Electric Field Controlled Columnar and Planar Patterning of Cholesteric Colloids

We study how dispersions of colloidal particles in a cholesteric liquid crystal behave under a time-dependent electric field. By controlling the amplitude and shape of the applied field wave, we show that the system can be reproducibly driven out of equilibrium through different kinetic pathways and navigated through a glassylike free energy landscape encompassing many competing metastable equilibria. Such states range from simple Saturn rings to complex structures featuring amorphous defect networks, or stacks of disclination loops. A nonequilibrium electric field can also trigger the alignment of particles into columnar arrays, through defect-mediated force impulses, or their repositioning within a plane. Our results are promising in terms of providing new avenues towards controlled patterning and self-assembly of soft colloid-liquid crystal composite materials.

Figure 1

Evolution of an equilibrated cholesteric colloid (top) subject to an electric field pulse starting at $t=0$ and ending at $t=0.24\mathrm{s}$. The asymptotic states at $t=0.24\mathrm{s}$ for the three increasing field strengths, ${\mathcal{E}}_1<{\mathcal{E}}_2<{\mathcal{E}}_3$, are shown in panels $A_1$, $B_1$, and $C_1$, respectively. The corresponding postpulse relaxed states at $t=0.42\mathrm{s}$ are shown in panels $A_2$, $B_2$, and $C_2$.

Figure 2

Steady-state evolution of a single cholesteric colloid in a periodically modulated electric field. The cyclic time evolution of the free energy and the field are shown in the first and third panel from the top for a square-wave and sine-wave modulation, respectively. For each case, four representative snapshots of the system dynamics are shown.

Figure 3

An equilibrated $\widehat{\mathbf{x}}$-aligned cholesteric dimer is driven to the orthogonal, $\widehat{\mathbf{z}}$-stacked configuration by a cyclic electric field modulated as a square wave.

Figure 4

Top Panel: Controlled columnar and planar alignment of several colloids with time-dependent electric fields. An equilibrated random dispersion of colloids ($A_1$) is first subjected to a cyclic field along $\widehat{\mathbf{z}}$, resulting in vertical columnar alignments ($A_2$). Next, the repeated on-off switching of the field in the $\widehat{\mathbf{x}}$ and $\widehat{\mathbf{y}}$ directions rearranges the colloids into horizontal planes ($A_3$). Bottom Panel: The same initial configuration ($A_1$) is only slightly perturbed ($B_1$) by the application of a static electric field in the $\widehat{\mathbf{z}}$ axis.