Microdroplets find versatile applications in the fields of chemistry, materials science, and biochemistry, particularly in chemical engineering and biochemical microfluidics like microreactors and biosensors. Achieving precise control over microdroplets in their shape, size, and contact angle (CA) is especially crucial for applications involving precise control of printing/coating patterns and chemical reactions. 

Current research leverages the capillary and edge effects of micropillared structured surfaces to achieve certain polygonal patterns of liquid droplets. However, when given a specific liquid-material combination, especially for superhydrophilic surfaces (or surfaces exhibiting complete wetting), the achievable CA is limited by the conventional Gibbs equation typically used to predict the CA of a macro-droplet on rough surfaces. The contact shapes of the microdroplets are limited to certain polygons. Achieving precise control over microdroplets with arbitrary shapes and a wide range of CAs has long been a challenge. 

In a study published in PNAS, Prof. GAO Yurui’s group from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences, collaborating with Prof. ZENG Xiaocheng from City University of Hong Kong and Prof. Joseph S. Francisco from the University of Pennsylvania, employed photolithography techniques and subsequent processing to fabricate a class of structured surfaces featuring concentric closed-loop microwalls/microchannels, thus allowing for precise control of microdroplets with a wide range of CAs and offering high shape and pattern tunability. 

Based on the notion of "topological wetting states" (J. Am. Chem. Soc. 2020, 142, 18491–18502), the researchers engineered a variety of surfaces with homocentric orthorhombic closed-loop microwalls/microchannels using lithography techniques. These surfaces exhibited precise microwall edge angles of 90° and, with the application of UV-Ozone treatment, achieved an intrinsic contact angle of 0°. On these surfaces with closed-loop structures, topological wetting states were observed. 

Due to the closed-loop topology of the surface structures, the microdroplets exhibited multiple Wenzel states with their three-phase contact lines pinned at the outer edge of the microwall, and the CA varied widely from 0 to 130°. By designing the shape of homocentric microwalls, the microdroplet CA and size can be effectively controlled as well, enabling formation not only of regular shapes such as circles, triangles, and squares, but also of irregular patterns like heart-shaped shapes. Furthermore, the researchers extended the control to the CA dimension. They proposed control across a broad range (from 0 to >130°), particularly for surface-liquid combinations that exhibit intrinsically complete wetting, by leveraging droplet evaporation and closed-loop geometry. 

Interestingly, the researchers uncovered a wetting phenomenon that challenges the traditional Gibbs equation in describing a droplet at its boundaries: Regardless of the shape of the closed-loop structure, the maximum CA of the microdroplet remains stable at around 130°, largely deviating from the angle limit predicted by the Gibbs equation based on macroscale edge effects. This finding suggested that the Gibbs equation, traditionally used for accessing the CA of macrodroplets on rough surfaces, may not be applicable at the micro or nano scale. This conclusion is applicable to various liquids, including isopropanol, ethanol, decane, and octane, considered in this study. 

Through independent molecular dynamics simulations, the researchers attributed this large deviation from the Gibbs equation prediction to the combined effects of water-surface interaction and the atomic structure at the edge. They suggested adding a correction term to the Gibbs equation to address the apparent deviation. 

"This work demonstrated closed-loop microstructures with well-controlled orthorhombic edges, enabling a comparative analysis of the droplet’s contact angle and the edge angle. It provides compelling evidence on the need for the modified Gibbs equation at the micro or nano scale, and the obtained droplets that can be precisely controlled offers a possibility for accurate measurement of droplets. It has implications for exploiting controllable microdroplets in fields such as microfluidics, chemical reactions, and biosensing, offering new opportunities for material manufacturing and green synthesis,” said Prof. GAO.