be understood based on a purely kinetic mechanism. We
also compare these correlations quantitatively to those obtained from
simulations done with the relevant experimental parameters and find them in
good agreement.Comment: 18 pages, 18 figures, submitted to Phys. Rev.
Active matter clusters at interfaces
Collective and directed motility or swarming is an emergent phenomenon
displayed by many self-organized assemblies of active biological matter such as
clusters of embryonic cells during tissue development, cancerous cells during
tumor formation and metastasis, colonies of bacteria in a biofilm, or even
flocks of birds and schools of fish at the macro-scale. Such clusters typically
encounter very heterogeneous environments. What happens when a cluster
encounters an interface between two different environments has implications for
its function and fate. Here we study this problem by using a mathematical model
of a cluster that treats it as a single cohesive unit that moves in two
dimensions by exerting a force/torque per unit area whose magnitude depends on
the nature of the local environment. We find that low speed (overdamped)
clusters encountering an interface with a moderate difference in properties can
lead to refraction or even total internal reflection of the cluster. For large
speeds (underdamped), where inertia dominates, the clusters show more complex
behaviors crossing the interface multiple times and deviating from the
predictable refraction and reflection for the low velocity clusters. We then
present an extreme limit of the model in the absense of rotational damping
where clusters can become stuck spiraling along the interface or move in large
circular trajectories after leaving the interface. Our results show a wide
range of behaviors that occur when collectively moving active biological matter
moves across interfaces and these insights can be used to control motion by
patterning environments.Comment: 15 pages, 7 figure
Statistically Locked-in Transport Through Periodic Potential Landscapes
Classical particles driven through periodically modulated potential energy
landscapes are predicted to follow a Devil's staircase hierarchy of
commensurate trajectories depending on the orientation of the driving force.
Recent experiments on colloidal spheres flowing through arrays of optical traps
do indeed reveal such a hierarchy,but not with the predicted structure. The
microscopic trajectories, moreover,appear to be random, with commensurability
emerging only in a statistical sense. We introduce an idealized model for
periodically modulated transport in the presence of randomness that captures
both the structure and statistics of such statistically locked-in states.Comment: REVTeX with EPS figures, 4 pages, 4 figure
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