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Displacement of the contact line and the drop a, b Variation of contact line length ratio K of an impinging drop on concentric microgroove arrays (r = d/4) on the left (red) and right (black) sides under We = 16.9 at T = 250 and 350 °C, respectively. The insets in a, b represent the side view of maximum spreading. c Lateral displacements Δl of bouncing drops as a function of temperature under various Weber numbers. A positive Δl, i.e., ΔlL, indicates that drops rebound towards the center of curvature, while a negative one, i.e., ΔlR, towards the opposite direction. The insets show the recoiling of impacting drops shortly after they reach the maximum spreading at different boiling states. The error bars of the data in a–c denote the standard deviation of three measurements. Source data are provided as a Source Data file.
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Guided drop transport is of great importance in various water and thermal management technologies. Unidirectional drop transport on a hot surface has been widely developed, but a bidirectional reversal is still challenging. Here, we report a steerable transport of drop impinging on heated concentric microgroove arrays, on which the directionality o...
Citations
... As such, achieving manoeuvrable droplet levitation on hot engineered surfaces will be of great benefit for many applications in highly demanding heat transfer devices 19,20 . One of the prominent examples is the purging of surface fouling agents 21 : that is, the physical deposition of contaminating particulates on heat exchanging surfaces, which severely impairs the performance of thermal systems like boilers, condensers and heat exchangers by impeding effective heat exchange between the working liquids and the solid surface. ...
... Figures 6a,b show time-lapse images of ejection of sessile water droplets on tilted substrates with different pillar heights. For the tilted substrate [D, L, H] = [20,120,20] μm, the vibrating droplet initiated out-of-plane jumping, and then the water droplet landed softly on the substrate, remaining in the low-friction Cassie state until it slid off the substrate. For the tilted substrate [D, L, H] = [20,120,80] μm, the explosive droplet jumping caused the droplet to jump off the substrate with a maximum height of 6 mm, which is three times as large as the droplet diameter. ...
... Figures 6a,b show time-lapse images of ejection of sessile water droplets on tilted substrates with different pillar heights. For the tilted substrate [D, L, H] = [20,120,20] μm, the vibrating droplet initiated out-of-plane jumping, and then the water droplet landed softly on the substrate, remaining in the low-friction Cassie state until it slid off the substrate. For the tilted substrate [D, L, H] = [20,120,80] μm, the explosive droplet jumping caused the droplet to jump off the substrate with a maximum height of 6 mm, which is three times as large as the droplet diameter. ...
The Leidenfrost effect—the levitation and hovering of liquid droplets on hot solid surfaces—generally requires a sufficiently high substrate temperature to activate liquid vaporization. Here we report the modulation of Leidenfrost-like jumping of sessile water microdroplets on micropillared surfaces at a relatively low temperature. Compared to traditional Leidenfrost effect occurring above 230 °C, the fin-array-like micropillars enable water microdroplets to levitate and jump off the surface within milliseconds at a temperature of 130 °C by triggering the inertia-controlled growth of individual vapour bubbles at the droplet base. We demonstrate that droplet jumping, resulting from momentum interactions between the expanding vapour bubble and the droplet, can be modulated by tailoring of the thermal boundary layer thickness through pillar height. This enables regulation of the bubble expansion between the inertia-controlled mode and the heat-transfer-limited mode. The two bubble-growth modes give rise to distinct droplet jumping behaviours characterized by constant velocity and constant energy regimes, respectively. This heating strategy allows the straightforward purging of wetting liquid droplets on rough or structured surfaces in a controlled manner, with potential applications including the rapid removal of fouling media, even when located in surface cavities.
... Figure 2 and Movie S2 (Multimedia available online) show the dynamic behavior of water droplets on the heated smooth Al surface (without laser treatment) at different temperatures. As the temperature increases, the droplets exhibit four different states: droplet firmly adhering to the smooth surface (the single-phase state) [ Fig. 2 [33][34][35][36][37][38] In the singlephase state, the droplets firmly adhere to the smooth surface. Since the evaporation process only occurs at the liquid-gas interface, the whole evaporation process is relatively slow. ...
