Zhongmin Liu

Low-temperature increases the groove-crossing energy barrier of droplets on microgrooved PDMS surfaces. This effect promotes local Cassie-to-Wenzel wetting transition through the accumulation of condensate droplets within the microgrooves.

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.

Wetting depth and the energy barriers induced by micro-textures play a critical role in surface hydrophobicity; however, their underlying mechanisms remain an open question in interfacial science. In this study, the lateral depinning force of a droplet on a micro-grooved PDMS substrate was measured using scanning droplet friction micrography. The experiments were conducted across a range of liquid surface tensions ( 42 to 80 mN/m) and interface temperatures ( 5 °C to 25 °C).Experimental results demonstrate that the wetting state of water droplets transitions from the Cassie-Baxter state to an intermediate state as the micro-groove gap increases from 10 to 130 μm at 25 °C, with the energy barrier reaching its minimum at a 100 μm gap. Furthermore, as the interfacial temperature decreases, the condensation of nano-droplets on the rough surface significantly reduces the surface free energy, triggering an earlier transition to the Wenzel state, specifically at the 10 μm gap. This study elucidates the complex mechanisms underlying energy barriers on micro-grooved hydrophobic surfaces and provides critical insights into the wetting state transitions within such geometries.

Wetting behavior of droplets on a substrate with micro-grooves plays a key role in super-hydrophobicity. This paper aims to study the influence of groove width, droplet size, and surface tension on the anisotropic wetting behavior of droplets on micro-grooved PDMS membranes. Three types of PDMS membranes are made by mold replication. Profiles of droplets along the circumferential direction are observed by HD camera, and the 3D model of the droplet is reconstructed. Capillary forces at the three-phase contact line are obtained using the surface tension integration method. Additionally, liquid’s wetting depth within the micro-grooves is calculated using an interfacial free energy equilibrium approach. The experiment shows that the micro-grooved PDMS membrane has anisotropic constrain to droplets due to wetting depth change in micro-grooves. Meta-equilibrium states at the three-phase contact line are influenced not only by substrate free energy, but also by wetting depth, interface pressure, and substrate structure.

The present paper aims to study the influence of pore-scale fiber structure on the Cassie-Baxter contact angle of droplets on PTFE fibrous hydrophobic membranes. The fiber structure is obtained by the Confocal Laser Scanning Microscope (CLSM), and an optical test bench is built to measure the Cassie-Baxter contact angles of a droplet on a PTFE membrane. The free energy of the solid-liquid interface is measured by the surface tension integral on the droplet profile. An iterative algorithm for liquid-vapor interface shrinking is brought forward to analyze the liquid-vapor interface area between the droplet and the membrane, and the interface parameters of the Cassie-Baxter model are recognized. The experiment shows that the Cassie-Baxter contact angles and the contact line radii along the droplet circumference are in a normal distribution due to random fiber structures, and the variance of the normal distribution increases with the membrane pore diameter. The normal distribution of Cassie-Baxter’s contact angle indicates the constraint of the solid-liquid interface to the liquid-vapor interface. On the same normal contact pressure condition, the liquid-vapor area ratio of the droplet and membrane interface changes little, and Cassie-Baxter’s contact angle increases with the solid-liquid interface area ratio.

The capillary force is critical to the moving and breaking of droplets on fibers. This study brings forward a 3-D model reconstruction method for a clam-shell droplet on fibers and obtains the capillary force by the surface integral of Laplace pressure on the whole droplet. The capillary force results are verified by the droplet gravity and axial drag force, respectively. Moreover, the tensile tangential stresses are analyzed to illustrate the top limits of Laplace pressure against droplet breaking or sliding on the fiber. The experiment shows that the capillary force obtained by the 3-D model accurately describes the vertical and tangential forces of the clam-shell droplet on the fiber. Sharp shrinking of the cross-section on the droplet's upper part results in an exponential increase in tensile and tangential stresses, which makes the droplet break or move on the fiber.

The purpose of this paper is to experimentally investigate the oscillate of a clam-shell shaped droplet on a horizontal fiber by the action of crosswind, quantitatively describe the swing and deformation of the droplet, and analyze the damping force by surface tension. The dynamic images of the droplet are recorded by high-speed cameras, the profile curves in the top view and side view of droplet are both approximated to multiple ellipse arcs. Four kinds of liquids and three nylon fibers are used in the experiments. The experimental results show that the droplet oscillates periodically around the fiber by the flow drag force, the swing frequency is inversely proportional to the droplet mass, but does not increase with Re. In addition to the swing around the fiber, the droplet also oscillates periodically in the radial direction, and the frequency of radial oscillation is higher than that of circular swing, which results in the swing amplitude fluctuation. By the flow pressure, the windward side and the leeward side of the droplet deform asymmetrically, the difference of surface tension force on both sides of the droplet forms a moment against the droplet swing around fiber, the resistance moment increases with the liquid surface tension coefficient and fiber diameter.