Evaluating Superhydrophobic Surfaces under External Pressures using Quartz Crystal Microbalance

Bubble Growth on Hydrophobic Rough Surfaces in the Shear Flow

It is well known that bubbles will form on a hydrophobic rough surface immersed in water, which can create a surface covered with bubbles and leads to drag reduction. However, it is still not clear how bubbles grow on the surface under flow conditions. In this work, a rotating flow field is created using a parallel-plate setup of a rotational rheometer, and sample surfaces with different roughnesses and wettabilities are examined with different shear rates. The growth of bubbles is exclusively observed on the hydrophobic rough surface, and subsequent drag reduction is also detected simultaneously. The growth of bubbles is attributed to heterogeneous nucleation in the crevices under a local pressure reduction generated by the shear flow. A geometric model is established to describe the profile evolution of the trapped bubble in the crevice based on the contact angle and the pressure balance across the gas-liquid interface, which involves the variations of the Laplace pressure resulting from changes in the local liquid pressure. The growth of bubbles on the hydrophobic rough surface does not need a large decrease of the surrounding pressure or a high moving speed, which will have potential applications in drag reduction under the condition of a moderate shear rate.

Design and Strengthening of Superhydrophobic Coatings: The Influence of Intermediate Layers

The employment of intermediate layer technology to improve the mechanical stability of superhydrophobic coatings (SHCs) is an acknowledged tool, but the mechanism by which intermediate layers, especially different ones, affect superhydrophobic composite coatings is not clear. In this work, a series of SHCs based on the strengthening of the intermediate layer were fabricated by employing polymers with different elastic moduli such as polydimethylsiloxane (PDMS), polyurethane (PU), epoxy (EP) resin, as well as graphite/SiO₂ hydrophobic components. Following that, the effect of different elastic modulus polymers as an intermediate layer on the durability of SHCs was investigated. From the perspective of elastic buffering, the strengthening mechanism of elastic polymer-based SHCs was clarified. Furthermore, from the perspective of self-lubrication, the wear resistance mechanism of self-lubricating hydrophobic components in the SHCs was elucidated. Also, the prepared coatings exhibited excellent acid and alkali resistance, self-cleaning, anti-stain, and corrosion resistance. This work confirms that low-elastic-modulus polymers can also play the role of buffering external impact energy by elastic deformation even as an intermediate layer, and provides theoretical guidance for the development of SHCs with robustness.

Unraveling the liquid gliding on vibrating solid liquid interfaces with dynamic nanoslip enactment

Slip length describes the classical no-slip boundary condition violation of Newtonian fluid mechanics, where fluids glide on the solid surfaces. Here, we propose a new analytical model validated by experiments for characterization of the liquid slip using vibrating solid surfaces. Essentially, we use a microfluidic system integrated with quartz crystal microbalance (QCM) to investigate the relationship between the slip and the mechanical response of a vibrating solid for a moving fluid. We discover a liquid slip that emerges especially at high flow rates, which is independent of the surface wetting condition, having significant contributions to the changes in resonant frequency of the vibrating solid and energy dissipation on its surface. Overall, our work will lead to consideration of ‘missing slip’ in the vibrating solid-liquid systems such as the QCM-based biosensing where traditionally frequency changes are interpreted exclusively with mass change on the sensor surface, irrespective of the flow conditions.

A fluid flowing in solid confinement will glide, rather than stick to, the solid’s surfaces. This is usually described by introducing a concept known as slip length. The liquid slip concept is now extended for the situation of a vibrating solid–liquid interface.