Unidirectional droplet motion is realized on heated asymmetric microgroove arrays prepared by femtosecond laser direct writing. The plasma expansion under laser ablation compresses the two sides of the induced microgroove differently, resulting in the formation of asymmetrical microgrooves. The asymmetry of the microgrooves can rectify the water vapor that ejects from the Leidenfrost droplet and generate a viscous shear force at the bottom of the droplet, causing the droplet to move in a certain direction (where the laser scanning line is added) when the substrate temperature is higher than a certain critical value (the transition temperature of disordered motion and unidirectional motion). The velocity of droplets can exceed 318 mm/s, and the droplets can even climb surfaces that are tilted 14°. With the advantages of femtosecond lasers in the flexible design of surface microstructures and patterns, this unidirectional droplet motion can support a variety of complex droplet-manipulation applications, such as droplet movement along designed trajectories, droplet accelerator devices, fixed-point capture of droplets, and fixed-point cooling of hot solid surfaces. Compared with traditional macroscopic ratchets, laser-written asymmetrical microgrooves make the Leidenfrost droplet motion more designable and controllable.
... The absence of liquid-solid contact eliminates the contact line pinning that hinders droplet motion, allowing droplets to be propelled with tiny force. This frictionless nature is highly advantageous for various applications, e.g., fluidic drag reduction, rapid droplet transport and chemical reaction enhancement [3][4][5]. Researchers have found that the Leidenfrost effect can be extended to the boiling point through the combination of superhydrophobic surfaces, forming a cold Leidenfrost regime that broadens the temperature range of frictionless droplet transport [6,7]. Moreover, to overcome the random movement of Leidenfrost droplets and realize directional self-propulsion, several asymmetric structures, such as ratchet-like structures [8], Janus-mushroom structures [9] and tilting nanowires [10], have been designed to effectively convert thermal energy into kinetic energy, achieving a transport velocity of the order of 10 cm/s. ...
The recent discovery of Leidenfrost droplet trampolining deviates from the traditionally accepted steady-state assumption and updates our understanding of Leidenfrost droplet state. However, the conditions of trampolining and its effect on heat transfer have not been fully understood. To address these issues, this study numerically investigates the dynamic behavior and heat transfer characteristics of Leidenfrost droplets under varying liquid viscosity and droplet size. A regime map for Leidenfrost droplet state with respect to Bond number and Ohnesorge is presented. Particularly, an unreported oscillation regime is discovered between equilibrium regime and trampolining regime. The results indicate that low viscosity and moderate droplet size favor the observation of trampolining. Besides, the oscillating droplet is modeled via a mass-spring-damper system in both equilibrium and oscillation regimes, with damping coefficient, spring constant and oscillation period quantitatively correlated with liquid viscosity and droplet size by simple scaling laws. In the trampolining regime, an intriguing phenomenon is observed as the maximum vapor layer thickness demonstrates two local maxima with increasing droplet size. We also quantitatively unravel that reducing liquid viscosity and increasing droplet size can lead to a thicker vapor layer thickness, thus inhibiting the heat transfer to the droplet.
... Among these approaches, the design of surfaces with non-uniform topographies has received special attention for manipulating the droplet bouncing behaviors. [23][24][25] In particular, recently, Li et al. 26 designed various Janus-textured surfaces with structural roughness gradients to realize the directed motion of the droplet. The authors pointed out that in contrast to the canonical case of droplet impact on a superhydrophobic surface at room temperature, the directed motion of the droplet in the Janus-textured heated surfaces is due to the conversion of its surface energy to kinetic energy rather than a viscous dissipation occurring the canonical droplet propulsion processes. ...
... Combining Eqs. (25) and (26) yields a general expression for the vapor pressure ...
Janus-textured substrates refer to surfaces with heterogeneous topographies, which have received particular attention recently due to their potential application in manipulating droplet-bouncing behaviors [Li et al., Nat. Phys. 12, 606–612 (2016)]. In this paper, the droplet impact dynamics on the Janus-textured heated substrates are numerically investigated with an improved thermal lattice Boltzmann method. A comprehensive parametric study is conducted by varying the wettability, the Jakob number, the Weber number, and the surface topographies. With different control parameters, three distinct boiling regimes are observed, i.e., the contact boiling regime, the transition boiling regime, and the film boiling regime (Leidenfrost state). To reveal the underlying physics, the distributions of the unbalance Young's force, the thermophoretic force, and the vapor pressure difference in the system are theoretically analyzed. As for the self-propulsion behaviors, it is find that the droplet tends to move toward the denser side (area with more pillar arrays) for the contact boiling regime. However, when the droplet is under the Leidenfrost state, its bouncing dynamics depend on the combined effects of the Weber number and the wettability, and a decrease in wettability induces the droplet to migrate toward the sparser side (area with fewer pillar arrays). These physical insights enrich the fundamental understanding of the droplet-bouncing dynamics on heated substrates and also provide guidelines for designing advanced surfaces to manipulate the droplet-bouncing behavior.
