I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. BASICS OF DROPLET IMP...III. MACROSTRUCTURE DESIG...IV. WETTABILITY PATTERN D...V. EFFECTS OF SUPERHYDROP...VI. SUMMARY AND PROSPECTSLotus leaves are famous for their unstained appearance from mud. Raindrops falling on the lotus leaves do not wet them either, but directly roll-off, picking up and carrying away dirt or other contaminants. This fascinating self-cleaning property is termed as the “lotus effect,”1–31. W. Barthlott and C. Neinhuis, Planta 202, 1 (1997).2. A. Marmur, Langmuir 20, 3517 (2004). https://doi.org/10.1021/la036369u3. A. N. Patankar, Langmuir 20, 8209 (2004). https://doi.org/10.1021/la048629t which can also be described in a more scientific terminology, i.e., superhydrophobicity.4–74. A. Lafuma and D. Quere, Nat. Mater. 2, 457 (2003). https://doi.org/10.1038/nmat9245. L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, and D. Zhu, Adv. Mater. 14, 1857 (2002). https://doi.org/10.1002/adma.2002900206. X. M. Li, D. Reinhoudt, and M. Crego-Calama, Chem. Soc. Rev. 36, 1350 (2007). https://doi.org/10.1039/b602486f7. X. Zhang, F. Shi, J. Niu, Y. Jiang, and Z. Wang, J. Mater. Chem. 18, 621 (2008). https://doi.org/10.1039/B711226B Although people began to study wetting very early and established different wetting modes with their equations (e.g., Young's equation,88. T. Young, Philos. Trans. R. Soc. London 95, 65 (1805). https://doi.org/10.1098/rstl.1805.0005 Wenzel's equation,99. R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936). https://doi.org/10.1021/ie50320a024 and Cassie–Baxter equation1010. A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944). https://doi.org/10.1039/tf9444000546) it was not until 1997 that the reason why lotus leaves have superhydrophobicity was recognized. Barthlott and Neinhuis captured scanning electron microscope (SEM) photos of the lotus leaves for the first time.11. W. Barthlott and C. Neinhuis, Planta 202, 1 (1997). The leaves are not only distributed with micrometer-scale mastoid structures but also densely covered with nanosized fluff on the mastoid structures. In addition to the two-tier micro/nano structures, the lotus leaves are covered with waxy layers of low surface energy. The hierarchical structures collaborate with the waxy layer, contributing to the superhydrophobicity of lotus leaves.1,51. W. Barthlott and C. Neinhuis, Planta 202, 1 (1997).5. L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, and D. Zhu, Adv. Mater. 14, 1857 (2002). https://doi.org/10.1002/adma.200290020 Since then, inspired by lotus leaves as well as other natural organisms, such as water striders,11,1211. X. Gao and L. Jiang, Nature 432, 36 (2004). https://doi.org/10.1038/432036a12. J.-S. Koh, E. Yang, G.-P. Jung, S.-P. Jung, J. H. Son, S.-I. Lee, P. G. Jablonski, R. J. Wood, H.-Y. Kim, and K.-J. Cho, Science 349, 517 (2015). https://doi.org/10.1126/science.aab1637 cicadas,1313. K. M. Wisdom, J. A. Watson, X. Qu, F. Liu, G. S. Watson, and C. H. Chen, Proc. Natl. Acad. Sci. U. S. A. 110, 7992 (2013). https://doi.org/10.1073/pnas.1210770110 and geckos,14,1514. Y. Wang, H. Lai, Z. Cheng, H. Zhang, E. Zhang, T. Lv, Y. Liu, and L. Jiang, Nanoscale 11, 8984 (2019). https://doi.org/10.1039/C9NR00154A15. K. Liu, J. Du, J. Wu, and L. Jiang, Nanoscale 4, 768 (2012). https://doi.org/10.1039/C1NR11369K diverse superhydrophobic surfaces have been engineered by a variety of advanced methods.16–2016. P. Roach, N. J. Shirtcliffe, and M. I. Newton, Soft Matter 4, 224 (2008). https://doi.org/10.1039/B712575P17. S. Wang, K. Liu, X. Yao, and L. Jiang, Chem. Rev. 115, 8230 (2015). https://doi.org/10.1021/cr400083y18. M. Ma and R. M. Hill, Curr. Opin. Colloid Interface Sci. 11, 193 (2006). https://doi.org/10.1016/j.cocis.2006.06.00219. Y. Y. Yan, N. Gao, and W. Barthlott, Adv. 