Manufacturing Technologies

Superhydrophobic and drag-reduction microstructures

Periodic micro- and nano-scale structures found in nature have sparked interest in the scientific community for their potential applications, particularly in the realm of reducing drag through superhydrophobic surfaces. A key challenge in employing these microstructures in practical applications has been the scalability of traditional microfabrication techniques.

This study introduces an innovative, template-free, and scalable manufacturing method for creating periodic microstructures by managing defects in the ribbing during forward roll coating. The approach involves the use of viscoelastic composite coating materials comprising carbon nanotubes (CNT) and polydimethylsiloxane (PDMS). This combination allows for precise control over the formation of periodic microstructures with a periodicity ranging from 114 to 700 micrometers. Depending on specific process parameters, the patterned microstructures can transition from linear alignment to a more randomized arrangement.

These periodic microstructures confer hydrophobic properties, as evidenced by water contact angles ranging from 128° to 158° on the coated samples. In a static water environment, a model boat coated with the microstructured film exhibited a 7% to 8% increase in speed compared to a boat with a flat PDMS film.

The inclusion of CNT in the material not only enhances its mechanical properties but also improves its electrical conductivity. In a mechanical scratch test, the CNT-PDMS film exhibited approximately 90% greater resistance to cohesive failure compared to bare PDMS. Additionally, the inclusion of CNT resulted in a significant reduction in sheet resistance, with values ranging from 747.84 to 22.66 ohms per square for CNT-PDMS composites with CNT concentrations ranging from 0.5% to 2.5%.

Daytime passive radiative cooling

The extremely low temperatures of deep space, around 4 Kelvin (4 K), offer a widespread and nearly limitless source of thermodynamic energy for reducing cooling energy consumption. However, achieving subambient radiative cooling during daylight hours in direct sunlight is a complex task. This requires a carefully designed photonic system that can both reflect a significant portion of solar energy (over 94%) and emit thermal energy effectively. Meeting these strict requirements for photonic microstructures while ensuring efficient large-scale manufacturing is a significant challenge.

In this study, we present a rapid and cost-effective roll-to-roll method, free from the constraints of templates, for creating photonic nanocomposite coatings with spike-shaped microstructures. These coatings incorporate nanoparticles of Al2O3 and TiO2 and demonstrate exceptional characteristics, with a solar reflectivity of 96.0% and a thermal emissivity of 97.0%. When subjected to direct sunlight during a typical spring day in Chicago (with an average solar intensity of 699 watts per square meter), these coatings exhibit a radiative cooling power of 39.1 watts per square meter.

Furthermore, these coatings possess superhydrophobic properties and resistance to contamination. We’ve numerically demonstrated that these attributes could result in a potential 14.4% reduction in cooling energy usage across the United States.

Passive radiative cooling for solar panel efficiency

Solar panels have played a substantial role in generating over 115,000 gigawatt-hours (GWh) of energy in the United States. The demand for solar panel energy has increased by 27% since the beginning of the 21st century. However, as solar panel efficiency tends to drop at higher temperatures and the need for clean energy continues to rise, there is a growing necessity to develop cost-effective cooling technology for solar panels.

One approach to achieve passive cooling involves radiating heat into space through infrared emission. Since solar arrays typically require large surface areas to generate significant power, any method aimed at reducing the temperature of solar panel surfaces must be scalable for practical and sustainable use.

In this study, we present a rapid, cost-effective roll coating method that does not rely on templates. It is used to create a photonic composite film with SiO2 nanoparticles. These nanoparticles possess high emissivity within the atmospheric transparent window, allowing them to release heat effectively while permitting visible and near-infrared light to reach the photovoltaic components beneath. When exposed to direct sunlight during a summer noon, these coatings lead to a temperature reduction of 3.5°C without sacrificing the efficiency of the photovoltaic system. Additionally, these coatings exhibit hydrophobic properties and resistance to contamination.