Role of anisotropic pinning and liquid properties during partial rebound of droplets on unidirectionally structured hydrophobic surfaces

Highlights

• Drop impact dynamics: a strong function of microstructures and liquid properties.

• In-depth physics of partial rebound on anisotropic microstructured surfaces.

• Importance of capillary wicking and contact line pinning during partial rebound.

• Partial rebound characteristics as a function of excess rebound energy.

Abstract

Droplet impact dynamics on super-hydrophobic micro-structured surfaces is vital in a multitude of processes such as spray coating, cooling, and inkjet printing etc. Majority of the research has been focused on the rebound phenomena while partial rebound is relatively underreported. Recent studies indicate that partial rebound is encountered where transition of wetting state may take place on super-hydrophobic surfaces. Herein, we report that satellite drops are formed during partial rebound of droplets even on hydrophobic surfaces with unidirectional topography created using a mask-less and stamp-less fabrication process. The simultaneous capillary driven retraction along the wrinkles and the strong contact line pinning across these anisotropic substrates leads to formation of satellite drops on hydrophobic surfaces. Excess rebound energy has been evaluated as functions of contact angle, contact angle hysteresis and the maximum spreading diameter to explain partial rebound on such surfaces with potential applications in the design of novel, functional surfaces.

Introduction

The unique features created by water, milk and mercury drop on their impact on solid surfaces have first been witnessed and reported by Worthington (1876). The instantaneous/rapid nature of droplet impact dynamics has intrigued scientists for a long time with diversified applications in a plethora of fields like inkjet printing (de Gans et al., 2004), metal spray coating (Yarin, 2006), agriculture (Wirth et al., 1991), fire controlling sprinklers (Mawhinnq and Associates, 1997), fabrication of self-cleaning hydrophobic surface (Matin et al., 2016) and metallurgical sectors (Ravikumar et al., 2014) among others. In general, the dynamic behavior of drop impact can be divided into three stages namely, spreading, retraction and equilibrium. High speed imaging has enabled researchers to have a better understanding of the rapid dynamics of the aforementioned phenomena (Chandra and Avedisian, 1991, Thoroddsen et al., 2008). When a drop impacts on a solid surface, it may experience deposition, splashing, rebound and partial rebound. These outcomes are due to several factors such as liquid property and condition (surface tension, viscosity, density, drop temperature) (Aytouna et al., 2010, Crooks et al., 2001, Šikalo et al., 2002, Zhang and Basaran, 1997), drop impingement height (drop impact velocity) (Bartolo et al., 2006, Patil et al., 2016c), droplet size (Visser et al., 2012), type of the liquids (newtonian/non-newtonian) (Boyer et al., 2016, Guémas et al., 2012), nature of the impingement surface (Kannangara et al., 2006) (hydrophobic, hydrophilic) and associated roughness (Kannangara and Shen, 2008) and temperature (Bhardwaj et al., 2010, Bhardwaj and Attinger, 2008, Tran et al., 2013, Tran et al., 2012) etc.

Bulk of the research on drop impact dynamics has been focused on the determination of drop spreading behaviour on flat surfaces (Fard et al., 1996, Fukai et al., 1995, Madejski, 1976, Roisman, 2009, Scheller and Bousfield, 1995, Seo et al., 2015). Numerous research groups have investigated the influence of surface wettability such as hydrophilicity, hydrophobicity, and super hydrophobicity on drop impact dynamics (Mech and Sci, 2006). Surface wettability can be altered either by chemical means (Almohammadi and Amirfazli, 2017, Antonini et al., 2012) or by varying topography (Bandyopadhyay and Sharma, 2010, Parihar et al., 2018a, Tsai et al., 2009). Recently topographically dependent wettability of surfaces has gained importance for various applications (Kumar et al., 2019, Liu et al., 2015, Patil et al., 2016a, Yeong et al., 2014).

Microstructures on flat substrates increase the surface roughness, thereby modifying the contact angle in the process and make the surfaces hydrophobic/super-hydrophobic. Some of the examples of naturally occurring anisotropic topographical surfaces include shark skin or butterfly wing for low-drag locomotion, whereas isotropic topography on lotus leaf results in lower wettability (Bixler and Bhushan, 2014). Artificial anisotropic surface induced wettability can be utilized to provide directional wetting/movement in microfluidics, and such structured surfaces can be used in designing self-cleaning, drag reduction, defrost, and antifouling surfaces etc. (Frankiewicz and Attinger, 2016). Droplet impact dynamics on topographical surface is a complicated process and due to the lower wettability, the droplet may rebound from the surface thereby causing problems such as unwanted splashing and uneven coverage, to name a few. Therefore, the phenomena need to be explored while designing relevant processes e.g., ink-jet printing, impingement cooling etc. Weisensee et al. (2016) have fabricated nanostructured surfaces using multiple coating of the commercially available hydrophobic nanoparticles on glass and polymer substrates and studied the effect of surface elasticity on droplet dynamics.

