Shaping nanoparticle fingerprints at the interface of cholesteric droplets

Abstract

The ordering of nanoparticles into predetermined configurations is of importance to the design of advanced technologies. Here, we balance the interfacial energy of nanoparticles against the elastic energy of cholesteric liquid crystals to dynamically shape nanoparticle assemblies at a fluid interface. By adjusting the concentration of surfactant that plays the dual role of tuning the degree of nanoparticle hydrophobicity and altering the molecular anchoring of liquid crystals, we pattern nanoparticles at the interface of cholesteric liquid crystal emulsions. In this system, interfacial assembly is tempered by elastic patterns that arise from the geometric frustration of confined cholesterics. Patterns are tunable by varying both surfactant and chiral dopant concentrations. Adjusting the particle hydrophobicity more finely by regulating the surfactant concentration and solution pH further modifies the rigidity of assemblies, giving rise to surprising assembly dynamics dictated by the underlying elasticity of the cholesteric. Because particle assembly occurs at the interface with the desired structures exposed to the surrounding water solution, we demonstrate that particles can be readily cross-linked and manipulated, forming structures that retain their shape under external perturbations. This study serves as a foundation for better understanding inter-nanoparticle interactions at interfaces by tempering their assembly with elasticity and for creating materials with chemical heterogeneity and linear, periodic structures, essential for optical and energy applications.

Fig. 1. Lipid and nanoparticle segregation at the cholesteric liquid crystal–water interface.

(A) Cholesterics (gray) must twist along the surface to have as much homeotropic anchoring as possible from the presence of lipids in the surrounding water phase. The hydrophobic tails of the lipids prefer the liquid crystal phase, causing liquid crystal molecules to lie parallel to the tail and thus perpendicular to the interface. Twist regions with molecules tangent to the interface exclude traditional, molecular surfactants, such as lipids (red). (B) Laser scanning confocal microscopy data of lipids (TR-DLPC, red) at the cholesteric-water interface demonstrate segregation of the lipids into stripes that follow the underlying cholesteric order. As lipid concentration increases (i to iii), surface stripes become wider and more disordered (ii) until twist regions are forced away from the surface as a result of the lipids saturating the interface (iii). (C) Surfactant-decorated nanoparticles, made surface active from the electrostatic grafting of molecular surfactants to the nanoparticle surface, are also found to align with molecules perpendicular to the interface, forming patterned assemblies. (D) Projections of laser scanning confocal microscopy z stacks of nanoparticles (green) on cholesteric droplets demonstrate how nanoparticles surface modified by surfactants can follow the underlying cholesteric patterning at the cholesteric-water interface. Electrostatic surface functionalization allows flexible surface chemistry. The data show that amine-functionalized silica nanoparticles, which have positive surface charge, now become surface active after modification by negatively charged surfactant, SOS. Increasing the SOS concentration to obtain nanoparticles with sufficiently negative zeta potentials is needed for the particles to segregate into stripes (iii). Numbers on the upper right corners of micrographs are system zeta potential measurements. Scale bars, 50 μm.

Fig. 2. State diagram for nanoparticle stripe segregation with varying concentrations of HCl and C₈TAB.

Segregation of surface-modified nanoparticles at the cholesteric-water interface takes place in a narrow pH and surfactant concentration range. Silica nanoparticles (30 nm) are in situ, surface functionalized with C₈TAB. Droplets of the cholesteric 5CB with 3 wt % CB15 are formed by simple vial shaking. Varying the concentration of C₈TAB and HCl shows three regions: Red regions indicate that particles do not attach to the cholesteric interface. Yellow regions signify unordered, interfacial assembly of nanoparticles. Green regions denote conditions where particles are surface active and have cholesteric ordering, forming stripes. Zeta potentials for selected systems are given at the top of micrographs. The bare silica nanoparticles have a slightly positive zeta potential that becomes more positive with increasing C₈TAB concentration, implying the presence of a slight C₈TAB double layer at their surfaces that further develops with increasing concentration. Exact acid concentrations may vary depending on the age of solutions, as systems are pH sensitive and become more acidic with time due to the absorption of carbon dioxide. Scale bars, 50 μm.

Fig. 3. Controlling the thickness of nanoparticle-filled stripes with CB15 or C₈TAB concentrations.

(A and D) Similar to the lipid results in Fig. 1B, the nanoparticle-filled stripe width can also be controlled with the C₈TAB concentration. (B and C) The stripe width can be tuned by adjusting the concentration of the chiral dopant CB15. By increasing the chiral dopant from 3 to 7 wt % at a fixed concentration of 10 mM C₈TAB, the stripe width decreases from ~1.7 μm to ~600 nm, corresponding to a decrease in the cholesteric pitch (A). Increasing the dopant concentration to 10 wt %, the cholesteric pitch decreases, with a projected surface stripe width on the order of ~100 nm. However, no stripe segregation at the cholesteric-water interface is evident from confocal data. Instead, nanoparticles organize into circular domains dictated by cholesteric double-spiral domains (B, bottom right). Lines in (C) and (D) are drawn to guide the eye. Scale bars, 25 μm.

Fig. 4. Time evolution of hydrophobic nanoparticles coating a cholesteric droplet.

Confocal data of nanoparticles coating two droplets with differing stripe widths are shown in (A), where the left column is a zoom in of the top of the droplet in (B). Scale bars, 25 μm. Preassembled nanoparticle clusters translate and coalesce along stripes that result from the cholesteric ordering of the droplet. The total stripe length growth rate is highest at the beginning of the interfacial attachment process and decreases with time due to the saturation of the surface with nanoparticle-filled stripes. The total stripe length of all nanoparticle stripes on the droplet shown in the left of (A) and in (B) is plotted against time in (C), where the total length is given by L_total = ∑N_i ⋅ L_i, where N_i is the number of stripes with the length L_i. The total stripe length normalized by the number of stripes N_i with the length L_i is given by L_n = (∑N_i ⋅ L_i)/ ∑N_i, which, when plotted against time, is shown in (D) to have a linear growth. Lines in (C) are drawn to guide the eye. Similar dynamics are seen in droplets coated with thinner nanoparticle stripes, shown in the right column of (A). A video of this process is in the supplementary materials.

Fig. 5. Cross-linking nanoparticle assemblies and destroying cholesteric ordering with temperature.

(A) A confocal micrograph shows the cross-linked nanoparticles on a cholesteric droplet. The integrity of the cross-linked nanoparticle stripes is tested by rapidly heating the droplet to and cooling from the isotropic phase to disrupt the cholesteric ordering. (B) The confocal micrograph of the droplet in (A) after quenching reveals that the nanoparticle assemblies are more disordered, but they still retain their linear shape, confirming their robust structure after cross-linking. Scale bars, 25 µm.