Superhydrophobicity enhancement through substrate flexibility

Abstract

Inspired by manifestations in nature, microengineering and nanoengineering of synthetic materials to achieve superhydrophobicity has been the focus of much work. Generally, hydrophobicity is enhanced through the combined effects of surface texturing and chemistry; being durable, rigid materials are the norm. However, many natural and technical surfaces are flexible, and the resulting effect on hydrophobicity has been largely ignored. Here, we show that the rational tuning of flexibility can work synergistically with the surface microtexture or nanotexture to enhance liquid repellency performance, characterized by impalement and breakup resistance, contact time reduction, and restitution coefficient increase. Reduction in substrate areal density and stiffness imparts immediate acceleration and intrinsic responsiveness to impacting droplets (∼350 × g), mitigating the collision and lowering the impalement probability by ∼60% without the need for active actuation. Furthermore, we exemplify the above discoveries with materials ranging from man-made (thin steel or polymer sheets) to nature-made (butterfly wings).droplet impact | superhydrophobicity | flexible | wetting transition | biomimicry H ydrophobic surfaces have gained much attention in recent years (1) for their unique attributes, such as self-cleaning behavior (2), extreme repellency to liquids (3, 4), and resistance to surface icing (5). For practical applications, repellency to impacting liquid droplets is of great importance, and numerous studies have investigated the physics of droplet impact on rigid surfaces and the diverse outcome of such events for a broad range of liquid properties and impact conditions [liquid viscosity (6, 7), surface tension (3), environmental pressure (8, 9), etc.]. Additionally, extensive work has been done on the role surface morphology plays in determining the outcome of such eventswith the goal being full rebound of an impacting droplet from the surface (10-17). In these studies, the emphasis was on texturing rigid materials to impart enhanced properties. On the other hand, there is a broad palette of surfaces in nature and technology that is characterized by some degree of flexibility [leaves (18), construction materials, textiles (19), etc.]. Studies have been reported with respect to dynamic wetting on hydrophilic, flexible materials (20-23); however, little work has addressed the interweaving effects of wetting behavior and material flexibility. In addition, the work that has been reported (24) did not focus on the role of surface compliance or flexibility in influencing the physics of the droplet collision process.Here, we investigate the effect of substrate flexibility on superhydrophobicity through the outcome of droplet impact events with respect to impalement resistance, droplet−substrate contact time, maximum droplet deformation, and restitution coefficient. We demonstrate, through appropriate modeling and experiments, that, by rational tuning of the substrate stiffness and areal density, flexibility can actually w...

Fluid-structure interaction with the entropic lattice Boltzmann method

“…Note however that most studies have focused on rigid surfaces and neglected the flexibility of the substrate, which is inherent to most naturally occurring repellent surfaces such as leaves, textiles or butterfly wings. Notable is the recent study of [63], where the effect of elasticity on hydrophobicity was investigated experimentally.…”

“…In this section, we aim to go beyond classical benchmark cases and explore the capabilities of the KBC-FSI solver in the context of multiphase flows by considering droplet impact on flexible superhydrophobic surfaces, similar to [63]. From the numerical point of view, simulations of such a kind are challenging.…”

“…This leads to an artificial negative pressure, which is compensated in our simulations by a regularization procedure, where we use a simple linear interpolation between the liquid and vapor density to evaluate the pressure. Motivated by the experimental study of [63], we investigate the effect of elasticity on the droplet impact on a superhydrophobic, elastic beam for a wide range of Weber numbers.…”

Dynamics of a flexible superhydrophobic surface during a drop impact

“…The impact force of the droplet, which scales as F ∼ ρU 0 2 πD 0 2 , 23 excites the flexible surface to vibrate at its first-mode natural frequency, f s = (1/2π) √ k s /m s . [24][25][26] Here, m s is the effective mass and k s is the flexural rigidity. During spreading, the initial kinetic energy of the droplet converts to the elastic energy of the flexible surface and the surface energy of the droplet.…”

Superhydrophobic Surfaces Based on Fractal and Hierarchical Microstructures Using Two‐Photon Polymerization: Toward Flexible Superhydrophobic Films

“…Additionally, natural superhydrophobic materials are usually of good flexibility, whose effect on the wetting performance has long been ignored. It is only recently found that substrate flexibility enhances superhydrophobic performance along with surface microstructures synergistically . For instance, extended water repellency can be attributed to substrate flexibility, and other droplet effects such as impalement resistance and droplet‐substrate contact time also benefit from flexible feature.…”