Interfaces of water with water-repellent, or hydrophobic, materials are relevant in numerous natural and applied contexts. Examples include lotus leaves, membranes-assisted separation processes, and oil–water emulsions. Typically, water repellence is realized with the help of hydrocarbons and perfluorocarbons. Although these materials present low adhesion to water, their interfaces with water are known to be electrically charged. Origins of the electrification of water–hydrophobe interfaces is a century-old mystery that has been intensely debated on. A number of competing hypotheses have been proposed: specific interfacial adsorption of hydroxide ions, water dipole moment, partial interfacial charge transfer, specific interfacial adsorption of protons, cryptoelectrons, bicarbonate ions, and surfactant contamination. Given the significance of these interfaces, we investigated the origin of water–hydrophobe electrification.
To disentangle the role of the various factors, we studied water’s interfaces with: solid hydrophobes, e.g., polypropylene; liquid hexadecane; and gas (air). Electrical charges incurred by water droplets formed using pipettes/tubes of hydrophobic (and hydrophilic) chemical make-up were quantified via electrometers and uniform electric fields. Specifically, we interrogated the contributions of water–hydrophobe surface area, surface chemistry, and water’s ionic strength, pH and dissolved CO2 content. We deduced that common solid hydrophobes have negatively charged surfaces even in air: when a hydrophobic pipette/tube is used to draw an aliquot of water from the bulk, hydrated cations form an electrical double layer at the liquid–solid interface. For the water–hexadecane interface, we tracked interfacial tensions over time. Our investigation revealed that trace amounts of impurities are present in the oil, despite purification, which interfere with purely interfacial effects. Lastly, we applied these fundamental insights to investigate slippery liquid-impregnated surfaces (SLIPS) realized using microtextured SiO2/Si wafers and sand dollar (Dendraster excentricus) templated PDMS surfaces. Recognizing the significant activity in triboelectric nanogenerators (TENGs), we conducted a parametric study of the device output and water–hydrophobe interfacial properties; and tested whether SLIPS could be incorporated in next-generation TENGs. The findings reported in this thesis address some long-standing questions on the spontaneous electrification of water–hydrophobe interfaces, and they should aid the rational development of practical applications such as SLIPS, TENGs, and beyond.
|Date of Award
|Mar 10 2022
- Biological, Environmental Sciences and Engineering
|Himanshu Mishra (Supervisor)
- Contact electrification