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Cheng, M. Song, H. Dong, and F. Shi, Small 11, 1665 (2015). https://doi.org/10.1002/smll.201402618 and energy harvesting.34,3534. N. Miljkovic, D. J. Preston, R. Enright, and E. N. Wang, Appl. Phys. Lett. 105, 013111 (2014). https://doi.org/10.1063/1.488679835. X. Gong, X. Gao, and L. Jiang, Adv. Mater. 29, 1703002 (2017). https://doi.org/10.1002/adma.201703002In many application scenarios of superhydrophobic surfaces, the interaction between droplets and surfaces occurs and causes key effects. Droplet impacting on solid surfaces is a typical form of droplet–surface interaction, which is widely observed in nature and industry.36,3736. A. L. Yarin, Annu. Rev. Fluid Mech. 38, 159 (2006). https://doi.org/10.1146/annurev.fluid.38.050304.09214437. C. Josserand and S. T. Thoroddsen, Annu. Rev. Fluid Mech. 48, 365 (2016). https://doi.org/10.1146/annurev-fluid-122414-034401 For example, on rainy days, raindrops hit the window surfaces; when the aircraft is flying, supercooled water droplets in the clouds hit the wings and freeze; during the printing process, ink droplets deposit on the surface to form patterns. Moreover, the droplet impacting is also being developed and applied to some emerging technologies, such as droplet-based power generation,3838. W. Xu, H. Zheng, Y. Liu, X. Zhou, C. Zhang, Y. Song, X. Deng, M. Leung, Z. Yang, R. X. Xu, Z. L. Wang, X. C. Zeng, and Z. Wang, Nature 578, 392 (2020). https://doi.org/10.1038/s41586-020-1985-6 chemical detection,3939. H. Li, W. Fang, Z. Zhao, A. Li, Z. Li, M. Li, Q. Li, X. Feng, and Y. Song, Angew. Chem., Int. Ed. 59, 10535 (2020). https://doi.org/10.1002/anie.202003839 and liquid film manipulation.4040. F. Chu, Z. Ni, D. Wen, Y. Feng, S. Li, L. Jiang, and Z. Dong, Adv. Funct. Mater. 32, 2203222 (2022). https://doi.org/10.1002/adfm.202203222 Therefore, deeply understanding the droplet impacting dynamics on superhydrophobic surfaces and realizing the precise regulation of droplet impact behavior based on the design and utilization of superhydrophobic surfaces will be the key steps before the practical applications of superhydrophobic surfaces and developments of advanced technologies based on droplet impacting.

In this Perspective, we first review the basics of droplet impacting on superhydrophobic surfaces and introduce the definitions and theories of the maximum spreading coefficient and the contact time; second, we pay attention to the strategies of using macrostructures to regulate the impacting droplets and conclude that the macrostructures have great advantages in reducing the contact time of impacting droplets; third, we focus on how to use wettability pattern to control droplet impacting behavior and introduce some special motion of droplets after rebounding; finally, we discuss the effects of moving superhydrophobic surfaces on the droplet impacting dynamics and propose the Perspectives of combining the motion control of superhydrophobic surfaces with the structure and wettability design to regulate the droplet impacting so that it can be applied in multiple application scenarios.

