Surface hydrophilicity mediated migration of nano/microparticles under temperature gradient in a confined space

Hypothesis: Particle transport by a temperature gradient is prospective in many biomedical applications. However, the prevalence of boundary confinement in practical use introduces synergistic effects of thermophoresis and thermo osmosis, causing controversial phenomena and great difficulty in understanding the mechanisms. Experiments: We developed a microfluidic chip with a uniform temperature gradient and switchable sub strate hydrophilicity to measure the migrations of various particles (d = 200 nm 2 lm), through which the effects of particle thermophoresis and thermo osmotic flow from the substrate surface were decou pled. The contribution of substrate hydrophilicity on thermo osmosis was examined. Thermophoresis was measured to clarify its dependence on particle size and hydrophilicity. Findings: This paper reports the first experimental evidence of a large enthalpy dependent thermo osmotic mobility chi similar to Delta H on a hydrophobic polymer surface, which is 1 2 orders of magnitude larger than that on hydrophilic surfaces. The normalized Soret coefficient for polystyrene particles, ST/ d = 18.0 K 1 mu m( 1), is confirmed to be constant, which helps clarify the controversy of the size dependence. Besides, the Soret coefficient of hydrophobic proteins is approximately four times larger than that of hydrophilic extracellular vesicles. These findings suggest that the intrinsic slip on the hydrophobic surface could enhance both surface thermoosmosis and particle thermophoresis. (c) 2023 Elsevier Inc. All rights reserved