Segregation of microspheres by applying a mechanical-driven force*

Recently, experiments have been performed using agglomerated hydrophilic silica, or less agglomerated hydrophobic polystyrene microspheres with diameters between 3 to 10 μm to study their self-organization by either agitating or manually rubbing these spheres on (non-) coated silicon substrates. We serendipitously observe that these microspheres preferentially adhere to and form monolayers on the fluorocarbon-coated substrates. Using Kelvin probe force microscopy, we show that the tribocharging mechanism lies at the heart of these observations. The particles and substrates acquire opposite charges, inducing an electrostatic attraction. Consequently, the particles adhere to the fluorocarbon layers. Our results are further corroborated with colloidal probe (CP) measurements performed at different humidity levels, in which a long-range electrostatic attraction can be measured between the CP and substrate.

Altogether, the results highlight that the relative humidity and, surprisingly, even the substrate material carrying the fluorocarbon layer strongly affects the tribocharging process.  Soon, the same type of experiments will be performed to explore the self-organization of binary mixtures into monolayers on different polymer surfaces driven by the tribocharging mechanism.