II. BASICS OF DROPLET IMPACTING ON SUPERHYDROPHOBIC SURFACES

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. BASICS OF DROPLET IMP... <<III. MACROSTRUCTURE DESIG...IV. WETTABILITY PATTERN D...V. EFFECTS OF SUPERHYDROP...VI. SUMMARY AND PROSPECTSThe complex hydrodynamics of droplet impacts on solid surfaces are influenced by the droplet parameters (i.e., diameter, velocity, and shape),41,4241. Q. Liu, J. Lo, Y. Li, Y. Liu, and L. Xu, Nat. Commun. 12, 3068 (2021). https://doi.org/10.1038/s41467-021-23138-442. S. Lin, D. Wang, L. Zhang, Y. Jin, Z. Li, E. Bonaccurso, Z. You, X. Deng, and L. Chen, Adv. Sci. 8, e2101331 (2021). https://doi.org/10.1002/advs.202101331 fluid properties (i.e., viscosity, surface tension, and density),43–4543. F. Yu, S. Lin, J. Yang, Y. Fan, D. Wang, L. Chen, and X. Deng, Adv. Sci. 7, 1902687 (2020). https://doi.org/10.1002/advs.20190268744. T. Mouterde, P. Lecointre, G. Lehoucq, A. Checco, C. Clanet, and D. Quere, Nat. Commun. 10, 1410 (2019). https://doi.org/10.1038/s41467-019-09456-845. M. Song, J. Ju, S. Luo, Y. Han, Z. Dong, Y. Wang, Z. Gu, L. Zhang, R. Hao, and L. Jiang, Sci. Adv. 3, e1602188 (2017). https://doi.org/10.1126/sciadv.1602188 and surface properties (i.e., wettability, microstructures, and stiffness),46–4946. C. Antonini, A. Amirfazli, and M. Marengo, Phys. Fluids 24, 102104 (2012). https://doi.org/10.1063/1.475712247. H. Li, A. Li, Z. Zhao, L. Xue, M. Li, and Y. Song, Acc. Mater. Res. 2, 230 (2021). https://doi.org/10.1021/accountsmr.1c0000548. P. Zhu, C. Chen, K. Nandakumar, and L. Wang, Small 17, e2006695 (2021). https://doi.org/10.1002/smll.20200669549. S. Mangili, C. Antonini, M. Marengo, and A. Amirfazli, Soft Matter 8, 10045 (2012). https://doi.org/10.1039/c2sm26049b resulting in a wide range of phenomena, such as deposition, rebounding, and splashing.36,50–5236. A. L. Yarin, Annu. Rev. Fluid Mech. 38, 159 (2006). https://doi.org/10.1146/annurev.fluid.38.050304.09214450. R. Rioboo, C. Tropea, and M. Marengo, Atomization Sprays 11, 155 (2001). https://doi.org/10.1615/AtomizSpr.v11.i2.4051. L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. 94, 184505 (2005). https://doi.org/10.1103/PhysRevLett.94.18450552. D. Richard and D. Quéré, Europhys. Lett. 50, 769 (2000). https://doi.org/10.1209/epl/i2000-00547-6 On superhydrophobic surfaces, the typical dynamic process of droplet impacting can be divided into two stages: spreading and retracting [Fig. 1(a)]. In the spreading stage, the droplet spreads to a circular film under an inertial force and reaches the maximum spreading. The spreading is dominated by inertia and less affected by the surface wettability, so that the spreading stage of impacting droplets on superhydrophobic surfaces is almost the same as that on hydrophilic surfaces. In the retracting stage, the circular film retracts toward the impact point under the control of surface tension. The retraction velocity in the early stage is constant, which follows the Taylor–Culick law: Vret ≈ σ1−cos θr/ρδ, where θr is the receding contact angle on the superhydrophobic surface, δ is the film thickness at the maximum spreading, ρ is the liquid density, and σ is the liquid surface tension.53,5453. D. Bartolo, C. Josserand, and D. Bonn, J. Fluid Mech. 545, 329 (2005). https://doi.org/10.1017/S002211200500718454. Z. Hu, F. Chu, and X. Wu, Extreme Mech. Lett. 52, 101665 (2022). https://doi.org/10.1016/j.eml.2022.101665 In the later stage of retraction, the retraction velocity decreases because the liquid film thickness increases gradually. At the end of retracting, the droplet departs from the surface, i.e., rebounding. The rebounding droplet does not remain spherical in general, but ellipsoidal, spindle-shaped, and even jet satellite droplets.50,51,5550. R. Rioboo, C. Tropea, and M. Marengo, Atomization Sprays 11, 155 (2001). https://doi.org/10.1615/AtomizSpr.v11.i2.4051. L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. 