... Although extensive studies have been conducted on flat superhydrophobic surfaces, their non-flat counterparts commonly exist in many actual situations [46][47][48][49][50][51][52]. Bird et al. [53] observed that droplets hitting a ridged surface could induce non-axisymmetric center-assisted recoil, which reduced the overall contact time of a bouncing droplet below the theoretical limit of 2.2τ. ...
... Currently, the means of controlling fluid for spontaneous and directional transportation can be divided into two methods: fluid transportation using external energy and spontaneous directional fluid transportation without external energy input. Directional transportation using external energy is often achieved by magnetic field [8][9][10], electric field [11][12][13], thermodynamics [14,15], optics [16][17][18], and so on. Despite the fact that these methods can achieve directional fluid transportation, their application areas are limited due to the need for external energy input. ...
The spontaneous directional transportation (SDT) of water and gas has functions such as efficient water collection, enhanced heat transfer, underwater drag reduction, and so on, having great application prospects in aerospace and navigation fields. Therefore, it is important to efficiently prepare spontaneous directional water droplet transportation (SDWT) surfaces and spontaneous directional gas bubble transportation (SDBT) surfaces and apply them in different fields. In recent years, researchers have used biological structures as the basis for their studies and have continued to analyze the SDT transport mechanism in depth, aiming to find more efficient transportation methods. In this review, we first summarize the important basic theories related to fluid transportation. Then, the related methods and the limitations corresponding to SDWT and SDBT are introduced and discussed. In addition, we review the applications of SDWT and SDBT. Finally, we highlight the challenges and future perspectives of SDWT and SDBT.
... For a drop on a hot surface, this stable state is known as the Leidenfrost state (6), in which the drop entirely floats above a thin vapor layer (7,8). This thin vapor layer makes the drop highly mobile, leading to various dynamic behaviors, such as oscillation (9)(10)(11), transportation (12)(13)(14)(15)(16), and bouncing (17)(18)(19)(20)(21). It is widely believed that a highly unstable transition state with a narrow temperature gap exists on hydrophilic surfaces before the Leidenfrost state forms. ...
When a water drop is placed on a hot solid surface, it either undergoes explosive contact boiling or exhibits a stable state. In the latter case, the drop floats over an insulating layer of vapor generated by rapid vaporization of water at the surface/drop interface; this is known as the Leidenfrost state. Here, we discuss a previously unrecognized steady state in which a water drop "stands" on a hot smooth surface. In this state, the drop stabilizes itself with partial adhesion on the hot surface, leading to unique deformation and rotation behavior reminiscent of Sufi whirling-a form of spinning dance. Our analysis of this standing Leidenfrost state reveals the underlying mechanisms that drive the drop's stable partial adhesion and subsequent deformation with rotation. The heat-transfer efficiency of this standing state is up to 390% greater than that of the traditional floating Leidenfrost state.
... Additionally, a steerable transport of droplet impinging on heated concentric microgroove arrays is reported, where the substrate temperature determines the direction of droplet bouncing. 19 Nevertheless, the high temperature is not applicable to some applications that cannot hold several hundred temperatures. First proposed by Chaudhury et al., [20][21][22] mechanical vibration could serve as a stimulating strategy for droplet transport and have a wide range of advantages, including no-contamination, facileness, as well as robustness and durability. ...
