Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review

Abstract

Mimicking the topographic structures and designs of living surfaces (e.g., lotus leaf, pitcher plant and beetle) onto the non-living surfaces (e.g., metallic plates, glass wafers, wood and fabrics) is known as bioinspiration. Consequently, the pristine topography of the non-living surfaces is robustly modified, known as bioinspired smart surfaces, providing novel surface regimes, i.e., wetting regimes and droplet dynamic regimes. Herein, factors affecting the droplet dynamics and its applications in bioinspired smart surfaces are presented. The droplet dynamics is a complicated phenomenon being affected by the various factors, encompassing the surface roughness, axial structural interspacing (ASI), structural apex layer (SAL), surface positioning, structural alignment, liquid droplet-surface interaction (LD-SI), and various stimuli, etc. Further, the droplet dynamics can be seen many applications, such as droplet manipulation, self-cleaning effect, design of controllable chemical reactors and electric circuits, water harvesting and condensation heat transfer, and oil/water separation, amongst others. The chapter has been mainly divided in three sections enclosed between the introduction and conclusion, comprehensively elaborating the classification of surface regimes, factors affecting the droplet dynamics and the applications at lab and industrial scales. In all, the contents are expected to serve as the guideline to accelerate advancement in the surface science.

1. Introduction

Surface science has revolutionized the modern technology based on bioinspired alternatives and novel applications, attempting to control and resolve academic, industrial and societal challenges across the globe. It is important herein to explicitly define the surface so as to remove the literature anomaly because it is elusive whether to consider the top-most region as a surface or the whole of it. On the one hand, the entity under study should be called the substrate, and its top-most region should be named the surface. On the other hand, if the entity under study is named as surface (as we see in the literature), its top-most region should better be termed as the topography, meaning that the region lying under the topography is the surface. Therefore, we are also likely to adopt and utilize the latter terminologies henceforward, i.e., the topography is the part of a surface where the water droplet interacts. Altogether, surface science is based on the fundamental surface models (Young’s model, Wenzel model and Cassie-Baxter model) that are employed to understand the wettability/wetting, droplet dynamics and the involved surface engineering processes [1, 2, 3].

Surfaces can be categorized as living and non-living surfaces.

Living surfaces include birds, insects, plants and animals. Upon careful observation, it has been found that the living surfaces behave very uniquely and differently with the water droplets. For example, the rainwater cannot wet the wings of the butterfly even during rain. Water striders can move on the water surface with great ease. Water can flow on the rice leaves very fast. Water droplets can dance on the lotus leaf and finally rolls off by taking the dust with them. Water droplets can stay on the rose petals even if it is vertically standing. Fish can travel through water very efficiently, and so on. When the microscopic examination of various living surfaces is carried out, the topographic morphologies have been found different in each case, consisting of micro/nano-structures of a wide variety of geometries, arrangements and interspacing. In addition, a particular kind of layer surrounding the micro/nano-scaffolds has been discovered, which is named the epicuticular wax [4]. Therefore, a constructive conclusion can be drawn that the topography of various surfaces is the main driver that affects the water droplets. In other words, living surfaces provide different levels of adhesions and slipperiness based on the epicuticular wax.

Non-living surfaces encompass metals, wood, polymer, glass, fabric and paper, etc. Until recently, rigorous efforts have been underway, aiming to modify the topographies of the non-living surfaces with help of bioinspiration. The bioinspiration is a broad technological and scientific concept in which the micro/nano-structures of living surfaces are studied, and then similar structures are created onto the topographies of non-living surfaces, which are termed the bioinspired smart surfaces. With the bioinspired conversion of wettability (i.e., bioinspired mimicking) as depicted in Figure 1(a–c), the newly-born topographic structures include extruding topographic structures (ETS) (Figure 1b) and intruding topographic structures (ITS) (Figure 1c). The major influencers for bioinspired wettability are the axial structural interspacing (ASI) and the structural apex layer (SAL). Based on the ETS/ITS and ASI, the newly-born area on the topography is called the projected area over which the contact angle is known as the apparent contact angle θ_a (°). By dividing θ_a (°) by the equilibrium contact angle θ_e (°) of the pristine surface, the roughness R can be obtained. The SAL can be defined as the sites existing on the top of ETS/ITS responsible for introducing the same properties as the epicuticular wax does in the living surfaces. Therefore, SAL can be obtained by coating certain materials having either high surface energy (providing hydrophilic or superhydrophilic characteristics such as silicon dioxide [5]) or low surface energy (providing hydrophobic or superhydrophobic characteristics such as silanes [6, 7]). An experimental case study of bioinspired smart surface, known as the slippery liquid-infused porous surface (SLIPSs), can be seen in Figure 1(d–f). SLIPSs are prepared by getting inspiration from the pitcher plants.

