Khalifa University researchers found a way to optimize capillary action – a process that moves liquid passively – in thin-film evaporators, which are used to generate steam and purify water with solar energy, cool buildings and electronics, and much more.

Evaporation is a process fundamental to everyday life. It keeps our bodies cool and the air moist, and it plays a critical role in a number of industrial systems that drive our society today, from providing power and purifying water, to cooling buildings and electronics. Thin-film evaporation is an extremely effective and energy-efficient way to transfer heat. For thin-film evaporation to work, however, a stable liquid film needs to be maintained on the surface, which can be a challenge.

Inspired by the same process used by plants to carry water up from their roots to their leaves, capillary-fed wicks offer an attractive means of moving liquid to the surface since it is a passive mechanism; it does not rely on an external power supply or a mechanical pump to deliver the fluid to the evaporator.

Researchers and engineers are continuously exploring ways to improve the performance of passive liquid propagation, solar energy-driven evaporation and water distillation. “Using wicks to supply liquid to the evaporating surface via a process called capillarity may be the solution to providing a constant, stable liquid film for thin-film evaporation,” explained Dr. Tiejun Zhang, Associate Professor of Mechanical Engineering. With funding support from a 2019 Abu Dhabi Award for Research Excellence (AARE), Dr. Zhang is leading a research team from Khalifa University to investigate how to improve wickability, or how efficiently liquid travels up through a wick, and in turn, the performance of thin-film evaporation. They recently published their work in the journal Advanced Engineering Materials.

Co-authors include Dr. Hongxia Li, Postdoctoral Fellow, Afra Al Ketbi and Qiangshun Guan, Graduate Students, and Dr. Mohamed Alhosani and Ablimit Aili, PhD and MSc Graduates, all from the Department of Mechanical Engineering.

Wicking is the absorption of a liquid by a porous material, with the liquid then transferred through the process of evaporation. Daily examples of capillary action can be seen when dipping a paintbrush in water where the liquid is drawn up between the brush hairs against gravity, or in a paper towel dipped in spilled coffee as the liquid moves up the pores of the paper. Rather than an external energy supply causing capillary action, intermolecular forces cause the surface tension of the liquid and the adhesive forces between the liquid and solid to propel the liquid through the solid material. This is how the tallest trees can pull water through the roots to the highest branches.

The crucial role of capillary pumping for thin-film evaporators has motivated KU researchers to explore ways to improve wickability.

Many factors, like surface wettability and permeability, affect a material’s ability to propagate, or spread of liquid and can significantly reduce the rate at which the liquid is absorbed in the wick. Viscous pressure drop, surface roughness, blockages, and twists and turns can all slow the movement of the liquid through the material.

Wickability can be enhanced by improving the intrinsic wettability of the wick surfaces to stop it from drying out during evaporation, and through designing a porous structure to maximize fluid flow.

Dr. Zhang’s team developed a wick with excellent capillary pumping ability by creating nanostructures made of copper onto a water-loving, hydrophilic surface, also made of copper. This created a large, porous surface area for thin-film evaporation. As an additional benefit, in solar-driven applications where the wicking porous material also acts as a solar absorber, these nano-structures can also help harvest the sunlight more efficiently.

The KU team then used their prototype to create a model to predict how effective a material’s wickability would be based on a number of different factors, including pore sizes, shapes and orientations. The model can help researchers design effective wicks in the future.

“We systematically characterized the water propagation dynamics from microscale to macroscale through experimental observation and theoretical modelling,” explained Dr. Li. “We fabricated a nanostructured porous wicking surface—essentially a copper micromesh attached to a flat copper substrate with a nanostructured surface. The micromesh improves wickability by acting as the wicking structure, providing capillary pressure with relatively high permeability, while the copper oxide nanostructures enhance the surface hydrophilicity.”

The team then observed the water propagation behaviors under optical and infrared thermal cameras to develop a capillary pressure model and permeability model to predict how efficiently the capillary-pumped water travelled along the porous surfaces. They also conducted studies with varying pore sizes before optimizing pore dimension to achieve the maximum capillary pumping rate.

The KU team’s technology offers outstanding solar-driven evaporation capability owing to their high liquid propagation rate and excellent light absorption. The proposed scalable nanostructured porous surfaces promise great potential in broad energy and sustainability applications.

Source: Khalifa University