(504c) Programmable Pattern Formation of Colloids in Two-Dimensions Using Diffusiophoresis: From Optimized Source-Sink Models to Spontaneously Induced Turing Patterns

Diffusiophoresis enables colloids to move in solute concentration gradients. Prior literature of diffusiophoresis has focused on colloidal banding, i.e., accumulation of particles in a specific region, in one spatial dimension. While these studies reveal physical insights into the process of diffusiophoresis, the emphasis of one dimension limits the colloidal patterns that can emerge. Therefore, we sought to investigate colloidal banding in two dimensions. Here, we computationally study two distinct solute particle systems by solving the coupled solute and particle transport equations.

First, we study the diffusiophoretic response of colloids in two-dimensional solute fields, generated by sources and sinks. For a dipole system, i.e., one source and one sink, we found that both the inter-dipole diffusion and solute flux decay timescales affect the banding of colloidal particles. A balance between these timescales yields a dipole separation distance which maximizes the number of particles enriched. We demonstrate that the optimal separation distance depends primarily on the partition coefficient, while the diffusivity ratio has a much smaller effect. We also analyze the colloidal patterns generated by four sources and four sinks arranged in a circle.

Second, we study how chemical reaction-diffusion instabilities, which manifest in the form of Turning patterns, can assemble colloids into patterns such as stripes and hexagons. We demonstrate that colloidal gradients can be significantly steeper than the chemical gradients, leading to the generation of much sharper patterns. We devise an analytical model, allowing us to collapse our computational data set onto a master curve describing how colloid length scale varies with Péclet number. Lastly, by including two colloidal particles which have opposite diffusiophoretic velocities, we uncover colloidal patterns which bear striking resemblance to the skin pattern of the Ornate Boxfish. This discovery underscores how diffusiophoresis might play an important role in pattern formation of biological species.

Overall, our findings underscore the ability to employ diffusiophoresis to program complex colloidal patterns in two dimensions and have significant implications for next-generation lab-on-a-chip technologies, adaptive materials, and biophysics.

References:

Raj, Shields, Gupta, Soft Matter, 19, 892-904, 2023

Alessio and Gupta, In preparation