Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is $\sim 4.5\times {10}^{-3}\text{C}/{\text{m}}^2$ for 20-nm nanocolloids, and two orders of magnitude lower, i.e., $\sim 2.3\times {10}^{-5}\text{C}/{\text{m}}^2$, for $1\text{−}\mu \text{m}$ colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.

Figure 1

Snapshots of pair interaction and the trajectories of two colloidal particles, initially situated at a separation of 2–$3 \mu \text{m}$. (a) Two 35-nm diameter silanated silica particles form a stable pair within one minute of interaction. (b) Blue circles and red triangles denote the separation of each particle from its origin. Black squares denote the time dependence of the separation between the two particles. (c) Set of consecutive images of interacting 20-nm particles, which form a metastable pair. (d) The corresponding separations of each particle from the origin (blue circles and red triangles); black squares are the time dependence of the separation between the two particles. Note little “bumps” in the separation at 80 s and 120 s, which show temporal dissociation of the pair. (e),(f) Two 20-nm diameter particles experience no interaction. Black squares show the distance between two particles. Blue circles and red triangles correspond to the separation of the first and second colloid from the initial position.

Figure 2

Distribution ratio of finding a stable pair, a metastable pair, and pairs of noninteracting particles, as a function of the diameter of the silanated silica nanoparticle in a planar 5CB. The lines are guides to the eyes only.

Figure 3

(a) Dark-field images of 20-nm DMOAP-coated silica particles moving along the applied electric field. (b) Time dependence of the particle velocity. The parameters of the applied electric field are the following: the electric-field strength is ${10}^5$ V/m, the wave form is rectangular, and the frequency is 1 Hz. (c) The dependence of the average particle's velocity on the applied electric-field strength.

Figure 4

(a) Total electric charge of DMOAP silanated silica nanoparticle as a function of its diameter. Dotted line is a guide to the eyes only. (b) Surface charge density of a particle, determined from data in panel (a), as a function of inverse diameter of the particle. The dotted line is the best linear fit.

Figure 5

(a) Dark-field images of a 20-nm silica colloid, approaching a surface defect. (b) The corresponding distance between the 20-nm particle and the surface defect. (c) Dark-field images of a single 20-nm colloid, being trapped by a larger cluster of nanoparticles. Note much larger brightness of the agglomerate. (d) Corresponding trajectories of 20-nm colloid and a particle agglomerate, initially separated by $3 \mu \text{m}$. Blue circles and red triangles show the distance between 20-nm colloid and particle agglomerate with respect to their start positions. Black squares show the distance between the 20-nm colloid and particle agglomerate.