Figure 1.

Bioinspired mimicking depicting (a) pristine copper surface and topography, growth of (b) ETS and (c) ITS [8]. An experimental case study demonstrating the (d) porous scaffold of pitcher plant [9], and (e) fabrication of bioinspired SLIPSs consisting of (f) porous scaffold alike pitcher plant. [Note: Microscopic images in (a), (b) and (f), as well as the contact angles and their images, belong to the author(s)].

To create ITS/ETS, there are many traditional and advanced surface engineering methods [10, 11, 12], such as chemical oxidation, chemical etching, reactive ion etching, grafting, dipping, spinning, photo-lithography, electron beam-lithography, electrodeposition, imprinting, templating, hot embossing, plasma treatment, vaporization, selective tunneling, anodizing, laser ablation, etc. Thus, by selecting the most suitable method or combination of several methods, the ETS and ITS having a wide variety of geometries can be created, for example: plate-like, wire-shaped, whisker, cone-like, square-arrays, fibre-like, vertical pillars, spikes, honeycomb-shaped, grooves, regular deep-pores, holes, channels, trenches, voids, etc., and the common feature of all geometries is their size which tends to exist at micro, nano or micro-nano mixed (hierarchical) scale [12]. Hence, droplet wetting and droplet dynamic regimes of the pristine non-living surfaces can be completely tailored. Droplet dynamics of the bioinspired smart surfaces greatly rely on the SAL and ASI of the ITS/ETS.

In brief, the bioinspired smart surfaces are more efficient and capable of controlling industrial challenges. Consequently, a wide variety of tangible applications and salient surface characteristics has been unveiled, for example, dust-free solar cells [13], non-wetting leathers [14], efficient and durable oil/water separators [15], anti-biofouling surfaces [16], anti-reflective surfaces [17], pump-free microfluidic and lab-on-chip devices [18], nanogenerators for energy harvesting [19], efficient water vapor condensation and enhanced heat transfer [20], as well as the stimuli-responsive surfaces [21], gas sensors [22], smart gating-based valves [23], and functionalized immunoassays [24].

This chapter shed a light on the bioinspired smart surfaces, and comprehensively elaborates the droplet dynamics. In between Section 1 (Introduction) and Section 5 (Conclusion), the main chapter breakdown provides three sections. By taking the examples of living surfaces, the classification of surface regimes, liquid droplet-surface interactions and the quantification criteria of wetting and dynamic regimes are included in Section 2. The major factors affecting the droplet dynamics have been unveiled in Section 3. The importance and role of droplet dynamics in emerging applications have been summarized in Section 4.

2. Classification of surface regimes and droplet-surface interactions

The surface regimes can be majorly categorized into wetting and dynamic regimes with reference to the interacting liquid droplets. In surface science, the living surfaces can provide four kinds of wetting regimes against water droplets, namely, hydrophilic, super hydrophilic, hydrophobic and superhydrophobic, as demonstrated in Figure 2. The droplet dynamics can be typically categorized into two main branches, namely, sticky and slippery regimes (Figure 2). The droplet dynamics is defined as the study of droplet growth, droplet mobility, droplet speed, droplet transport range and the underlying forces.

Figure 2.

Bioinspired classification of droplet wetting and droplet dynamic regimes. Liquid droplet-dry surface interaction (LD-DSI) and liquid droplet-wet surface interaction (LD-WSI) are depicted depending on the topographic states.

