Probing the adsorption and interaction mechanisms of green bio-lubricants on model surfaces: Using okra mucilage on mica as a model system

Okra is an important vegetable that is rich in mucilage, which is a promising and environment friendly bio-lubricant due to its excellent physicochemical and lubrication properties. In this work, the adsorption behavior and lubrication mechanism of purified okra mucilage on model surfaces ( e.g., freshly-cleaved mica) were systematically investigated using nano-mechanical techniques such as surface force apparatus and atomic force microscopy. Experimental results indicate that thin layers of okra-mucilage coatings are built up within 120 min after injecting diluted mucilage solution between two mica surfaces. Moreover, the confined mucilage layers can serve as good bio-lubricants and exhibit anti-wear performance. Because of the varying film thickness and configuration of adsorbed mucilage films, the measured friction coefficient (μ) is lower on hydrophobic silane ( e.g., octadecyltrichlorosilane) modified mica (μ ∼ 0.07) than that on hydrophilic mica (μ ∼ 0.12). Additionally, it is demonstrated that the incorporation of okra mucilage into the polyacrylamide hydrogel network could further reduce the interfacial friction by one order of magnitude at high loads (friction coefficient μ < 0.01). This work provides fundamental insights into the adsorption and lubrication behavior and mechanisms of water-based bio-lubricants on solid substrate, which contributes to the developing of environment friendly bio-lubricants for various applications.