Many plants and insects exhibit self-cleaning properties as a result of the hydrophobic nature of their leaves and wings. Instead of spreading out on these surfaces, water beads up into a tight ball that minimizes its contact with the surface. This “superhydrophobic” interaction is commonly referred to as the lotus effect because it was first studied in lotus plants. The top of the lotus leaf consists of hierarchical micro- and nanofeatures that enhance the hydrophobicity of the waxy leaf. To demonstrate, I grew a lotus leaf and imaged the surface and its interactions with water droplets, seen in Figure 1. If this natural phenomenon can be harnessed, for example in food safe non-stick coatings, anti-icing surfaces for airplanes, or oil-repellent surfaces for camera lenses, it can improve our everyday lives.

Figure 1. SEM images of SMS's lotus leaf and a photo of the leaf diplaying hydrophobicity

But what causes water to show a fear of the surface and bead up in hydrophobic interactions but spread out readily in hydrophilic ones? The answer lies in both the chemistry and texture of the surfaces. Every surface or interface holds energy due to the broken or dangling bonds searching for a chemical mate. The most stable state of the droplet on the surface is the state that minimizes the area of the high-energy interfaces while maximizing the area of the low-energy interfaces.

When considering a droplet on a solid surface, there are three interface energies to consider: the liquid-vapor, solid-liquid, and solid-vapor interface energies. A simple way to evaluate the interaction between these three values on an untextured surface is Young’s contact angle - the angle of the liquid-vapor interface measured through the droplet. Contact-angle measurements are often used as a measure of the hydrophobicity of a planar surface due to challenges in measuring the surface interactions.

Figure 2. The states of a water droplet on a surface and the associated contact angles.

The solid-liquid interfacial energy ( γ_sl) can be used to predict changes in Young's contact angle, assuming the solid and vapor remain the same. A high γ_sl leads to a higher contact angle because the liquid and solid don’t want to be in contact with each other - this causes the water to attempt to find stability elsewhere by creating a bead of water (Figure 2a,b). The inverse is also true - a low γ_sl leads to a low contact angle as the interface is stable and the liquid and solid want to remain in contact with each other (Figure 2c,d).

Texturing a surface can enhance the existing interaction between a liquid and a surface - in fact, it is currently only possible to achieve superhydrophobicity with a textured surface. This enhancement occurs because applying a texture increases the surface area and therefore increases the intensity of surface energy on the solid, changing the balance between the three interfaces. Assuming the interaction started hydrophobic, the water droplet will continue wanting to have minimal contact with the solid. The radius of the droplet will shrink from R_P on the planar surface to R_T on the textured surface, resulting in the enhanced hydrophobic interaction seen in Figure 3.

Figure 3. The hydrophobic enhancement between planar and textured surface. The dashed lines surround the solid-liquid interface of the droplets.

There are two possible states for a droplet on textured surfaces, the Wenzel state and the Cassie-Baxter state (Figure 4). In both states the interaction can be measured via the apparent contact angle (θ^*). In the Wenzel state, the droplet has fully wetted the texture; a hydrophobic surface will become more hydrophobic and a hydrophilic surface will become more hydrophilic. In the Cassie-Baxter state, the texture is partially wetted with air trapped between the bottom of the texture and the droplet. Textures with drops in the Cassie-Baxter state always increase the hydrophobicity of hydrophobic surfaces, but may not increase the hydrophilicity of hydrophilic surfaces.

Figure 4. A water droplet on a planar surface, a droplet in the Wenzel State, and a droplet in the Cassie-Baxter state. Note that the droplet reaches the bottom of the texture in the Wenzel state, but air is trapped below the droplet in the Cassie-Baxter state.

Figure 5. Contact angle of water on a patterned vs smooth surface (silane treated polycarbonate).

The hydrophobic surfaces found in nature can be replicated and engineered for individual applications ranging from liquid-repellent fabrics to drag-reduced surfaces. For example, the Kota Research Group at NCSU has worked to create omniphobic fabrics that can be used as chemical shields against almost all contacting liquids. While SMS has not optimized a surface for hydrophobicity, an improvement in the apparent contact angle has been achieved in small experiments with existing replicas (Figure 5).

Roll-to-roll, also referred to as reel to reel, technology began in the 1800’s as a way to mass produce newspapers and photographs. Since then it has been incorporated into the production of goods ranging from food packaging to flexible electronics, and more recently in emerging nanotechnology applications. Smart Material Solutions primary mission is to supply the seamless nanopatterned drum molds needed for roll-to-roll nanoimprint lithography (R2R NIL).

R2R manufacturing is a method of fabrication in which a continuously moving, flexible substrate is processed. The processing looks different depending on the application but can include printing, patterning, coating, or lamination. In all cases, the substrate starts wound on one roll, undergoes the processing steps, and then the finished product is wound onto a second roll (Figure 1). Nanoimprint lithography (NIL) is the process of replicating nanoscale patterns from a mold into another material. When combined, R2R processing and NIL result in a high-speed, continuous process to produce large areas of nanopatterned surfaces.

Figure 1. R2R production diagram, where the processing steps are represented by block S1 through S5 and a tensioning step is represented by roller T.

There are two main styles of R2R NIL: photolithography and thermal imprinting. For photolithography, uncured photopolymer is applied to a smooth substrate. The photopolymer-covered substrate is wrapped around the drum mold; while in contact with the mold, the photopolymer is exposed to UV light, curing the polymer. The cured polymer sticks to the backing resulting in a patterned film that is peeled off the mold (Figure 2a). For thermal imprinting, also known as thermal embossing, the patterned drum is heated to raise the temperature of the substrate above the glass transition temperature. Meanwhile, a second “nip” roller applies pressure to the backside of the substrate, causing the softened substrate to fill the pattern on the surface of the drum mold. The web is then cooled and released from the drum (Figure 2b).

Figure 2. Illustrations of a R2R photolithography setup (a) and a R2R thermal embossing setup (b).

This continuous process allows for replication at rates much faster than step and repeat or “batch” imprinting due to the cycle time required in both thermal and UV processes. A seamless mold also allows the final patterned area in the substrate to be much larger than the pattern created in the mold. These advantages are especially potent in applications that require mass production of films. As an example, if Apple wanted to cover every iPhone sold in 2020 with an antireflective nanopattern, it would take approximately 47 million feet (8,900 miles) of six inch wide substrate. Smart Material Solutions’ advantage is its ability to seamlessly nanopattern cylindrical drum molds for R2R NIL hundreds of times faster than electron-beam lithography using its patented diamond indenting process - nanocoining.

Figure 3. Nanocoined molds from SMS being replicated at UMass Amherst as part of an NSF Phase II SBIR 1738387 (a) and at MicroContinuum, Inc. as part of our current Army Phase II STTR W15QKN19C0044 (b).

Hydrophobicity: Why Lotus Leaves Fear Water

Roll-to-Roll Nanoimprint Lithography to Add Function to Surfaces