Slippery Liquid-Infused Porous Surfaces and Droplet Transportation by Surface Acoustic Waves

On a solid surface, a droplet of liquid will stick due to the capillary adhesion, and this causes low droplet mobility. To reduce contact line pinning, surface chemistry can be coupled to micro- and/or nanostructures to create superhydrophobic surfaces on which a droplet balls up into an almost spherical shape, thus, minimizing the contact area. Recent progress in soft matter has now led to alternative lubricant-impregnated surfaces capable of almost zero contact line pinning and high droplet mobility without causing droplets to ball up and minimize the contact area. Here we report an approach to surface-acoustic-wave- (SAW) actuated droplet transportation enabled using such a surface. These surfaces maintain the contact area required for efficient energy and momentum transfer of the wave energy into the droplet while achieving high droplet mobility and a large footprint, therefore, reducing the threshold power required to induce droplet motion. In our approach, we use a slippery layer of lubricating oil infused into a self-assembled porous hydrophobic layer, which is significantly thinner than the SAW wavelength, and avoid damping of the wave. We find a significant reduction (up to 85%) in the threshold power for droplet transportation compared to that using a conventional surface-treatment method. Moreover, unlike droplets on superhydrophobic surfaces, where interaction with the SAW induces a transition from a Cassie-Baxter state to a Wenzel state, the droplets on our liquid-impregnated surfaces remain in a mobile state after interaction with the SAW.

Figure 1

Three states of surface wetting influenced by topography. (a) The Cassie-Baxter state involves the droplet bridging between the tips of the surface features, thus, reducing the solid-liquid interfacial area. The droplet has a small footprint and high mobility. (b) The Wenzel state involves the droplet penetrating between the surface features, thus, increasing the solid-liquid interfacial area. The droplet sticks to the surface and has low mobility. (c) The lubricant-impregnated surface retains lubricant due to it wetting a high-surface-area nanoparticle porous structure whose hydrophobic properties ensure the lubricant is not displaced by water. The droplet has a large footprint and high mobility.

Figure 2

(a) Schematic illustration of a SAW device with a SLIP surface. (b) Schematic illustration of the interaction between a SAW and a liquid droplet on a SLIP surface. The Rayleigh angle ${\theta }_R$ and a larger droplet footprint enabling effective transfer of SAW energy to drive the droplet motion are indicated. (c) Illustration of the enlarged interface between the droplet and the lubricant oil.

Figure 3

SEM images of hydrophobic silica nanoparticle surfaces created using different numbers of Glaco coatings: (a) single coating, (b) double coating, and (c) four coats. Insets are the larger magnification SEM images. (d)–(f) The corresponding images for a $2\text{−}\mu \mathrm{l}$ droplet of water on each of the above surfaces.

Figure 4

Left-hand $y$ axis shows the sliding angles (i.e., angles of the substrate tilt needed to initiate the droplet sliding) of $2\mu \mathrm{l}$ DI water on a SLIP surface created using an oil withdrawal speed of $0.1\mathrm{mm}/\mathrm{s}$ as a function of the number of Glaco coatings. Right-hand $y$ axis shows the advancing and receding contact angles of $2\mu \mathrm{l}$ DI water on the surface before oil impregnation as a function of the number of Glaco coatings and also shows the apparent contact angle of $2\mu \mathrm{l}$ DI water on a SLIP surface created using an oil withdrawal speed of $0.1\mathrm{mm}/\mathrm{s}$ as a function of the number of Glaco coatings.

Figure 5

The SAW microfluidic performance of a $2\mu \mathrm{l}$ DI water droplet on SAW devices with double Glaco coatings and an applied SAW power of 2.5 W at a frequency of 22.44 MHz after various durations. (a)–(e) show the SAW device without silicone oil. (a) Before applied SAW power, (b) applied SAW power of 0.05 s, (c) 0.12 s, (d) 0.19 s, and (e) after turning off the SAW power. (f)–(j) show the SAW device impregnated with silicone oil and the oil treatment of $0.1\mathrm{mm}/\mathrm{s}$. (f) Before applied SAW power, (g) applied SAW power 0.05 s, (h) 0.12 s. (i) 0.19 s, and (j) after turning off the SAW power.

Figure 6

(a) The threshold powers for droplet transportation of $2\mu \mathrm{l}$ DI water using a frequency of 22.44-MHz rf signal for surfaces created with two oil withdrawal speeds depending on the number of Glaco coatings. (b) Droplet velocity of $2\mu \mathrm{l}$ DI water on SAW devices using rf power of 7 W at the frequency of 22.44 MHz as a function of the number of coatings.