Directed droplet manipulation is paramount in various applications, including chemical micro-reaction and biomedical analysis. The existing strategies include some kinds of gradients (structure, inherent wettability, and charge density), whereas they suffer from several limitations, such as low velocity, limited volume range, poor durability, and inefficient environmental suitability. Moreover, active bi-directional reversal of omni-droplets remains challenging because one kind of microstructure at a single scale cannot acquire two kinds of net results of mechanical interaction. Herein, we report an active and directional steering of omni-droplets utilizing bi-directional (vertical and horizontal) vibration on slippery cross-scale structures consisting of macro millimeter-scale circular arc arrays and micro/nanometer-scale slant ratchet arrays, which are fabricated by femtosecond laser patterned oblique etching and lubricant infusion. The physical mechanism of active droplet steering lies in the relative competition between the forces under vertical and horizontal vibration, which mainly arise from the circular arc arrays and slant ratchet arrays, respectively. Various steering modes, including climbing and programmable manipulation, can be realized. Our work is applicable to a wide range of potential applications, including circuit on/off and droplet-based chemical micro-reaction, particularly in the field of high-throughput omni-droplets operation.
... Anisotropic wettability also enables fish to reduce drag [7], water striders to walk on water [8], and beetles to capture water in the desert [9]. In addition, the wettability can change once the temperature of the surface or the environment changes, which has attracted increasing attention in recent years [10][11][12]. For instance, with the one-dimensional distribution of the mastoid structure in the parallel direction of leaf veins and uneven distribution in the vertical direction [3], water droplets accumulate on rice leaf when the temperature is low in the morning and roll along the veins as the temperature increases. ...
... When the infrared-light irradiation was focused on the substrate at one side of a droplet, a temperature difference was generated between the two sides of the droplet, which resulted in the unbalanced surface tension and Marangoni force, driving the droplet towards the side without light irradiation [26]. When the surface temperature was close to the Leidenfrost point, the droplet levitated on a vapor layer; the movements of a droplet on surfaces have been intensively investigated [10,11,13,30]. The existence of a vapor layer would result in a negligible normal adhesion or lateral friction of the water droplet on the surface, which, however, would cause a high thermal resistance. ...
... Wang et al. [13] fabricated a micropillar surface with gradient periods and thus the coefficient of heat transfer and realized the directional transport of a high-temperature (close to Leidenfrost point) droplet towards the region with a higher heat-transfer coefficient. Liu et al. [11] found an interesting phenomenon in which a droplet showed a steerable bouncing on heated concentric microgrooves arrays under different temperatures, which is believed to originate from the synergistic action of the surface structure and boiling states. That is, the motion of a water droplet could be manipulated by controlling the temperature (close to the Leidenfrost point) and the topography substrate surface. ...
Anisotropic surfaces with special wettability under various temperatures are of both fundamental interest and practical importance in many fields. However, little attention has been paid to the surfaces at temperatures between room temperature and the boiling point of water, which is partially due to the lack of a suitable characterization technique. Here, using the MPCP (monitoring of the position of the capillary’s projection) technique, the influence of the temperature on the friction of a water droplet on the graphene-PDMS (GP) micropillar array (GP-MA) is investigated. The friction forces in the orthogonal directions and the anisotropy in the friction decrease when the GP-MA surface is heated up, based on the photothermal effect of graphene. The friction forces also decrease along the pre-stretching direction but increase in the orthogonal direction when the stretching is increased. The change in the contact area, the Marangoni flow inside a droplet, and the mass reduction are responsible for the temperature dependence. The findings strengthen our fundamental understanding of the dynamics of drop friction at high temperatures and could pave the way for the design of new functional surfaces with special wettabilities.
... Controllable droplet manipulation is valuable in various practical applications, such as biological detection (1,2), chemical reactions (3,4), water harvesting (5-7), and heat management (8)(9)(10). Various external stimuli including magnetism (11)(12)(13)(14)(15), electricity (16)(17)(18)(19), and light (20)(21)(22) are introduced to achieve more flexible and precise droplet manipulation on superhydrophobic surfaces. ...
Spatiotemporally controllable droplet manipulation is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite considerable advances, droplet manipulation without surface or droplet pretreatment is still challenging in terms of response and functional adaptability. Here, a droplet ultrasonic tweezer (DUT) based on phased array is proposed for versatile droplet manipulation. The DUT can generate a twin trap ultrasonic field at the focal point for trapping and maneuvering the droplet by changing the position of the focal point, which enables a highly flexible and precise programmable control. By leveraging the acoustic radiation force resulting from the twin trap, the droplet can pass through a confined slit 2.5 times smaller than its own size, cross a slope with an inclination up to 80°, and even reciprocate in the vertical direction. These findings provide a satisfactory paradigm for robust contactless droplet manipulation in various practical settings including droplet ballistic ejection, droplet dispensing, and surface cleaning.