The droplet wetting regimes are conventionally quantified via equilibrium contact angle θ_e (°) that a liquid droplet makes with the surface, varying in a range of values (Figure 1a). It should be noted that, in several other studies, the inherent wettability of the pristine surfaces is denoted by the equilibrium contact angle θ_e (°), while the artificial wettability of the smart surfaces is denoted by the apparent contact angle θ_a (°). It means that the way to differentiate the pristine and smart surfaces can also be understood by the notation of contact angles. Guided by Young’s model, θ_e (°) defines the static interaction of a stationary droplet with the flat topography of a horizontally-positioned solid substrate. Apparently, the solid substrate surfaces have a compact topography, but microscopically, the voids exist consisting of certain interspacing that affects the wetting interaction and create the philic (referring to hydrophilic and superhydrophilic) and phobic (referring to hydrophobic and superhydrophobic) regimes based on the extent of droplet penetration. For the philic regime, the micro-interspacing is supposed to exist into which droplet penetrates more intensely, and the opposite holds true for the phobic regime, i.e., nano-interspacing prohibits the droplet penetration. Thus, the surface topography affects the liquid droplet-surface interaction (LD-SI) that plays a decisive role regarding the droplet shape, i.e., with dominant micro-interspacing, the extent of droplet penetration is high, so the LD-SI area tends to be large with hemi to quarter-spherical droplet shape, specifying the philic regime in range of 0° ≤ θ_e ≤ 90° [25, 26, 27]. While with dominant nano-interspacing, the LD-SI area is deemed to be small with hemi to full-spherical shape, which is called the phobic regime in the range of 90° ≤ θ_e ≤ 180° [25, 26, 27]. The LD-SI can further be divided into two classes liquid droplet-dry surface interaction (LD-DSI) and liquid-droplet wet surface interaction (LD-WSI), as depicted in Figure 2. Depending on the LD-DSI and LD-WSI, the mechanism of droplet dynamics can be entirely changed as discussed below.

The droplet dynamic regimes can be quantified by the rolling/sliding angle α (°), which defines the dynamic interaction of a mobile droplet with the underlying tilted substrate. It should be noted that the rolling and sliding angles are specified just to distinguish the different surfaces due to dissimilar dynamic mechanisms depending on the droplet-surface interactions, e.g., the droplet is driven by the rolling mechanism on superhydrophobic surfaces due to LD-DSI (case of lotus leaf-inspired surfaces), while that on the slippery liquid-infused porous surfaces (SLIPSs), the sliding mechanism occurs due to LD-WSI (case of pitcher plant-inspired SLIPSs). But in both cases, α (°) is measured by the same method, i.e., the droplet is inserted onto the surface after which it is gradually tilted, and the angle is measured when the droplet starts moving. α is mainly helpful to demarcate the slippery and sticky regimes, regardless of the dynamic mechanisms of the involved surfaces types (superhydrophobic surfaces or SLIPSs).

The slippery dynamic regime, where virtually no droplet adhesion/pinning is considered, allows the ease of droplet mobility with value of α varying in range of 0° ≤ α ≤ 5° [28]; while the sticky dynamic regime, where the droplet adhesion/pinning is variably considered, may provide different mobility behaviors with a range of 5° ≤ α ≤ 90°. A low adhesion Wenzel state or slippery Wenzel state can be conceived with a range of 5° ≤ α ≤ 50° where the droplet pinning is relatively stronger compared with that of slippery regime [29]. A high adhesion Wenzel state corresponds to the range of 50° ≤ α ≤ 90° [30], inducing the highest pinning against the droplet. A full adhesion Wenzel state occurs when α = 90° where the droplet does not move [31]. Therefore, the traditional norm defining that the Wenzel state induces the complete sticky regime (i.e., the droplet does not move at all), can be negated henceforth based on the above categorization and experimental proofs.

3. Factors affecting the droplet dynamics

Droplet dynamics is a complicated phenomenon, which is drastically affected by several salient features such as surface roughness, ASI, SAL, surface position (horizontal and inclined), topographic structural alignment (isotropic, anisotropic and gradient), LD-SI and droplet shape, and stimuli. They are elaborated as follows:

3.1 Roughness, ASI and SAL

The droplet dynamics is a very complicated phenomenon that simultaneously takes the topographic structural geometry, structural arrangement, structural direction, size scale, wetting regime, ASI (axial structural interspacing) and SAL (structural apex layer) into account.

