(722d) Electric-Field Driven Assembly of Polarizable Colloids Confined to a Surface

Electric-field
driven assembly of polarizable
colloids confined to a surface

Joseph
Maestas^1*, Ning Wu², David T. Wu^1,2

1. Department
of Chemistry, Colorado School of Mines, Golden CO, US

2. Department
of Chemical & Biological Engineering, Colorado School of Mines, Golden CO,
US

jomaesta@mines.edu, ningwu@mines.edu, dwu@mines.edu, (303) 384 2066

Abstract

The
assembly of colloids into unique structures has been the subject of intense research
for several decades, as colloids are a convenient system for studying self and
directed assembly, and the resulting structures are expected to further the
development of new functional materials, such as sensors, electronics, and
metamaterials. One way of controlling the particle interactions that direct
colloidal assembly is through the use of an externally applied electric-field.
Recently, this method has been used to assemble a wide assortment of elaborate
phases. [1-4] However, while electric-field directed assembly
efficiently orders colloids into complex structures, the mechanisms governing
the interparticle interactions are not well understood. As such, we have
turned to simulation and theory in order to gain insight about the assembly
process. Monte Carlo simulations were conducted to model the assembly of
colloids confined near the surface of an electrode under an applied
electric-field. Particle pair interactions were modeled using a Stockmayer
potential to capture the dipole-dipole interactions induced by the
electric-field, as well as the steric repulsion between the spherical colloids.
By carefully tuning particle concentration, field strength, temperature, and fraction
of particles in direct contact with the surface, we observed a wide array of
unique structures. Isothermal phase diagrams were constructed as a function of
dipolar strength and concentration for different ratios of particles in contact
with the electrode surface. We found that the phase boundaries obtained from
our simulations were in excellent agreement with an analytical model for the
free energies of several idealized lattices. Many of the simulated results had
been observed experimentally, including a honeycomb-like network, and a square
and triangular bilayer. Moreover, several new phases identified in simulation
were later found experimentally, such as networks of zig-zag stripes and
rectangular bands. A comparison between the simulation and experimental phase
behavior as a function of the dipole strength and concentration are in good
qualitative agreement. Therefore, the theoretical and simulation results gave
us insight into experimentally observed phase behavior and provided guidance
for identifying structurally rich new phases accessible for confined colloids
under applied fields.

References

[1] Ma, F.; Wu, D. T.; Wu, N. JACS.
2013, 135, 7839.

[2] Ma, F.; Wang, S.; Wu, D. T.; Wu,
N. PNAS. 2015, 112(20), 6307.

[3] Heatley, K. L.; Ma, F.; Wu, N. Soft
Matter. 2017, 13, 436.

[4] Gong, J.; Wu, N. Langmuir. 2017,
33, 5769.