Adv. Funct. Mater.http://doi.org/fz46bp (2012)

Credit: © 2012 WILEY

The stable state and self-assembly kinetics of an ensemble of colloidal particles depends on the interaction potential between the particles and on any competing external potential. Because the latter can be easily tuned, it is, in principle, possible to actively guide the self-assembly process towards the desired state. This is what Jaime Juárez and Michael Bevan have achieved. The researchers show that the assembly and disassembly of a charged colloidal monolayer in an electric field can be controlled by manipulating the electric potential (which competes with the colloids' chemical potential) so as to correct for the difference between actual and desired values of a crystallization order parameter (such as that for bond orientation). This is realized through a feedback control loop that involves measuring in real time the order parameter (by tracking particle positions with an optical microscope) and setting its desired value according to an a priori computed free-energy landscape model (constructed from measured histograms of the order parameter). This feedback-control approach should also be applicable to more complex colloidal systems.