Critical Casimir levitation of colloids above a bull's-eye pattern.

This article investigates the use of critical Casimir forces—fluctuation-induced forces arising near the critical point of a binary liquid mixture—to trap and levitate colloidal particles above a chemically patterned substrate featuring a bull's-eye design. Employing the Derjaguin approximation and mean-field theory, the study identifies four possible colloid configurations: far from the surface, stuck to the ring, levitating above the ring, and levitating above the bull's-eye center (point levitation). The authors present a detailed morphological phase diagram showing how these configurations depend on the pattern geometry, colloid size, and temperature, revealing that the stability region for point levitation shrinks as the system moves away from criticality, although the trapping potential deepens. Potential applications include sorting colloids by size and developing a "critical Casimir thermometer" to measure the thermodynamic distance to criticality, with the theoretical predictions deemed experimentally accessible using current microfabrication and microscopy techniques.