Overcoming Kinetic Bottlenecks of Colloidal Self-Assembly*

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, 16023–16028.
[2] Swan, J. W.; Bauer, J. L.; Liu, Y.; Furst, E. M. Soft Matter 2014, 10, 1102–1109.