Shear-Induced Alignment of Janus Particle Lamellar Structures

Control over the
alignment of colloidal structures plays a crucial
role in advanced reconfigurable materials. In this work, we study
the alignment of Janus particle lamellar structures under shear flow
via Brownian dynamics simulations. Lamellar alignment (orientation
relative to flow direction) is measured as a function of the Péclet
number (Pe)the ratio of the viscous shear
to the Brownian forcesthe particle volume fraction, and the
strength of the anisotropic interaction potential made dimensionless
with thermal energy. Under conditions where lamellar structures are
formed, three orientation regimes are observed: (1) random orientation
for very small Pe, (2) parallel orientationlamellae
with their normals parallel to the direction of the velocity gradientfor
intermediate values of Pe, and (3) perpendicular
orientationlamellae with their normals parallel to the vorticity
directionfor large Pe. To understand the
alignment mechanism, we carry out a scaling analysis of competing
torques between a pair of particles in the lamellar structure. Our
results suggest that the change of parallel to perpendicular orientation
is independent of the particle volume fraction and is caused by the
hydrodynamic and Brownian torques on the particles overcoming the
torques resulting from the interparticle interactions. This initial
study of shear-induced alignment on lamellar structures formed by
Janus colloidal particles also opens the door for future applications
where a reversible actuator for structure orientation is required.