  Microfluidic technology has revolutionized the control of flows at small
scales giving rise to new possibilities for assembling complex structures on
the microscale. We analyze different possible algorithms for assembling
arbitrary structures, and demonstrate that a sequential assembly algorithm can
manufacture arbitrary 3D structures from identical constituents. We illustrate
the algorithm by showing that a modified Hele-Shaw cell with 7 controlled
flowrates can be designed to construct the entire English alphabet from
particles that irreversibly stick to each other.
