Colloidal Crystallization—The Effect of Active Colloids on Interfacial and Sedimented Crystals

Colloidal crystals are a structured arrangement of self-organized particles suspended in a fluid forming a regular, repeating pattern mimicking the behaviour of crystalline solids. Colloidal crystals offer immense potential application in a broad range of scientific and industrial innovation, including photonic materials (Huang et al., 2023; Li et al., 2019), biosensing (Wan & Qian, 2023) and drug delivery and biomedical applications (Stein et al., 2013).
Colloidal crystallization requires a specific environment to occur. It can broadly be classified according to phase transition and the nature of the system. Sedimented crystallization and interfacial crystallization follow different mechanisms as they occur in different environments. Sedimented colloidal crystallization process takes place within the liquid environment where the colloids settle due to gravity above a solid substrate, nucleate and grow to form crystal arrangement (Russo et al., 2013). Conversely, for surface colloidal crystallization, the process takes place at the air-water interface, and involves spreading colloids at the meniscus which self- assemble to form crystal arrangements (Kohoutek et al., 2020). Understanding the unique characteristics and conditions under which each type of crystallization occurs is vital for optimizing processes in manufacturing, environmental management, and materials science.
Active colloids are able to propel themselves independently by using energy from outside sources. Here we study catalytic Janus colloids powered by hydrogen peroxide decomposition, which due to a self-generated chemical gradient, move spontaneously through their environment and exhibit non-equilibrium behaviour. Combining active and passive colloids to enable complex crystallization processes, where both the hindered active colloid motion and catalytically induced interactions alter the overall order, has new potential to synthesize responsive smart materials and overcome common issues in passive colloid self-assembly such as polycrystallinity and glass arrest (Altemose et al., 2020; Lin et al., 2022).
Here, the formation of mixed active-passive colloidal crystals from Polymethyl Methacrylate (PMMA) spheres is reported under both sedimented and interfacial conditions. The passive sedimented system self-organized into close-packed hexagonally ordered monocrystalline arrangements in the absence of active colloids. In the interfacial system, notable colloidal interactions and crystal formation were observed during surface crystallization. The variation in the formation of colloidal crystal arrangements at the air-water interface shows the influence of spreading solvents that play a crucial role in sustaining the colloids at the interface, thus influencing the interaction between each colloid and crystal parking pattern.
The influence of active colloids activity on crystallization formation was analysed in both systems through a series of static optical microscopy images and videos. This allowed the effect of active colloids on local and global order to be quantified. Various factors, including catalytically induced interactions and shear effects (Saud & Solomon, 2023), were found to have significant effects on colloidal crystallization in both the sedimented and interfacial states. The presence of active colloids within the passive crystals also influenced crystal defects.

These findings indicate the ability of colloidal systems to form crystal arrangements with a small doping of catalytically active colloids into the system, generating new understanding of out of equilibrium colloidal crystallization in two different geometries.