94, 184505 (2005). https://doi.org/10.1103/PhysRevLett.94.18450555. P. Tsai, S. Pacheco, C. Pirat, L. Lefferts, and D. Lohse, Langmuir 25, 12293 (2009). https://doi.org/10.1021/la900330qFor the droplet impacting on superhydrophobic surfaces, the maximum spreading coefficient (βmax) is an important parameter, defined as the ratio of the maximum spreading diameter to the initial droplet diameter, i.e., βmax = Dmax/D0. As mentioned before, the spreading of impacting droplets is driven by inertia and mainly resisted by surface tension, so the maximum spreading coefficient βmax is related to the Weber number (We = ρD0U02/σ, where U0 is the droplet impact velocity), which is a measure of the relative importance of the fluid's inertia compared to its surface tension. For droplets with high viscosity, the effect of Reynolds number (Re = ρD0U0/μ, here μ is the liquid viscosity) should be considered. In the past decades, various scaling laws of βmax suitable for different conditions have been developed, as shown in Fig. 1(b).56–6256. J. Madejski, Int. J. Heat Mass Transfer 19, 1009 (1976). https://doi.org/10.1016/0017-9310(76)90183-657. S. Chandra and C. Avedisian, Proc. R. Soc. London A 432, 13 (1991). https://doi.org/10.1098/rspa.1991.000258. M. Pasandideh‐Fard, Y. Qiao, S. Chandra, and J. Mostaghimi, Phys. Fluids 8, 650 (1996). https://doi.org/10.1063/1.86885059. C. Clanet, C. Beguin, D. Richard, and D. Quéré, J. Fluid Mech. 517, 199 (2004). https://doi.org/10.1017/S002211200400090460. C. Ukiwe and D. Y. Kwok, Langmuir 21, 666 (2005). https://doi.org/10.1021/la048128861. I. V. Roisman, Phys. Fluids 21, 052104 (2009). https://doi.org/10.1063/1.312928362. N. Laan, K. G. de Bruin, D. Bartolo, C. Josserand, and D. Bonn, Phys. Rev. Appl. 2, 044018 (2014). https://doi.org/10.1103/PhysRevApplied.2.044018The contact time (τc) is another parameter, which is defined as the time from a droplet contacting the surface to its complete rebounding. In 2002, Richard et al. first measured the droplet contact time on superhydrophobic surfaces after impacting, and they found τc was proportional to the inertial-capillary time, τ0 = (ρD03/8σ)1/2.6363. D. Richard, C. Clanet, and D. Quéré, Nature 417, 811 (2002). https://doi.org/10.1038/417811a They as well as many other researchers have proved that the prefactor is 2.6 ± 0.1, i.e., τc ≈ 2.6τ0.63–6563. D. Richard, C. Clanet, and D. Quéré, Nature 417, 811 (2002). https://doi.org/10.1038/417811a64. J. C. Bird, R. Dhiman, H. M. Kwon, and K. K. Varanasi, Nature 503, 385 (2013). https://doi.org/10.1038/nature1274065. C. Hao, J. Li, Y. Liu, X. Zhou, Y. Liu, R. Liu, L. Che, W. Zhou, D. Sun, L. Li, L. Xu, and Z. Wang, Nat. Commun. 