Título: Directing colloidal self-assembly

Procedencia: Department of Chemical and Biomolecular Engineering
Center for Molecular and Engineering Thermodynamics
University of Delaware

Colloidal and nanoparticle self-assembly is a promising approach to the
nanomanufacture of advanced functional materials capable of controlling
the transport of heat, light, and chemical species. But while
thermodynamics dictates the structures that form by self-assembly, the
kinetics of colloidal assembly are often trapped into arrested, non-
equilibrium states.

In this talk, I will discuss the use of directing electric and magnetic
fields to circumvent kinetic bottle necks during colloidal self-assembly.
A useful model system has been suspensions of superparamagnetic colloids.
In a strong, steady magnetic field, paramagnetic colloids form system-
spanning, kinetically arrested networks similar to a gel. From this state,
it is possible to phase separate and condense the suspension by toggling
the external field [1]. In its evolution towards the equilibrium state,
the suspension undergoes a Rayleigh-Plateau instability for a range of
field strengths and toggle frequencies [2]. The particles initially chain
together to form a percolated network that coarsens diffusively. With
time, the surface of the growing domains in the network become unstable.
The amplitude of the waves eventually reaches a critical value and the
columns pinch off and condense into ellipsoidal structures.

A key advantage of directed self-assembly in toggled fields is the
relatively large range of field-strengths, analogous to effective
temperatures, that lead to phase separation. Our results demonstrate how
kinetic barriers to a colloidal phase transition are subverted through
measured, periodic variation of driving forces while retaining the
strengths of a “bottom-up” self-assembly process.

[1] Swan, J. W. et al., Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 16023–
[2] Swan, J. W.; Bauer, J. L.; Liu, Y.; Furst, E. M. Soft Matter 2014, 10,