Hierarchical Nanotexturing Enables Acoustofluidics on Slippery yet Sticky, Flexible Surfaces

Abstract

The ability to actuate liquids remains a fundamental challenge in smart microsystems, such as those for soft robotics, where devices often need to conform to either natural or three-dimensional solid shapes, in various orientations. Here, we propose a hierarchical nanotexturing of piezoelectric films as active microfluidic actuators, exploiting a unique combination of both topographical and chemical properties on flexible surfaces, while also introducing design concepts of shear hydrophobicity and tensile hydrophilicity. In doing so, we create nanostructured surfaces that are, at the same time, both slippery (low in-plane pinning) and sticky (high normal-to-plane liquid adhesion). By enabling fluid transportation on such arbitrarily shaped surfaces, we demonstrate efficient fluid motions on inclined, vertical, inverted, or even flexible geometries in three dimensions. Such surfaces can also be deformed and then reformed into their original shapes, thereby paving the way for advanced microfluidic applications.

Nanoscale “Earthquake” Effect Induced by Thin Film Surface Acoustic Waves as a New Strategy for Ice Protection

“…Meanwhile, the streaming or microflow inside the droplet is also enhanced with the increased SAW power. [ 10,18,32 ] The stronger acoustic streaming force also enhances the detachment of ice nucleolus from the surface of devices and leads to the delayed icing generation with the increase of power.…”

“…A hydrophobic layer was further coated onto the ZnO film to enhance the hydrophobicity of the working surface, as illustrated in Figure a,b (in a 3D view with different dimension scales). At a given freezing temperature, when a water droplet is positioned on the ZnO thin film/Al surface, the surface hydrophobicity could offer a hybrid state of Wenzel and Cassie–Baxter states, [ 18,19 ] as illustrated in Figure 1b. Ice nucleation and growth will happen when a thermodynamic equilibrium is reached at the water/structure interface, although the icephobic coating with the nano–micro‐structured ZnO/Al surface could potentially delay the ice formation.…”

Acoustic Waves for Active Reduction of Contact Time in Droplet Impact

“…Additionally, as described in the results section, another resistive force is formed due to the CAH along the TPCL. By assuming that the contact area is a complete circle during the impingement, the tangential CAH resistive force, can be calculated by [59]:…”

Slippery liquid‐infused porous surfaces: The effect of oil on the water repellence of hydrophobic and superhydrophobic soils

“…As shown by the Wenzel state (Figure 1c), a high contact angle surface is not necessarily a slippery surface with good water‐shedding properties, where water‐shedding refers to the ease with which a droplet moves on a surface (via rolling or slipping) with minimal force (Quére et al, 2003). Excellent water‐shedding abilities can be achieved through low contact angle hysteresis irrespective of the absolute value of the static contact angle (see Tao et al, 2020) and is typically seen when a droplet interacts with a surface in a Cassie‐Baxter state where there is less solid–liquid contact (McHale, Newton, & Shirtcliffe, 2005). However, water‐shedding surfaces can also be created by exposing structured, rough or granular/porous hydrophobic surfaces to oil, to create bio‐inspired slippery liquid‐infusedporous surfaces (Wong et al, 2011) and lubricant‐impregnated surfaces (LIS) (Lafuma & Quéré, 2011; Smith et al, 2013).…”