6, 7986 (2015). https://doi.org/10.1038/ncomms8986 Actually, due to the symmetry in the processes of spreading and retracting, the Rayleigh limit of contact time (2.2τ0) can hardly be broken on flat superhydrophobic surfaces.6666. L. Rayleigh, Proc. R. Soc. London 29, 71 (1879). It should be noted that not all impacting droplets can rebound from superhydrophobic surfaces. If the kinetic pressure of impacting droplets overcomes the capillary pressure within the microstructures, the droplets penetrate into the microstructures (i.e., complete or partial water entrapment occurs),67–7067. M. McCarthy, K. Gerasopoulos, R. Enright, J. N. Culver, R. Ghodssi, and E. N. Wang, Appl. Phys. Lett. 100, 263701 (2012). https://doi.org/10.1063/1.472993568. J. Y. Ho, K. Fazle Rabbi, S. Khodakarami, X. Yan, L. Li, T. N. Wong, K. C. Leong, and N. Miljkovic, Nano Lett. 22, 2650 (2022). https://doi.org/10.1021/acs.nanolett.1c0446369. T. Maitra, M. K. Tiwari, C. Antonini, P. Schoch, S. Jung, P. Eberle, and D. Poulikakos, Nano Lett. 14, 172 (2014). https://doi.org/10.1021/nl403709270. S. Ryu, P. Sen, Y. Nam, and C. Lee, Phys. Rev. Lett. 118, 014501 (2017). https://doi.org/10.1103/PhysRevLett.118.014501 which may lead to complete or partial adhesion on superhydrophobic surfaces. Under low pressure conditions,7171. H. Lambley, T. M. Schutzius, and D. Poulikakos, Proc. Natl. Acad. Sci. U. S. A. 117, 27188 (2020). https://doi.org/10.1073/pnas.2008775117 or for hot water droplets,4444. T. Mouterde, P. Lecointre, G. Lehoucq, A. Checco, C. Clanet, and D. Quere, Nat. Commun. 10, 1410 (2019). https://doi.org/10.1038/s41467-019-09456-8 due to the condensation effect inside the microstructures, the probability of water entrapment becomes much larger. The current literature has demonstrated that through the reasonable microstructure design (e.g., adjustments of structure size, form, spacing, hierarchy, etc.), the entrapped water within the structures can be discharged again, or the droplet penetration can be avoided, thereby ensuring the complete droplet rebound on superhydrophobic surfaces.67,71–7467. M. McCarthy, K. Gerasopoulos, R. Enright, J. N. Culver, R. Ghodssi, and E. N. Wang, Appl. Phys. Lett. 100, 263701 (2012). https://doi.org/10.1063/1.472993571. H. Lambley, T. M. Schutzius, and D. Poulikakos, Proc. Natl. Acad. Sci. U. S. A. 117, 27188 (2020). https://doi.org/10.1073/pnas.200877511772. R. Zhang, P. Hao, and F. He, Langmuir 33, 3556 (2017). https://doi.org/10.1021/acs.langmuir.7b0056973. N. D. Patil, R. Bhardwaj, and A. Sharma, Exp. Therm. Fluid Sci. 74, 195 (2016). https://doi.org/10.1016/j.expthermflusci.2015.12.00674. F. Chu, X. Yan, and N. Miljkovic, Langmuir 38, 4452 (2022). https://doi.org/10.1021/acs.langmuir.2c00373 From a practical view, rebounding the impacting droplets completely and reducing the contact time as much as possible are of great significance for many engineering applications such as anti-icing and energy harvesting;38,75–7838. W. Xu, H. Zheng, Y. Liu, X. Zhou, C. Zhang, Y. Song, X. Deng, M. Leung, Z. Yang, R. X. Xu, Z. L. Wang, X. C. Zeng, and Z. Wang, Nature 578, 392 (2020). https://doi.org/10.1038/s41586-020-1985-675. L. Mishchenko, B. Hatton, V. Bahadur, J. A. Taylor, T. Krupenkin, and J. Aizenberg, ACS Nano 4, 7699 (2010). https://doi.org/10.1021/nn102557p76. S. H. Lee, M. Seong, M. K. Kwak, H. Ko, M. Kang, H. W. Park, S. M. Kang, and H. E. Jeong, ACS Nano 12, 10693 (2018). https://doi.org/10.1021/acsnano.8b0510977. X. Wang, S. Fang, J. Tan, T. Hu, W. Chu, J. Yin, J. Zhou, and W. Guo, Nano Energy 80, 105558 (2021). https://doi.org/10.1016/j.nanoen.2020.10555878. Y. Jin, C. Wu, P. Sun, M. Wang, M. Cui, C. Zhang, and Z. Wang, Droplet 1, 92 (2022). https://doi.org/10.1002/dro2.22 therefore, many design strategies of superhydrophobic surfaces are proposed to reduce the contact time of impacting droplets, among which the addition of macrostructures is a successful strategy, as introduced below.