The existing literature based on the drop impact dynamics on hydrophobic surfaces mainly stresses the determination of maximum spreading ratio, drop contact time and rebound criteria (Izbassarov and Muradoglu, 2016); whereas, there are scant reports on the partial rebound criteria. Complete rebound refers to the detachment of the entire droplet from the surface, post retraction, while partial rebound represents the formation of an elongated liquid column during retraction followed by the generation of a daughter (satellite drop) droplet as it pinches off from the parent drop and the residual parent drop remains stuck to the surface. Antonini et al. (2014) have observed the partial rebound of a droplet at higher values of the Weber number (We). They have also reported that the genesis of partial rebound is due to the meniscus penetrating the surface topography (at higher values of We). Further, Chen et al. (2011) have witnessed the partial rebound phenomenon on a dual scale roughened super hydrophobic surfaces. The origin of the aforementioned observations is owed to the partial pinning of the impinging droplet for intermediate values of impact velocities. Tsai et al. (2009) have worked on the combined effect of the Weber number and surface roughness on micro and nano structured super hydrophobic surfaces. They have found that for We ≤ 120, the effect of surface roughness (r) at nanoscale plays an insignificant role on drop impact dynamics, whereas, for We > 120, ‘r’ acts as the determining factor on post impact drop dynamics.

Megaridis et al. (2004) have observed that the partial rebound phenomenon is favoured for smaller drop diameter and high impingement velocity (for Sn/Pb molten metal drop). Thus, so far the partial rebound has been explained on the basis of impact velocity (Z Wang et al., 2007), We (Zhang et al., 2016), viscoelasticity (Tsai et al., 2009), addition of particles (Qiao et al., 2013), liquids type (e.g. water, molten metal) and the transition of the wetting state on isotropic microstructured surfaces (superhydrophobic) (Bange and Bhardwaj, 2016, McCarthy et al., 2012, Patil et al., 2016b). A recent work emphasized the role of unidirectional structured surfaces and reported results with varying pitch. Complete rebound was observed due to the presence of an air gap at low values of pitch. Rebound with a small drop left over on the surface or no rebound at higher pitch is attributed to the transition of wetting states (Malla et al., 2017). In another recent work insight is provided into the petal bouncing and fragmentation phenomena (Guo et al., 2018) on super-hydrophobic ridge microstructures. However, the partial rebound phenomena, especially in terms of anisotropic pinning at the ridges and capillary wicking on the underlying channels is still being explored.

In the present work, a mask-less and stamp-less fabrication process has been employed to obtain a unidirectional wrinkled topography on nichrome surfaces (hydrophilic). These topographies are then successfully transferred to polydimethylsiloxane (PDMS) by soft lithography technique to observe the relative importance of hydrophobicity vis-à-vis the unidirectionality of topography serving as hydrophobic micro channels. The wrinkles are characterized by two parameters: (i) Wavelength (λ), which is the peak to peak distance of the two wrinkles (ii) Amplitude (A), defined as the height of wrinkles as shown in Parihar et al. (2018b). The fabrication parameters are adjusted to ensure that one of the resulting surfaces is having a lower amplitude (height of the channels; A = 3.55 µm) and higher wavelength (width of the channels; λ = 48 µm), with an aspect ratio of 13.52. The other channel is made wider with an aspect ratio of 2.15 (A = 13 µm, λ = 28 µm). The first substrate is named as S1 while the second will be referred to as S4. In between S1 and S4, two additional surfaces, S2 (A = 10 µm, λ = 32 µm) and

S3 (A = 12 µm, λ = 30 µm) having intermediate topography have also been constructed for comparison. Thus, the focus of the current work encompasses a comparative study of the drop impact dynamics in between the flat PDMS and unidirectional wrinkled PDMS (hydrophobic) surfaces having topographical variation. The deposition and partial rebound criteria for different liquids on flat and unidirectional micro-wrinkled PDMS surfaces having variable state of wetting have also been studied herein.

We have employed the unidirectional wrinkled surfaces with topographical variation to understand the role of the varying anisotropy. Moreover, the effect of anisotropy assisted contact angle hysteresis (CAH), hydrophobicity, and capillarity driven contact line motion on the partial rebound phenomena are explored in detail. To analyze the effect of the liquid properties in a comprehensive manner, the relevant dimensionless numbers have been calculated highlighting the underlying physics of the droplet impact phenomena. Since the topographies are anisotropic in nature, the drop impact dynamics have been captured from two mutually perpendicular directions - one parallel and the other perpendicular to the channels. The temporal variations in the spreading diameter and height are measured to explore the anisotropic shape after impact that will be correlated to the partial rebound phenomena. The role of unidirectional microstructures on the contact line dynamics of an impacting droplet and the possibility of partial rebound on a hydrophobic surface are the focus and novelty of the present study as compared to the information available in the literature where the transition of wetting states on a superhydrophobic surface is featured to be the main reason behind the rebound phenomena. The results will be useful in a number of microfluidic applications and will provide a new insight in the field of droplet impact dynamics.