Superhydrophobic hemispherical array which can realize droplet pancake bouncing phenomenon

Abstract

A superhydrophobic hemispherical array which can realize droplet pancake bouncing phenomenon is provided. The superhydrophobic hemispherical array shows an arc-shape structure which is narrow at the top and wide at the bottom, where a is the angle that substrate-gas interface goes across the gas and reaches substrate-hemisphere interface, d refers to the diameter of the contact area between hemispherical structure and substrate, s represents the space between two adjoining hemispheres, h denotes the vertical height from the top of hemisphere to substrate surface, and 70°≤a≤90°, 900 μm≤d ≤1700 μm, s≤550 μm, 600 μm≤h≤1100 μm, respectively. The superhydrophobic hemispherical array has a water contact angle larger than 150° and roll-off angle lower than 10°.

Claims

1. A superhydrophobic hemispherical array which can realize droplet pancake bouncing phenomenon, wherein the superhydrophobic hemispherical array shows an arc-shape structure which is narrow at the top and wide at the bottom, where a is the angle that substrate-gas interface goes across the gas and reaches substrate-hemisphere interface, d refers to the diameter of the contact area between hemispherical structure and substrate, s represents the space between two adjoining hemispheres, h denotes the vertical height from the top of hemisphere to substrate surface, and 70°≤a≤90°, 900 μm≤d≤1700 μm, s≤550 μm, 600 μm≤h≤1100 μm, h/d≥0.48, respectively; the superhydrophobic hemispherical array has a water contact angle larger than 150° and roll-off angle lower than 10°.

Description

DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic illustration of the structural parameters of a superhydrophobic hemispherical array.

(2) FIG. 2 shows a structural diagram of the superhydrophobic hemispherical array with a=71°, d=1570 μm, s=160 μm, and h=890 μm.

(3) FIG. 3 shows the bouncing dynamics of a 21.0 μL water droplet impacting on the superhydrophobic hemispherical array with a=71°, d=1570 μm, s=160 μm, and h=890 μm.

(4) FIG. 4 shows a SEM image of a magnesium (Mg) alloy mold with hemispherical micro-dimple array on a scale of 300 μm.

DETAILED DESCRIPTION

(5) The specific embodiments of the present invention will be further described below in conjunction with the drawings and technical solutions.

Embodiment

(6) A superhydrophobic hemispherical array which can realize droplet pancake bouncing phenomenon is presented in FIG. 2. It shows an arc-shape structure which is narrow at the top and wide at the bottom, where the angle that substrate-gas interface goes across the gas and reaches substrate-hemisphere interface is a=71°, the diameter of the contact area between hemispherical structure and substrate is d=1570 μm, the space between two adjoining hemispheres is s=160 μm, the vertical height from the top of hemisphere to substrate surface is h=890 μm, and the height-diameter ratio reaches 0.56. The aforementioned superhydrophobic hemispherical array has a water contact angle of 160° and roll-off angle of 3°. As shown in FIG. 3, water droplet of 21.0 μL impacts on such a superhydrophobic hemispherical array and exhibits a pancake bouncing state.

(7) The preparation process of the aforementioned superhydrophobic hemispherical array capable of realizing droplet pancake bouncing phenomenon is as follows: (1) Pre-treatment: a Mg alloy plate of 30 mm×40 mm×2 mm was cleaned with acetone to degrease, mechanically polished using #800 and #1500 abrasive paper to remove surface oxide layer, then ultrasonically rinsed with deionized water, and drying. (2) Mask preparation: the pre-treated Mg alloy plate was sequentially attached with photopolymer resist dry film HT200 and a mask with 600 μm hole diameter and 1.9 mm space, then exposed to a UV irradiation (360 nm) for 30 s to initiate photopolymerization, subsequently developed in a 5 wt % NaCO.sub.3 solution for 2 min, and finally the masking patterns were copied onto the dry film. (3) Electrochemical machining: the anodic marked Mg alloy plate and cathodic Cu plate of equal size were installed on the side punching fixture, which were separated by a distance of 1 mm With the pulse parameters of 14 A.Math.cm.sup.−2 for current density, 20 kHz for frequency and 30% for duty cycle, the marked Mg alloy plate was electrochemical etched in a 15 wt % NaNO.sub.3 electrolyte solution for 2 min which could fill the gap between electrodes through an electrolyte circulating system. Then, the Mg alloy plate was taken out and immersed into a 5 wt % NaOH solution for 4 min to remove the film. After subsequent cleaning and drying, the Mg alloy mold with hemispherical micro-dimple array was just prepared, as shown in FIG. 4. (4) Micro/nano-scale structures construction: the Mg alloy mold obtained in step 3 was processed by nanosecond laser scanning at the frequency of 20 kHz, power of 50 W and scanning speed of 500 mm.Math.s.sup.−1 followed by ultrasonic cleaning with deionized water and drying. (5) Replication: PDMS solution was poured on the Mg alloy mold obtained in step 4, deaerated in vacuum for 2 h, then baked at 60 C for 6 h for the curing process, and finally the PDMS hemispherical array was easy to demould by hand. (6) Superhydrophobic treatment: the PDMS hemispherical array obtained in step 5 was immersed into a 1 wt % ethanol solution of fluoroalkylsilane for 40 min and dried by heating. Thus, a superhydrophobic hemispherical array was fabricated, as shown in FIG. 2.