III. MACROSTRUCTURE DESIGN FOR DROPLET CONTACT TIME REDUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. BASICS OF DROPLET IMP...III. MACROSTRUCTURE DESIG... <<IV. WETTABILITY PATTERN D...V. EFFECTS OF SUPERHYDROP...VI. SUMMARY AND PROSPECTSThe symmetries of both droplet spreading and retracting on flat superhydrophobic surfaces greatly limit the reduction of droplet contact time.6464. J. C. Bird, R. Dhiman, H. M. Kwon, and K. K. Varanasi, Nature 503, 385 (2013). https://doi.org/10.1038/nature12740 Therefore, it is an effective way to reduce the contact time of impacting droplets by constructing macrostructures on the superhydrophobic surface, which could break the symmetry of the droplet morphology evolution and change the mass distribution of droplet flow. In 2013, Bird et al. designed a macrostructure of submillimeter scale on superhydrophobic surfaces.6464. J. C. Bird, R. Dhiman, H. M. Kwon, and K. K. Varanasi, Nature 503, 385 (2013). https://doi.org/10.1038/nature12740 Upon impacting, the droplet flows away from the structure with the liquid mass and moment modulated; as a result, the contact time is reduced by 37% compared to flat superhydrophobic surfaces, which is below the Rayleigh limit.6464. J. C. Bird, R. Dhiman, H. M. Kwon, and K. K. Varanasi, Nature 503, 385 (2013). https://doi.org/10.1038/nature12740 Henceforward, researchers have made many attempts to reduce the contact time of impacting droplets by using macro-structured superhydrophobic surfaces.79–8379. Z. Hu, X. Zhang, S. Gao, Z. Yuan, Y. Lin, F. Chu, and X. Wu, J. Colloid Interface Sci. 599, 130 (2021). https://doi.org/10.1016/j.jcis.2021.04.07880. P. Chantelot, A. Mazloomi Moqaddam, A. Gauthier, S. S. Chikatamarla, C. Clanet, I. V. Karlin, and D. Quere, Soft Matter 14, 2227 (2018). https://doi.org/10.1039/C7SM02004J81. A. Gauthier, S. Symon, C. Clanet, and D. Quere, Nat. Commun. 6, 8001 (2015). https://doi.org/10.1038/ncomms900182. R. Zhang, P. Hao, and F. He, Langmuir 32, 9967 (2016). https://doi.org/10.1021/acs.langmuir.6b0264883. D.-J. Lin, L. Wang, X.-D. Wang, and W.-M. Yan, Int. J. Heat Mass Transfer 132, 1105 (2019). https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.087 Since the number, shape, and size of structures all play important roles in the solid–liquid interaction, diverse macrostructures have been designed, including cylindrical ridge [Fig. 2(a)],8181. A. Gauthier, S. Symon, C. Clanet, and D. Quere, Nat. Commun. 6, 8001 (2015). https://doi.org/10.1038/ncomms9001 wedge-shaped ridge,8383. D.-J. Lin, L. Wang, X.-D. Wang, and W.-M. Yan, Int. J. Heat Mass Transfer 132, 1105 (2019). https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.087 rectangular ridge [Fig. 2(b)],8484. Z. Hu, F. Chu, and X. Wu, Phys. Fluids 34, 092104 (2022). https://doi.org/10.1063/5.0105634 craterlike ridge,8585. C. Lin, K. Zhang, X. Chen, L. Xiao, S. Chen, J. Zhu, and T. Zou, Phys. Rev. Fluids 6, 083602 (2021). https://doi.org/10.1103/PhysRevFluids.6.083602 and conical convex [Fig. 2(c)].86–8886. Y. Shen, S. Liu, C. Zhu, J. Tao, Z. Chen, H. Tao, L. Pan, G. Wang, and T. Wang, Appl. Phys. Lett. 110, 221601 (2017). https://doi.org/10.1063/1.498423087. J. Luo, F. Chu, Z. Ni, J. Zhang, and D. Wen, Phys. Fluids 33, 112116 (2021). https://doi.org/10.1063/5.007304988. G. Durey, Q. Magdelaine, M. Casiulis, H. Kwon, J. Mazet, P. Chantelot, A. Gauthier, C. Clanet, and D. Quéré, Phys. Rev. Fluids 5, 110507 (2020). https://doi.org/10.1103/PhysRevFluids.5.110507 These macrostructures are usually in the size scale ranging from ∼100 μm to ∼1 mm, which is equivalent to or slightly smaller than the droplet size. However, due to the diversity of macrostructure shapes, it seems difficult to quantify the effect of structural forms on droplet impacting dynamics using a unified principle.The macrostructure array also shows great potential in reducing the droplet contact time because superhydrophobic macrostructure arrays have the functions of storing and releasing the surface energy of droplets. In 2014, Liu et al. fabricated superhydrophobic surfaces patterned with lattices of submillimetre-scale posts