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New Concepts at the Interface: Novel Viewpoints and Interpretations, Theory and Computations
Tunable Droplet Breakup Dynamics on Micro-Pillared Superhydrophobic Surfaces Rui Zhang, Pengfei Hao, Xiwen Zhang, Fenglei Niu, and Feng He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01480 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Tunable Droplet Breakup Dynamics on Micro-Pillared Superhydrophobic Surfaces Rui Zhang1,2, Pengfei Hao*1,2, Xiwen Zhang1,2 Fenglei Niu3 and Feng He*1,2 1
Department of Engineering Mechanics, Tsinghua University, Beijing, 100084, China 2
3
Key Laboratory of Applied Mechanics, China
School of Nuclear Science and Engineering, North China Electric Power University, Beijing 102206, China
Abstract Functional materials with controllable droplet breakup property have extensive application prospects in aircraft anti-icing, spraying cooling, surface coating and so on. Here we show that introducing micro-pillar arrays with various morphology to fabricate superhydrophobic surfaces could either facilitate or suppress droplet splitting. The spacing and height of micro-pillars play an essential role in tuning the splitting patterns. Delayed splashing occurs on dense pillars which support the liquid lamella and provide channels for air to escape. A novel droplet breakup mechanism is found on sparse tall pillars, which rises from the instability of lateral liquid jets and significantly reduces the droplet breakup threshold. The critical Weber number of the rupture of low-viscous liquid is solely determined by the geometric parameters of micro-pillars and droplets. This work unveils the impact of ordered microstructures on the droplet breakup dynamics and provides a quantitative analysis of the geometric parameters in revising the breakup criteria.
1. Introduction Droplet breakup is of great importance in a wide variety of industrial applications, such as inkjet printing, pesticide deposition, liquid fuel combustion, spray cooling, surface coating, aircraft anti-icing, vehicle soiling and so on.1-8 Depending on the application, either promotion or suppression of the liquid breakup is desired. Droplet breakup could be divided into two types: rim splash and internal rupture. Since the pioneering exploration of Worthington in 1876,9 a lot of efforts have been made on understanding the liquid splashing phenomenon on both smooth and rough solid surfaces.10-34 The splashing dynamics is dominated by a complex interaction between the liquid properties (droplet size, velocity, surface tension, viscosity…), 1
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the wetting property and roughness of the surface, together with the ambient gas pressure. Many stability analysis methods have been proposed to interpret the complicated physical mechanisms behind splashing on both smooth surfaces and textured surfaces,10-17 however, the phenomenon of droplet splashing remains far from being fully understood. Two distinct categories of splashing have been proposed: one is the prompt splash, which releases droplets from the rupture of the liquid rim just at the impact moment; the other is corona splash, in which the liquid sheet is levitated from the solid and breaks up into tiny droplets subsequently. To express the splashing threshold in both patterns, various empirical splashing criteria have been proposed recently.18-29 It has been proved that the splashing threshold could be controlled to some extent, by tuning the surface roughness, the impact angle and the surrounding gas pressure.18,29-34 In the collision between a droplet and a solid surface, the surface roughness of the same size might have a distinctly different effect on droplets with different sizes. However, the influence of the scaling effect between the droplet diameter and the surface roughness (or the geometric parameters of regular micro-pillar arrays) on droplet splashing or breakup criteria was seldom mentioned or discussed. The surface morphology plays an essential role in achieving the water-repellency of superhydrophobic surfaces.35-37 High-speed droplets tend to penetrate into and wet the microstructures on textured superhydorphobic surfaces, which is also noted as the Cassie-to-Wenzel
wetting
state
transition
and
results
in
the
loss
of
the
superhydrophobicity.36,37 Since the anti-penetrating pressure provided by the capillary force on micro-pillared surfaces can be expressed as Pan~σ/S,36 reducing the spacing S of micro-pillars could effectively improve the pressure stability of the superhydrophobic substrates against droplet collision. Besides, the textured surfaces with nanoscale roughness close to the critical nucleus radius were found to exhibit better icephobicity than those with a larger
feature
size.38,39
Moreover,
hierarchical
superhydrophobic
surfaces
with
micro/nano-structures could also lead to the rapid removal of small condensed droplets due to the coalescence induced jumping.40,41 Therefore reducing the geometric parameters of textured superhydrophobic surfaces was popularly proposed in most previous studies, to improve the pressure stability against droplet collision, and the self-cleaning and anti-icing property of superhydrophobic surfaces.36-41 2
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It was reported recently that the droplet impact dynamics could be remarkably modified by introducing macrotextures to superhydrophobic surfaces,42-48 where the rectified bouncing pattern of droplets,42 the symmetry breaking of the liquid spreading 43-47 or the fragmentation of droplets 44-49 led to a large reduction of contact time. We focus on here the droplet breakup and bouncing behaviors on micro-pillared superhydrophobic surfaces with a wide range of sizes, from micrometer-scale to sub-millimeter-scale, and try to control the droplet breakup threshold by manipulating the geometric arrangement of the micro-pillars. A new droplet breakup mode, which was quite different from both prompt splash and corona splash, was found on superhydrophobic surfaces with sparse tall pillars. The droplet breakup threshold was markedly decreased due to the direct disturbance of the sparse tall pillars, making large droplets splitting into smaller satellite droplets, which bounced off the surfaces in a shorter time. These findings provide new ideas in both maximization of water-repellency of superhydrophobic materials in potential applications, such as aircraft anti-icing and vehicle anti-soiling, and enhanced heat exchange in spray cooling and liquid fuel combustion. On the contrary, suppressing the splashing or breakup of high-speed droplets, which increases the deposition rate of liquid, also has a wide range of potential applications, such as in inkjet printing, pesticide spraying and surface coating.
