The proposed exploratory EAGER project aims at the first quantitative measurement of interfacial interactions between a droplet and a patterned substrate. A novel experimental technique is developed to quantitatively study the interaction forces as a function of the geometry and defect characteristics of the micropatterns. The system provides a quantitative testbed for long-standing theories of interfacial processes such as contact-line pinning under different geometric and chemical conditions for the first time, thus benefitting a huge variety of fields, including solid state physics, surface chemistry, and microfabrication.
Intellectual Merit
Substrates with micropatterns, particularly those with a "forest" of micropillars interacting with small-scale droplets, have garnered enormous interest in recent years for their versatility and unusual properties, including wettability, adhesive energy, conductivity or capacitance. These patterns have potential applications in widespread industrial processes that rely on non-wetting surfaces that reject dirt, have low adhesive energy, reject water (e.g. coatings for windshields), resist condensation (e.g. in refrigeration devices), or are useful in pore filtration of gases (e.g. in micro fuel cells). Nevertheless, a quantitative understanding of droplet shapes and dynamics lags behind a large number of proof-of-principle experiments. In particular, very little is known about the effect of pattern and pillar geometry on the dynamics of contact line motion and the forces needed to sustain (or arrest) such motion.
The proposed work will apply novel experimental techniques for simultaneous quantitative measurements of droplet shape and contact-line pinning forces, both with a spatial resolution at the single-defect level and capable of fast time resolution. The interaction of isolated defects of defined shape with contact lines has long been the subject of pinning theories, perceived as an idealization of the description of real contact line behavior. With high-speed photography and sensitive force sensors, forces and deformations of droplets and substrates in relative motion will be determined simultaneously by making crucial measurements for an accurate description of dynamical contact angle hysteresis as well as droplet repulsion, fragmentation, and coalescence on hydrophobic surfaces. The experiments can access and analyze a wide range of speeds beyond current experiments, in a regime highly relevant for applications.
The proposed EAGER research proposal has the following objectives: (i) to seek an accurate understanding of contact line pinning and depinning from isolated defects, in an experimental system that can serve as a paradigm for defect pinning in broader contexts of interfacial processes; (ii) to acknowledge the effect of defect distribution and defect interaction on the contact line as a whole; (iii) to explore an innovative combination of experimental techniques, promising an improved set of tools for analyzing contact line motion on the microscale.
Broader Impacts
The Broader Impacts of the proposed work include those on the societal, group, and individual scales. The research provides fundamental insight in fields of great societal need: clean water, refrigeration, energy, and advanced manufacturing. The graduate and undergraduate students involved in the project will be trained in the areas of microfabrication, soft lithography, surface patterning, and other processes that are of great importance. The PI will also incorporate research results in existing courses and demonstrations. The PI has plans in place to boost the participation of members from under-represented groups by proactively participating in several on-campus/off-campus programs, including Women in Engineering Program, the Minority Engineering Program, and the McNair Scholar Program.
Superhydrophobic surfaces - surfaces that strongly repell water - made from patterned arrays of micropillars are promising novel materials for applications in self-cleaning surfaces, refrigeration, or pore filtration. The behavior of liquid droplets on these substrates has so far largely been described in macroscopic terms, explaining properties like large contact angles from coarse-grained variables like pillar density. Understanding of the dynamical behavior of the droplets, microscopic parameters such as the shape, orientation, and arrangement of pillars or the precise topography of the contact line is crucial. In this project, we carried out an integrated investion of experimental and model research to study the interactions between water droplet and micro-patterned surfaces. We develop an imaging method and a force measurement setup to study contact line (CL) evolution and contact angle hysteresis (CAH) induced resistant force for a water droplet sliding on PDMS micro-pillar arrays. The topography of the CL between droplet and surface is imaged using fluorescence microscopy in combination with high-speed video. To measure the CAH induced resistant force, a micro-force sensor is attached to the droplet and the substrate moved relative to the droplet with prescribed velocity. The resultant force-time curve displays an initial maximum and subsequently a dynamic steady state with a sawtooth-like shape. The experimental results showed interesting features of the processes during droplet-surface interactions. Based on these observations, we carried numerical simulations and modeling. We have developed an energy–based model of a droplet in the Cassie-Baxter state using Surface Evolver. Simulations have been carried out for water on PDMS substrates with square-shaped pillars to assess the changes in droplet energy upon deformation and displacement, carefully evaluating the importance of droplet-scale shape changes and changes to the topography of the contact line, with particular attention to the pinning and depinning from individual pillars. We find that the majority of shape distortion and energy change occurs in close proximity to the substrate, encouraging a simplified theoretical description using concepts of 2D contact-line pinning in solid state physics. The integrated experimental and modeling study revealed the fundamental mechanisms of interactions between the droplet – micro-structured surface, and enriched our knowledge of this phenomenon. Broader impacts of this research include potential applications in self-cleaning surface, water condensation collection and waste water management. In these applications, the fundamental understanding of the water-surface interaction can guide engineers to design and fabricate structures and surfaces with significantly improved functions. The NSF award has also been used to train graduate students. Two students working on the project have received MS degrees. One of them is female. The research training they received by working on the project prepared them well for their future career.