and observed that droplets were able to depart from the surface in a flattened, pancake shape in the spreading stage, which is known as the pancake bouncing.8989. Y. Liu, L. Moevius, X. Xu, T. Qian, J. M. Yeomans, and Z. Wang, Nat. Phys. 10, 515 (2014). https://doi.org/10.1038/nphys2980 For pancake bouncing, the droplet does not need to go through the retracting stage, so the contact time of the droplet can be greatly reduced by more than 80% compared to flat superhydrophobic surfaces.8989. Y. Liu, L. Moevius, X. Xu, T. Qian, J. M. Yeomans, and Z. Wang, Nat. Phys. 10, 515 (2014). https://doi.org/10.1038/nphys2980 The core mechanism of the pancake bouncing can be explained as follows. When a droplet penetrates the macrostructure array, its kinetic energy is stored in the capillary energy, and then under the effect of Laplace pressure, the stored capillary energy is transformed back into kinetic energy, making the spreading droplet leaving the surface. In addition, the pancake bouncing can also be observed on the array of ridges or cylindrical pillars, resulting in a reduction of contact time by more than 60%.90,9190. J. Song, M. Gao, C. Zhao, Y. Lu, L. Huang, X. Liu, C. J. Carmalt, X. Deng, and I. P. Parkin, ACS Nano 11, 9259 (2017). https://doi.org/10.1021/acsnano.7b0449491. M. Song, Z. Liu, Y. Ma, Z. Dong, Y. Wang, and L. Jiang, NPG Asia Mater. 9, e415 (2017). https://doi.org/10.1038/am.2017.122 However, the pancake bouncing greatly depends on droplet impact conditions and surface structures, in other words, the geometry, height, and spacing of macrostructures all need to be carefully designed.92,9392. Y. Liu, G. Whyman, E. Bormashenko, C. Hao, and Z. Wang, Appl. Phys. Lett. 107, 051604 (2015). https://doi.org/10.1063/1.492705593. Y.-S. Ko, J. Kim, S. Ryu, J. Han, Y. Nam, and C. Lee, Int. Commun. Heat Mass Transfer 137, 106235 (2022). https://doi.org/10.1016/j.icheatmasstransfer.2022.106235 Some criteria for different geometric arrays have been established to predict the pancake bouncing and guide the design of macrostructure arrays,92,94,9592. Y. Liu, G. Whyman, E. Bormashenko, C. Hao, and Z. Wang, Appl. Phys. Lett. 107, 051604 (2015). https://doi.org/10.1063/1.492705594. H. Wu, K. Jiang, Z. Xu, S. Yu, X. Peng, Z. Zhang, H. Bai, A. Liu, and G. Chai, Langmuir 35, 17000 (2019). https://doi.org/10.1021/acs.langmuir.9b0315395. Z. Hu, F. Chu, and X. Wu, Int. Commun. Heat Mass Transfer 136, 106167 (2022). https://doi.org/10.1016/j.icheatmasstransfer.2022.106167 but these are still far from enough.

IV. WETTABILITY PATTERN DESIGN FOR DROPLET IMPACTING REGULATION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. BASICS OF DROPLET IMP...III. MACROSTRUCTURE DESIG...IV. WETTABILITY PATTERN D... <<V. EFFECTS OF SUPERHYDROP...VI. SUMMARY AND PROSPECTSSurface wettability is a significant factor that affects the retracting process of impacting droplets. The impacting droplet on superhydrophobic surfaces usually fully retracts and then rebounds; while on hydrophilic surfaces, the droplet only retracts a little (or not at all), and then its three-phase contact lines get pinned on the surface.46,9646. C. Antonini, A. Amirfazli, and M. Marengo, Phys. Fluids 24, 102104 (2012). https://doi.org/10.1063/1.475712296. J. B. Lee, D. Derome, R. Guyer, and J. Carmeliet, Langmuir 32, 1299 (2016). https://doi.org/10.1021/acs.langmuir.5b04557 Hence, the uniform retraction of impacting droplets can be changed by setting a certain wettability pattern on the surface, thereby resulting in different droplet behaviors or motion forms.The impacting and rebounding of droplets on homogeneous superhydrophobic surfaces can be regarded as a one-dimensional process, that is, the movement of droplets is always in a straight line. In 2015, Schutzius et al. designed a hydrophilic arc on a superhydrophobic surface, and such an asymmetric wettability could trigger a lateral rebounding if the droplet impacts the center of the arc.9797. T. M. Schutzius, G. Graeber, M. Elsharkawy, J. Oreluk, and C. M. Megaridis, Sci. Rep. 4, 7029 (2014). https://doi.org/10.1038/srep07029 In other words, the one-dimensional