2. Experimental Section 2.1 Material Preparation. To prepare the hierarchical superhydrophobic surfaces, we first fabricated various micro-pillars on silicon surfaces using standard photolithography technology and etching of inductively coupled plasma (ICP). Three essential geometric parameters of the designed textured surfaces were varied: the edge length P (25µm, 50µm, 100µm, 200µm), the spacing S (25µm, 50µm, 100µm, 200µm, 300µm, 500µm, 700µm, 900µm), and the height h (20µm, 50µm, 100µm, 230µm, 330µm). After the preparation of the surface topography, all the prepared textured surfaces with micro posts together with the contrasted smooth silicon wafers were given the same superhydrophobic treatment. The detailed procedures of the nano-coating treatment are described in Refs.[49,50]. The microscopic morphology of several typical textured surfaces with various micro-posts (edge length P=100µm, height h=330µm, spacing S=25µm, 50µm, 100µm, 200µm, 300µm, and 3
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500µm) after superhydrophobic treatment is shown in Figure 1a. Figure 1b shows the Scanning Electric Microscope (SEM) image of the single-tier superhydrophobic silicon surface with nano-coating. The average roughness of the nano-coating on both the micro-pillared surfaces and the single-tier surfaces was measured to be approximately 11nm, using the surface morphology measurement system (Talysurf PGI 1230, made in UK). The static and dynamic contact angle on the substrates was measured from water drops (with a needle) of 5µL with a standard contact angle goniometer. The apparent contact angle and contact angle hysteresis of the single-tier superhydrophobic surfaces are 150.2°±1.3° and 8.2°±0.6°, respectively. The apparent contact angle and contact angle hysteresis of the hierarchical superhydrophobic surfaces with various micro-pillars are approximately 147.5°±2.1°~159.4°±1.8° and 10.3°±0.9°~28.9°±2.1°. The geometric parameters and wetting properties of the single-tier superhydrophobic surface and several typical hierarchical superhydrophobic surfaces with micro-pillars are shown in Table 1. These values are the averages of three measurements. Since the intrinsic chemical properties of all the superhydrophobic substrates are similar, the different macroscopic wetting properties of the various prepared micro-pillared superhydrophobic surfaces simply result from the different geometric arrangements of the micro-post primary structure, instead of the intrinsic chemical properties. Therefore the effect of wetting properties on droplet collisions can be reflected by that of the geometric parameters of various micro-pillared surfaces.
Figure 1. Microscopic morphology of experimental substrates. (a) SEM images of the surface morphology of several typical hierarchical superhydrophobic surfaces with micro-pillar arrays of the same edge length (P=100µm) and height (h=330µm), and at varied spacing (S=25µm, 50µm, 4
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100µm, 200µm, 300µm, 500µm). (b) Microscopic morphology (SEM images) of the single-tier superhydrophobic surface.
Table. 1 Geometric parameters and wetting properties of the single-tier superhydrophobic surface and several typical hierarchical superhydrophobic surfaces with micro-pillars Substrate
Edge length P (µm)
Spacing S (µm)
Height h (µm)
Solid Fraction (Before nano-treatme nt) φ
Contact Angle
Contact Angle Hysteresis
Nano-Si P100S25 P100S50 P100S100 P100S200 P100S300 P100S500 P100S700 P100S900
100 100 100 100 100 100 100 100
25 50 100 200 300 500 700 900
330 330 330 330 330 330 330 330
1 0.64 0.44 0.25 0.11 0.063 0.028 0.016 0.01
149.4°±1.2° 150.8°±1.9° 152.6°±1.4° 155.5°±1.6° 159.4°±1.8° 157.0°±1.4° 147.5°±2.1° 148.5°±1.9° 150.1°±1.7°
10.9°±0.5° 28.2°±1.3° 22.3°±1.8° 27.5°±1.5° 23.4°±0.9° 21.2°±1.5° 20.7°±1.4° 18.4°±1.8° 17.6°±1.6°
Notes: φ is the initial geometric solid fraction of the micro-pillared surfaces without nano-coatings, which is defined as the ratio between the top area of the primary structure and the apparent area of the surface φ=2 ⁄P+S2 here. The final solid fraction of the substrates after nano-particle treatment is smaller than the initial geometric solid fraction φ. The initial geometric solid fraction φ is applied here to express and emphasize the characteristics of the primary structure, which are significant in determining the wetting properties of different micro-pillared superhydrophobic surfaces.
2.2 Experimental Setup. The experiments were performed by dripping deionized water droplets with millimetric size (D0=1mm±0.05mm, 1.4mm±0.07mm, 2mm±0.1mm) onto the prepared superhydrophobic substrates. The droplets were generated from a fine capillary tube which was equipped with a syringe pump. The height of the tube was adjusted to change the impact speed. The impact velocity ranges from 0.9m/s to 4.4m/s, corresponding to 10
