Biomimetics inspired surfaces for drag reduction and oleophobicity/philicity

Abstract

The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices, and processes which provide desirable properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature and possess properties of interest. There are a large number of objects including bacteria, plants, land and aquatic animals, and seashells with properties of commercial interest. Certain plant leaves, such as lotus (Nelumbo nucifera) leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical surface roughness and presence of a wax layer. In addition to a self-cleaning effect, these surfaces with a high contact angle and low contact angle hysteresis also exhibit low adhesion and drag reduction for fluid flow. An aquatic animal, such as a shark, is another model from nature for the reduction of drag in fluid flow. The artificial surfaces inspired from the shark skin and lotus leaf have been created, and in this article the influence of structure on drag reduction efficiency is reviewed. Biomimetic-inspired oleophobic surfaces can be used to prevent contamination of the underwater parts of ships by biological and organic contaminants, including oil. The article also reviews the wetting behavior of oil droplets on various superoleophobic surfaces created in the lab.

Keywords: aquatic animals; biomimetics; drag; lotus plants; shark skin; superhydrophobicity; superoleophobicity.

Figure 1

Two examples from nature: (a) Lotus effect [12], and (b) scale structure of shark reducing drag [21].

Figure 2

Schematic of velocity profiles of fluid flow without and with boundary slip. The definition of slip length b characterizes the degree of boundary slip at the solid–liquid interface. The arrows represent directions of fluid flow.

Figure 3

Schematic of the experimental flow channel connected with a differential manometer. The thickness, width, and length of the channel are H, W, and L, respectively.

Figure 4

(a) SEM micrographs taken at top view, 45° tilt angle side view and 45° tilt angle top view, show shark skin (Squalus acanthias) replica. The shark skin replica shows only three ribs on each scale. (b) optical microscope images (shown at two magnifications) show the rib-patterned surface fabricated as a model of artificial shark skin surfaces [21].

Figure 5

SEM micrographs taken at 45º tilt angle (shown using three magnifications) of nanostructure on flat replica, microstructures, and hierarchical structure. Nano and hierarchical structures coated with 0.8 μg/mm² of Lotus wax after storage for seven days at 50 °C with ethanol vapor. Flat epoxy resin and microstructure were covered with flat Lotus wax [21].

Figure 6

Pressure drop as a function of flow rate in the channel with various surfaces using water flow. The figure in the bottom is magnified in flow rate between 0 and 500 μL/s. Data are compared with predicted pressure drop values for a hydrophilic surface obtained using Equation 1 for laminar and turbulent flows (solid lines) [21].

Figure 7

Bar chart showing the slip length in the channel with various surfaces using water flow in laminar flow (0 < Re < 300). The slip length was calculated using Equation 8 and the pressure drop measured on various surfaces. The error bars represent one standard deviation [21].

Figure 8

Pressure drop as a function of flow rate in the channel with flat acrylic resin and rib-patterned surface using water flow. The figure in the bottom is magnified for flow rate between 0 and 500 µL/s. Data are compared with predicted pressure drop values for a hydrophilic surface obtained using Equation 1 for laminar and turbulent flows (solid lines) [21].

Figure 9

Pressure drop as a function of flow rate in the channel with various surfaces using air flow. The figure at the bottom is magnified for flow rate between 0 and 50 mL/s. Data are compared with predicted pressure drop values for a hydrophilic surface obtained using Equation 1 for laminar and turbulent flows (solid lines) [21].

Figure 10

Pressure drop as a function of flow rate in the channel with flat acrylic resin and rib-patterned surface using air flow. Data are compared with predicted pressure drop values for a hydrophilic surface obtained using Equation 1 for laminar and turbulent flows (solid lines) [21].

Figure 11

Schematics of a droplet of liquid showing philic/phobic nature in three different phase interface on the surface – θ_W, θ_O, and θ_OW are the static contact angles of a water droplet, an oil droplet, and an oil droplet in water, respectively [20].

Figure 12

Schematics of a solid–water–oil interface system. A specimen is first immersed in water phase, then an oil droplet is gently deposited using a microsyringe, and the static contact angle in the system measured [20].

Figure 13

SEM micrographs taken at a 45° tilt angle showing two magnifications of (a) the micropatterned surface, (b) hierarchical structure and nanostructure with three-dimensional platelets on the surface fabricated with 0.2 µg/mm² mass of n-hexatriacontane, and (c) shark skin (Squalus acanthias) replica. [20].

Figure 14

Optical micrographs of droplets in three different phase interfaces on flat epoxy resin and micropatterned surface without and with C₂₀F₄₂. Left images: a water droplet is placed on a surface in air. Middle images: an oil droplet is placed on a surface in air. Right images: an oil droplet is placed on a solid surface in water [20].

Figure 15

Static contact angle as a function of geometric parameters for water droplet (circle) and oil droplet (cross) in air (top), and oil droplet in water (triangle) (bottom) compared with predicted static contact angle values obtained using Wenzel and Cassie–Baxter equations (solid lines) with a measured value of θ₀ for the micropatterned surfaces [20].

Figure 16

Static contact angle as a function of geometric parameters for water droplet (circle) and oil droplet (cross) in air, and oil droplet in water (triangles) compared with predicted static contact angle values obtained using the Wenzel and Cassie–Baxter equations (solid lines) with a measured value of θ₀ for the micropatterned surfaces with C₂₀F₄₂ [20].

Figure 17

Optical micrographs of droplets in three different phase interfaces on nanostructure and hierarchical structure fabricated with 0.2 µg/mm² mass of n-hexatriacontane. Left images: a water droplet is placed on a surface in air. Middle images: an oil droplet is placed on a surface in air. Right images: an oil droplet is placed on a solid surface in water [20].

Figure 18

Optical micrographs of droplets in three different phase interfaces on shark skin replica without and with C₂₀F₄₂. Left images: a water droplet is placed on a surface in air. Middle images: an oil droplet is placed on a surface in air. Right images: an oil droplet is placed on a solid surface in water [20].