(266c) Directed Colloidal Motion Via Re-Usable Thermoresponsive Polymeric Gradients

The ability to control colloidal motion and organization is a central challenge in the field of soft matter, with wide-reaching implications for microfluidics, materials design, and active matter systems. Here, we introduce a reversible and re-usable strategy for directed colloidal motion using concentration gradients generated by thermoresponsive polymers. Our approach leverages thermal activated diffusiophoresis—particle motion in response to solute gradients—without the need to modify passive colloids or rely on non-renewable chemical reactions.

We fabricate semi-permeable sol-gel colloids, referred to as shepherd particles, by nucleating POSS-based droplets on magnetic haematite cores. These shepherds reversibly absorb and release hydrophobic triblock copolymers in response to temperature changes. Upon cooling, the expelled polymers create localized solute gradients that direct the motion of nearby unmodified, fluorescent tracer particles via diffusiophoresis. Using a custom-built thermal stage on an inverted microscope, we finely control temperature to modulate droplet swelling, gradient formation, and trapping behavior.

Brightfield microscopy, particle tracking, DSC, and DLS reveal that passive particle trapping occurs only when multiple shepherd particles cooperate to form colloidal rafts—individual shepherds do not generate sufficiently strong gradients alone. Notably, passive particles accumulate in the interstitial regions between shepherds, suggesting gradient overlap is crucial for efficient trapping. When a magnet is introduced post-trapping, these rafts migrate collectively, transporting the captured passive particles over ~100 µm within six minutes. This transport window coincides with the micelle-to-polymer transition observed via DSC, indicating a strong coupling between thermal responsiveness and directed motion.

Our findings introduce a tunable, fuel-efficient, and contact-free mechanism for directing unmodified colloids. By mapping critical relationships between micellar concentration, polymer emission, and cooperative particle behavior, this work lays the groundwork for programmable colloidal manipulation using thermal stimuli alone. This technique holds promise for applications in self-assembled materials, drug delivery, and dynamic microfluidic control, offering a scalable, reconfigurable alternative to existing manipulation strategies.