Based on the values of θ_e/θ_a (°) and α (°), the wetting and dynamics can be inter-linked in the above-categorized regimes (philic, phobic, slippery and sticky), but this link is supposed to be weak, especially when studying the droplet dynamics. Tuning the structural geometry with size from micro-scale to nano-scale (roughness) [31] can result in the philic to phobic regime, meaning that θ_a (°) can be symmetrically increased from 0° to 180°, but α (°) cannot be obtained in this order, i.e., it may either increase or decrease instead.

With a hierarchical structural arrangement even in the phobic regime, the droplet may lie in sticky regime where the adhesion can cause partial or even no mobility. The first example in such a case is the rose petal effect where the wetting lies in the extreme superhydrophobic regime but droplet does not move even at the tilt angle of 90°, evidencing the full adhesion Wenzel state [32]. The second example includes the butterfly wings and rice leaves with a superhydrophobic wetting regime where the structural direction simultaneously builds the sticky (full adhesion Wenzel state) and slippery regimes [33]. In addition, the efficient droplet dynamics do not completely rely on the wetting regime (phobic or philic), i.e., rose petal and lotus leave both have a hierarchical structural arrangement and lie in an extreme superhydrophobic regime [34], but the former does not support droplet mobility, while the latter is the most effective for droplet mobility. Indeed, the axial structural interspacing (ASI) is a prevalently crucial parameter [32] and with ASI as small as possible, the droplet dynamics can be made efficient. In general, the nano-scaled ASI [35] (producing the slippery regime with efficient droplet dynamics) is relatively encouraged compared with microscopic ASI; nonetheless, this principle does not always hold true, because now it is likely to convert the sticky regime even with high micro-scaled ASI into the perfect slippery regime with help of artificial SAL as discussed above. Thus, we conclude with reasoning that ASI and SAL are the main drivers together with entrapped-air as the secondary driver in all kinds of structural voids, establishing the exclusive droplet wetting and dynamic regimes.

3.2 Horizontal surface position

Horizontal surface position refers to the droplet lying in the static state. Indeed, the contact line [36] is the key factor influencing the extent of droplet adhesion, especially in the horizontal surface position. Herein, the droplet dynamics can be inferred by slowly evaporating the droplet resting on the surface, which can also be named evaporative droplet dynamics, as shown in Figure 3. It follows the simple rule, that is, the contact line immobility implies the sticky regime that works under the constant contact radius (CCR) mode [37], while the contact line mobility (normally inward) confirms the slippery regime working under the constant contact angle (CCA) mode [38]. In special cases, CCR and CCA mixed modes can also occur [39]. Quantitatively, the contact line is extracted in terms of the base diameter through experiments, and the depinning force F_d [40] can be measured.

Figure 3.

Study of droplet dynamics under evaporation in horizontal surface position, depicting the (a) contact line (CL) pinning in CCR mode, and (b) depinning in CCA mode.

In horizontal surface positions, the growth of droplets can also be carried out and controlled. For instance, the droplets tend to undergo frosting/icing/evaporation [41], and in such cases, the target is to delay or enhance the droplet growth rate.

In horizontal surface position, following factors also affect the droplet dynamics:

3.2.1 Wettability gradient

The specific arrangement of wetting regimes spanning from superhydrophobic to superhydrophilic can create the difference in wettability, known as the wettability gradient [42]. It plays an important role, providing the droplet mobility from a low wettability regime to a high wettability regime even when the surface is in horizontal position. This leads to self-propelled droplet mobility, i.e., no external energy in the form of gravity or stimulus is required [43]. Generally, with a high wettability gradient, the droplet dynamics are deemed to be efficient, and vice versa. An example of a wettability gradient can be seen in the beetle surface where the droplet dynamics play an important role in the water transport toward mouth [44].

3.2.2 Droplet impact

On the horizontal surface, the droplet impact can be obtained in two ways, i.e., by dropping the droplet from a certain height [45] or by coalescence of two or more droplets [46]. The droplet impact may exhibit a very unique phenomenon that strongly depends on the wetting regimes. While dropping the droplet, if the droplet rebounds several times, the slippery dynamics in superhydrophobic regime can be inferred; which is also known as droplet jumping, enabled by the conversion of energies [45, 46]. If the droplets do not rebound or show coalescence, the sticky dynamics are exhibited, which may lie in any wetting regime.

