Optimization Of Photophoretic Forces In Coated-Hollow Microspheres

Abstract

Photophoresis, a thermal phenomenon, generates non-contact forces on microparticles in a fluid when exposed to a light beam, such as a laser. These forces depend on the geometrical, thermal, and optical properties of the particles, and aligning these properties in inhomogeneous particles can enhance photophoretic forces. The use of coated-hollow microspheres offers a promising approach to this challenge. This work presents a photophoresis model for a threelayered microsphere in the slip-flow regime, applying Navier-Stokes equations with corrected boundary conditions. We used numerical approaches to compute the heat source function using Lorenz-Mie theory and validated the results against previous works. Applied to a copper-coated glass bubble, the model analyzed the photophoretic force as a function of coating thickness, considering several shell thicknesses. Results show that nanometric scale coatings enhance the force up to a maximum, after which the high thermal conductivity of copper reduces it. For coatings above 100 nm, the force becomes insensitive to shell thickness, demonstrating copper’s dominance in optical and thermal phenomena. The study suggests depositing excess coating for higher photophoretic forces, providing a framework for optimizing microparticle design for photophoretic applications. Future work includes further validation through numerical models and experiments and finding analytical solutions for integrals associated with terms in the boundary coefficients using Lorenz-Mie Theory, which holds great promise for advancing our understanding of photophoresis.