Lateral Retention Force and Directional Energy Barrier of a Droplet on a Microgrooved Hydrophobic Surface

While microstructures on a hydrophobic substrate can effectively enhance its hydrophobicity, a comprehensive understanding of the resulting wetting states and interfacial energy barriers remains a long-standing scientific challenge. This study aims to investigate the directional energy barriers induced by microgrooves on a polydimethylsiloxane (PDMS) substrate using lateral retention force measurements. Droplet lateral retention forces on the PDMS substrate were quantified using a scanning droplet friction force microscope (SDFM). Furthermore, a series of comparative experiments was conducted to systematically evaluate the influence of microgroove geometries, liquid surface tensions, and chemical bonding types (ionic versus polar covalent) on the interfacial energy barriers. The energy barrier perpendicular to the microgrooves is significantly greater than that parallel to them. Interestingly, this barrier gradually decreases as the system transitions from the static to the kinetic stage, due to an increase in the interfacial modulus. Furthermore, the strong affinity between hydroxyl groups and the Si-O groups in PDMS results in a retention force several times greater than theoretical predictions. To our knowledge, this is the first report detailing the dynamic interfacial modulus of a droplet moving on microgrooved PDMS substrates, denoting that the solid-liquid interfaces between droplets and PDMS substrates are not completely separated, and an interfacial transition region exists where molecular interpenetration and intermolecular interactions occur.