3.3 Inclined surface position

Depending on the wetting regime and droplet adhesion, the inclined mode allows the droplet movement under the effect of gravity, i.e., the gravitational force overcomes the under-lying resistive forces, which can be quantified by measuring the α (°). The droplet does not move in the sticky regime, while it moves along the incline in the slippery regime, as exhibited in Figure 4. The droplet pinning/depinning is exhibited at the contact line (defined as the interface between the droplet and the surface topography) across the front/rear ridges. The contact angle at the front ridge is called the advancing angle θ_a, while at the rear ridge, it is known as the receding angle θ_r. The difference between the advancing angle and receding angle is called the contact angle hysteresis [31], i.e., the interfacial resistive forces attempting to prohibit the droplet mobility. In general, the smaller the CAH, the greater the droplet mobility.

Figure 4.

Study of droplet dynamics in the inclined surface position.

The contact line plays the main role to overcome the interfacial resistive forces encountered by the droplets. The contact line tends to be discontinuous if the surface is in the slippery regime [33], requiring a smaller α that simply demonstrates the small interfacial resistive forces. Conversely, the contact line becomes continuous and stable (Wenzel state) in the sticky regime [33] due to which a larger α is indispensable.

In inclined surface position, further factors are important as discussed below:

3.3.1 Isotropic alignment

Isotropic is the surface feature in which the topographic structures are aligned equally, enabling droplet mobility in every direction. An example of such a case is the movement of a rain-drop on slippery surfaces such as taro leaf and lotus leaf [47].

3.3.2 Anisotropic alignment

Anisotropic is the surface feature in which the topographic structures are directionally-aligned, meaning that the droplet can only move along certain directions. For example, the rain droplet can move on the butterfly wings and rice leaves in only one direction [48].

3.3.3 Liquid transport modes

There are two most common liquid transport modes during the condensation process (conversion of the gaseous phase into the liquid phase), naming the filmwise mode and dropwise mode. They are studied normally on inclined surfaces. Each mode provides different droplet dynamics depending on the wetting regimes. In filmwise mode, droplets come into existence, soon after which the droplet coalescence (merging of small neighboring droplets to form a big droplet is called coalescence) starts occurring. Consequently, a thin liquid film is developed underneath that remains affixed to the surface topography, providing the under-layered path over which the bulk of liquid keeps on transporting. This happens due to the high surface energy that prompts the sticky regime where the filmwise mode occurs. Comparatively, the slippery regime, empowers the dropwise mode depending on the surface type underneath (i.e., either dry or wet). In the dropwise mode, the drops come into existence and move as soon as they attain the critical droplet size. The dry slippery hydrophobic/superhydrophobic surfaces give rise to the dropwise mode under the influence of rolling/jumping mechanisms [49] where the liquid drops tend to be transported either in the discrete pattern or coalesced pattern. While the wet slippery hydrophobic/hydrophilic surfaces support the dropwise mode enabled by the sliding mechanism [50] of droplets which can also either be in discrete or coalesced patterns.

3.4 LD-SI and droplet shape

The combined effect of LD-SI (droplet-surface interaction) and droplet shape may be considered a factor, however, droplet dynamics may not be fully predicted based on their combined effect. For example, the droplet usually adopts a full spherical shape (e.g., superhydrophobic lotus leaf and rose petals), hemispherical shape (e.g., hydrophobic cloverleaf) or quarter spherical shape (e.g., and hydrophilic herb) on LD-DSI, but some of them provide slippery dynamics, while some provide sticky dynamics. The droplet shape always tends to be a hemispherical shape (e.g., hydrophobic pitcher plant) or quarter spherical shape (e.g., hydrophilic beetle or fish surface) on LD-WSI, and they can all support slippery dynamics.

In the slippery regime, both LD-DSI and LD-WSI play a pivotal role in eventually deciding the two dynamic mechanisms or patterns of a mobile droplet (i.e., on the inclined surface). Owing to the underlying dry surface, the droplet tends to roll off, and the rolling mechanism is established as can be seen in the case of lotus leaf (superhydrophobic regime) and cloverleaf (hydrophobic regime). In contrast, because of the underlying wet surface, the droplet tends to slide-off over the liquid interface, and the sliding mechanism is produced as can be seen in case of pitcher plant and beetle surface.