droplet motion has been regulated to a two-dimensional transport by the arc wettability pattern.9797. T. M. Schutzius, G. Graeber, M. Elsharkawy, J. Oreluk, and C. M. Megaridis, Sci. Rep. 4, 7029 (2014). https://doi.org/10.1038/srep07029 Zhao et al. changed the hydrophilic arc to the hydrophilic straight line and established the correlation between the droplet lateral momentum and the surface area of a geometric region that depends on the position-coupling between the droplet maximum spreading and the wettability pattern.9898. Z. Zhao, H. Li, X. Hu, A. Li, Z. Cai, Z. Huang, M. Su, F. Li, M. Li, and Y. Song, Adv. Mater. Interfaces 6, 1901033 (2019). https://doi.org/10.1002/admi.201901033 We also realized the directional transport of droplets impacting on the boundary between the superhydrophobic surface and the hydrophilic surface [Fig. 3(a)].9999. F. Chu, J. Luo, C. Hao, J. Zhang, X. Wu, and D. Wen, Langmuir 36, 5855 (2020). https://doi.org/10.1021/acs.langmuir.0c00601 We demonstrated that a net lateral force was generated on the droplet contact line due to the wettability difference during the droplet retracting stage and developed a mechanical model to calculate lateral force and predict the lateral velocity.9999. F. Chu, J. Luo, C. Hao, J. Zhang, X. Wu, and D. Wen, Langmuir 36, 5855 (2020). https://doi.org/10.1021/acs.langmuir.0c00601 The droplet transportation distance could be more than ten times the droplet size if the adhesion length (i.e., covering length on the hydrophilic part by the droplet at the maximum spreading) is optimized.9999. F. Chu, J. Luo, C. Hao, J. Zhang, X. Wu, and D. Wen, Langmuir 36, 5855 (2020). https://doi.org/10.1021/acs.langmuir.0c00601 By the way, we also achieved the long-distance bidirectional transport of splitting droplets on the superhydrophobic surface with a single ridge, and the response of the bidirectional transport is very fast due to the reduction of contact time.100100. Z. Hu, F. Chu, X. Wu, S. Ding, and Y. Lin, Phys. Rev. Appl. 18, 044057 (2022). https://doi.org/10.1103/PhysRevApplied.18.044057By designing more complex wettability patterns, more plentiful droplet behaviors can be stimulated. Li et al. set several superhydrophilic lines on a superhydrophobic surface according to certain rules and showed that the translational motion of an impacting droplet could be converted to gyration [Fig. 3(b)].101101. H. Li, W. Fang, Y. Li, Q. Yang, M. Li, Q. Li, X. Q. Feng, and Y. Song, Nat. Commun. 10, 950 (2019). https://doi.org/10.1038/s41467-019-08919-2 The gyration behavior, with a maximum rotational speed exceeding 7300 rpm, is triggered by the synergetic effect of the asymmetric pinning forces originated from surface heterogeneity and the excess surface energy of the spreading droplet after impact.101101. H. Li, W. Fang, Y. Li, Q. Yang, M. Li, Q. Li, X. Q. Feng, and Y. Song, Nat. Commun. 10, 950 (2019). https://doi.org/10.1038/s41467-019-08919-2 Taking the opposite strategy, by designing superhydrophobic stripes on superhydrophilic surfaces, precise splitting of the impact droplets can be achieved [Fig. 3(c)], and this droplet splitting is attributed to the anisotropic retracting on the wettability-patterned surface.3939. H. Li, W. Fang, Z. Zhao, A. Li, Z. Li, M. Li, Q. Li, X. Feng, and Y. Song, Angew. Chem., Int. Ed. 59, 10535 (2020). https://doi.org/10.1002/anie.202003839 Recently, we proposed a liquid film sculpture strategy that can precisely sculpt a liquid film into any graphic pattern relying on the design of microstructured wettability-patterned surfaces [Fig. 3(d)].4040. F. Chu, Z. Ni, D. Wen, Y. Feng, S. Li, L. Jiang, and Z. Dong, Adv. Funct. Mater. 32, 2203222 (2022). https://doi.org/10.1002/adfm.202203222 The mechanism of the liquid film sculpture is that, upon droplet impacting on the liquid film covered wettability-patterned surfaces, the rupture of the residual film at the bottom of the impact cavity triggers the dewetting of the remaining liquid film.4040. F. Chu, Z. Ni, D. Wen, Y. Feng, S. Li, L. Jiang, and Z. Dong, Adv. Funct. Mater. 32, 2203222 (2022). https://doi.org/10.1002/adfm.202203222