Flow-Driven Assembly of Microcapsules into Three-Dimensional Towers

By harnessing biochemical
signaling and chemotaxis, unicellular
slime molds can aggregate on a surface to form a long, vertical stalk.
Few synthetic systems can self-organize into analogous structures
that emerge out of the plane. Through computational modeling, we devise
a mechanism for assembling tower-like structures using microcapsules
in solution as building blocks. In the simulations, chemicals diffusing
from a central patch on a surface produce a concentration gradient,
which generates a radially directed diffusioosmotic flow along the
surface toward the center. This toroidal roll of a fluid pulls the
microcapsules along the surface and lifts them above the patch. As
more capsules are drawn toward the patch, some units are pushed off
the surface but remain attached to the central microcapsule cluster.
The upward-directed flow then draws out the cluster into a tower-like
shape. The final three-dimensional (3D) structure depends on the flow
field, the attractive capsule–capsule and capsule–surface
interaction strengths, and the sedimentation force on the capsules.
By tuning these factors, we can change the height of the structures
that are produced. Moreover, by patterning the areas of the wall that
are attractive to the capsules, we can form multiple vertical strands
instead of a single tower. Our approach for flow-directed assembly
can permit the growth of reconfigurable, 3D structures from simple
subunits.