3.5 Surface durability

Surface durability is of practical importance that can drastically affect the droplet dynamic regimes. The surface durability can be defined as the withstanding capability of bio-inspired surfaces deciding the active lifespan, and it can be realized with reference to the impact of several parameters and phenomena, including the solvents (i.e., neutral, acids, alkalis), weather conditions (i.e., temperature, pressure, airflow), operational conditions (i.e., condensation, evaporation, shearing, friction), etc. Depending on the applications and the types of involved surfaces, different parameters and phenomena can affect the surface durability up to different extents. In general, the low surface durability can be caused by many factors, naming a few, the poor growth of ETS/ITS, the poor attachment of ETS/ITS with the parent substrate (pristine topography), the poor adherence/infusion of SAL with the ETS/ITS, the poor chemical compatibility, amongst others. For example, the SLIPSs have shown practical challenges and low durability due to oil depletion through cloaking (the encapsulation of water droplets by the oil), evaporation (oil is evaporated at ambient or at high temperature) or physical shearing (the mobile droplets continuously induce friction at the droplet-surface interface and keeps on removing the oil with the passage of time) [30], finally resulting in low durability.

3.6 Stimuli

In practice, there are three well-defined major stimuli that affect droplet dynamics. First is the gravitational force (external stimulus) that comes into action when the surface is in an inclined position; Second is the active stimulus (external stimulus) provided by several sources including temperature, photon (light), pH, stress, mechanical vibration, gas, reactive solvents, magnetic field and electric potential [51]. Active stimulus can enable tunable droplet dynamics. For example, tuning the movement of a droplet on the paraffin-infused surfaces by changing the electric potential [52] or temperature [30]. It should however be noted that the tunable droplet dynamics are hard to be found on living surfaces. Third is the self-stimulus that is enabled by the wettability gradients. The various stimuli of droplet dynamics can co-exist depending on the wetting regimes and surface positions.

4. Droplet dynamics-lab scale and industrial applications

Based on the intended applications of non-living surfaces, the droplet dynamic regimes (whether sticky or slippery) are vital, e.g., the corrosion-combating surfaces [53] or anti-icing surfaces [54] are intensively required where the main target is to prohibit the contact between the water and surface [55], thus the sticky or moderately slippery dynamics are favorable. While in other cases such as water vapor condensation [56] and vehicle transportation through the water [57], the main target is the quick transport of water droplets on the surface and the drag reduction on the object, respectively, both necessitating the slippery dynamic regime. Therefore, droplet dynamic regimes are mainly decided by the applications. Generally, there are many applications, but a few emerging applications of droplet dynamics are presented underneath.

4.1 Droplet manipulation

Droplet manipulation can be defined as the control of droplets through various design strategies of under-lying surfaces as well as droplets, such as rendering the droplet mobile, immobile, directional movements, static growth, etc. A few case studies are listed underneath.

4.1.1 Droplet guided-tracks

Guided tracks are the directional pathways where the droplet can move with great ease, demonstrating the efficient droplet dynamics. Following the bioinspired mimicking, guided tracks can be made in various shapes, such as straight, inclined, s-shape, and zig-zag amongst others. The example is given in Figure 5 [58], where the s-shaped track is depicted. The under-lying surface owns a slippery superhydrophobic regime, which has been precisely carved. Most importantly, the carved region and the droplet volume need to be effectively controlled so that the droplet cannot stay out of the track.

Figure 5.

Droplet transport along the guiding track [58].

4.1.2 Droplet segmentation/merging

Droplet segmentation/merging can be carried out in several ways, i.e., by making the segmented pathways over which the droplets can move or by merged pathways helping droplets come together and coalesced. This can be controlled by the bioinspired surface modifications as well as by the geometry modifications. For example, by modifying the surface of a common steel-made blade [58], the superhydrophobic regime can be obtained, which is then helpful to segment a big-sized droplet into two smaller droplets, as depicted in Figure 6.

Figure 6.

Droplet segmentation via superhydrophobic blade [58].

4.1.3 Stimuli-responsive manipulation

As described above, various stimuli can affect the droplet mobility and droplet transport range. The droplets can be either oil or water. On the inclined phase change slippery liquid-infused porous surface (PC-SLIPSs) [30], the droplet mobility has been achieved in solid, mush and liquid phases under the temperature-stimulus which is also known as thermo-responsiveness, as shown in Figure 7a. The wetting regime is hydrophobic, while the dynamic regimes are influenced by the phases and temperature. The solid phase provides the low adhesion Wenzel (LAW) state. Particularly, at the melting temperature of PC-SLIPSs, the droplet mobility suddenly changes from high adhesion Wenzel (HAW) state into a slippery state. The oil-droplet mobility has been realized underwater by using the electric stimulus [59], depicting the controllable dynamics, as depicted in Figure 7b.

