Rotator-to-Lamellar Phase Transition in Janus Colloids Driven by Pressure Anisotropy

We demonstrate through Brownian dynamics simulations a phase transition in plastic crystalline assemblies of Janus spheres through controlled pressure anisotropy. When the pressure in plane with hexagonally ordered layers is increased relative to that normal to the layers, a rapid first-order rotator-to-lamellar transition of Janus sphere orientation occurs at constant temperature. We show that the underlying mechanism closely follows the Maier-Saupe theory, originally developed for isotropic-to-nematic transition in positionally disordered materials but here applied to positionally ordered ones. Since the transition involves almost no translational diffusion or volume change, and occurs rapidly by particle rotation, the results should help guide the design of rapidly switchable colloidal crystals.

Figure 1

Temporal variation of pressure components $p$, volume fraction $\phi$, mean interparticle separation $\lambda$ in three directions, and orientational order parameter $⟨P_2⟩$ along with equilibrated renderings, for a system of Janus colloids with $\alpha =117°$, $ϵ/k_{B}T=1.05$ subject to two different anisotropic pressure conditions: $p_{zz}=8k_{B}T/{\sigma }^3$, and $p_{xx}=p_{yy}=12$ (open symbols) or $p_{xx}=p_{yy}=16$ (closed symbols).

Figure 2

(a) Variation of pressure components, volume fraction, mean interparticle separations, and orientational order parameter for a system of Janus colloids with $\alpha =117°$, $ϵ/k_{B}T=1.05$ subject to a constant lateral to normal pressure ratio of 2 while reducing pressure magnitude over time; (b) phase diagrams of color-coded order parameter as a function of applied pressures for particles with $\alpha =117°$ and two different interaction strengths.

Figure 3

(a) Director order parameter $S$ as a function of normal pressure $p_{zz}$ for Janus colloids with $\alpha =117°$, $ϵ/k_{B}T=1.05$ subject to several lateral to normal pressure ratios, with the dashed red line indicating the order parameter at the transition according to the Maier-Saupe theory; (b) variation of $S$ with $ϵ$ for colloids with different Janus balance subject to fixed pressures of $p_{xx}=p_{yy}=16$, $p_{zz}=8k_{B}T/{\sigma }^3$; solid lines are polynomial-linear fits connected by a sigmoidal function. The inset shows a variation of the Maier-Saupe parameter $u_{\mathrm{MS}}$ with $ϵ$ within the lamellar region.

Figure 4

Histogram of the orientation distribution of Janus particles with $\alpha =117°$, $ϵ/k_{B}T=1.05$ simulated under fixed pressures of $p_{xx}=p_{yy}=16$, $p_{zz}=8k_{B}T/{\sigma }^3$; the solid red line is the prediction of the Maier-Saupe theory from Eq. (5); the inset shows variation of the overall potential in spherical coordinates for an extreme sample case with a rotating central particle.