Figure 7.

(a) Thermo-responsive water droplet manipulation [30], (b) electro-responsive underwater oil droplet mobility [59].

4.1.4 Self-propelled droplet manipulation

The droplet manipulation can be achieved without any external energy drivers or stimuli. The wettability gradient is the main driver, stimulating the self-mobility from low wettability toward high wettability. The example of steam condensation [60] is demonstrated (Figure 8a) where the liquid droplets are generated and move from left to right along the horizontal wires, leading to the droplet coalescence along the way. Eventually, droplet shedding occurs when the critical size of droplet has been achieved.

Figure 8.

Wettability gradient-induced self-propelled (a) droplet merging and shedding [60], as well as droplet uphill movement [43].

By means of the wettability gradient, the uphill droplet manipulation (Figure 8b) can also be achieved [43]. For example, the droplet can move upward on different tracks, including circular geometry, straight geometry and s-shaped geometry. The droplet shape changes from the start until the end while covering the different transport ranges, depicting the various wetting regimes which are the characteristics of the wettability gradient.

4.2 Self-cleaning

When the mobile droplets move on the surfaces, they can carry the dust and dirt particles with them, which is named the self-cleaning effect. Therefore, the slippery dynamic regime is the key, that can be executed either on dry or wet slippery surfaces. For example, the droplet moves on the phase change material-based superhydrophobic surface, carrying the dust away as can be seen in Figure 9 [61].

Figure 9.

Self-cleaning via mobile droplets [61].

4.3 Chemical reactor and circuit controller

The mixed slippery and sticky regimes of the droplet dynamics can help build chemical reactors and electric circuit controllers. A dual stimuli-responsive SLIPSs have been presented [62], influenced by the temperature and force. A droplet (8 μL) of sodium hydroxide (NaOH) can be made mobile under the optimum effect of temperature and strain-induced force, letting it mix with the phenolphthalein droplet where the chemical reaction takes place, as exhibited in Figure 10a. Likewise, under the optimum effect of temperature and strain-induced force, the sliding and pinning of NaOH droplet can switch off and on the lamp, exhibiting the feasible electric controller (Figure 10b).

Figure 10.

Droplet sliding/pinning helping build (a) chemical reactor [62] and (b) electric circuit controller [62].

4.4 Condensation-water harvesting and heat transfer

The droplet dynamics play the most influential role in the condensation, where the filmwise mode and dropwise mode are considered. By means of condensation, water harvesting and heat transfer applications can be realized. In both of them, the dropwise mode is of great interest, providing the high droplet nucleation and ease of droplet transport which can be achieved through slippery dynamic regimes. For example, the dropwise mode has been demonstrated in the superhydrophobic regime and hydrophilic regime, enabling the fast droplet transport suitable for the efficient water harvesting (Figure 11(a and b)) [63]. Likewise, the heat transfer (Figure 11c) has been obtained higher in the slippery hydrophobic regime of phase change slippery liquid-infused porous surfaces (PC-SLIPSs) and slippery superhydrophobic regime compared to that in the sticky hydrophilic regime of pristine copper plate [20]. It should be particularly noted that efficient droplet dynamics are necessary during condensation, i.e., the dropwise mode may also underperform if the droplet shedding is slow as concluded in Ref. [20].

Figure 11.

Water harvesting under dropwise mode in (a) superhydrophobic (SHPo) regime and (b) hydrophilic (HPi) regime [63], (c) steam condensation heat transfer on pristine copper plate, superhydrophobic copper plate and PC-SLIPSs [20]. Scale bar, 5 mm.

4.5 Oil/water separation

The oil/water separators are prepared by developing smart coatings on the porous networks. Depending on the wetting regimes of the separator, one phase is blocked, while the other is permitted through the porous network. Sticky and slippery regimes of droplet dynamics are crucial for oil/water separators; however, it should be particularly noted that these regimes may be insignificant during the separation mechanisms of vertically-aligned separators. It is because the blocking of one phase is not driven by the sticky dynamic regime, as well as the permitting of the other phase is not driven by the slippery dynamics. Instead, the separation mechanism is driven by the positive and negative capillary effects of two phases, inducing physical absorption and physical adhesion, as shown in Figure 12a [15]. Thus, in vertically-aligned separators, the sticky and slippery dynamics put forward a positive contribution in other ways. For example, the low sticky regime or moderately-high sticky regime is relatively effective, which can create easiness in the recovery process of separation medium after the oil/water separation. The self-recovery is important because the smart coatings can retain either oil or water during the separation process. With low sticky dynamics, the retained oil or water can easily evaporate, providing efficient self-recovery, as depicted in Figure 12a. The slippery dynamics can also play a vital role before or after the separation process, for example, a superhydrophobic-oleophilic copper mesh has been presented which is an effective oil/water separator. The slippery dynamics are of great significance to resolve the cleaning challenges of the separators (Figure 12b) [64].

Figure 12.

Oil/water separators. (a) Low sticky dynamics depict the ease of self-recovery [15], (b) self-cleaning of separator [64].

4.6 Spraying

The spraying of different liquids is encountered almost everywhere, for example, spray of water, fuels, solvents, paints and perfumes, amongst others. In the spraying process, a multitude of droplets, consisting of various sizes, are generated. The droplet dynamics in the spray process can be affected by many factors, naming a few, the droplet type (i.e., oil, fuel, water, acids, alkalis, etc.), the droplet traveling speed, atomizer distance, and the surfaces position (horizontal, vertical or inclined). Upon impact with the surfaces, different droplets can behave differently depending on the wetting regimes and surface roughness. The impact of fuel droplets and surface roughness have been studied during the fuel spray in combustion engines [65]. It has been concluded that the various wetting regimes (depicted by the two values of surface roughness of R_a ~ 2.5 and 7.7) can produce various levels of fuel film thickness, depicting the unique film development mechanism (Figure 13). It shows that the droplet dynamics and the wetting regimes together play a pivotal role during the spraying, urging deeper studies in this field especially addressing the contact angles, sliding angles and contact angle hysteresis, etc.

Figure 13.

Spray process of fuel droplets onto the horizontally-positioned glass slides having different roughness (R_a)values [65].

5. Conclusion

Bioinspired smart surfaces have been reported, with a special focus on the droplet dynamics. The surface regimes have been fundamentally classified into four droplet wetting and two droplet dynamic regimes. The droplet dynamics is defined as the study of the droplet growth, droplet mobility, droplet speed, droplet transport range and the underlying forces. The droplet dynamics is of great interest to a wide variety of scientific areas. With help of bioinspired smart surfaces, the droplet dynamics, either in sticky or slippery regimes, have been greatly improved. There are many factors that affect the droplet dynamics, that is, it is hard to rely on a single factor. In particular, the specific dynamic regime and the influential factors need to be modified depending on the applications, i.e., some applications may need the sticky regime, some may need the slippery regime, and some may need the co-existence of both regimes.

This chapter implies that the droplet dynamics are potentially significant, however further efforts are necessary to sustain the efficient droplet dynamics. It indeed depends on the chemical and mechanical strength of the droplet wetting regimes of the under-lying bioinspired surfaces, i.e., the surface durability. All bioinspired surfaces (superhydrophobic, SLIPSs, etc.), as presented so far, suffer from the bottlenecks corresponding to the durability, rendering the droplet dynamics inefficient up to various levels. It is therefore recommended to use alternative materials (e.g., solid slippery materials such as waxy and non-waxy phase change materials) and advanced surface engineering approaches to enhance the durability of the bioinspired surfaces that can sustain the droplet dynamics during real-time applications. Briefly, the efficient droplet dynamics can potentially enhance the performance of those systems intending to minimize the global energy crisis (e.g., less drag-inducing pipes can help save energy during the pumping process, slippery surfaces of photovoltaics can prohibit/postpone the ice accretion as well as avoid the formation of dust layers), save the operational time (e.g., condensers can provide high output in less time based on the high droplet transport) and provide the high throughput (e.g., microfluidic devices).

Acknowledgments

The support provided by the IntechOpen, as well as by the Postdoctoral Program of Southeast University are highly acknowledged.