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Perumanath S, Chubynsky MV, Pillai R, Borg MK, Sprittles JE. Rolling and Sliding Modes of Nanodroplet Spreading: Molecular Simulations and a Continuum Approach. PHYSICAL REVIEW LETTERS 2023; 131:164001. [PMID: 37925699 DOI: 10.1103/physrevlett.131.164001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/12/2022] [Revised: 05/11/2023] [Accepted: 08/15/2023] [Indexed: 11/07/2023]
Abstract
Molecular simulations discover a new mode of dynamic wetting that manifests itself in the very earliest stages of spreading, after a droplet contacts a solid. The observed mode is a "rolling" type of motion, characterized by a contact angle lower than the classically assumed value of 180°, and precedes the conventional "sliding" mode of spreading. This motivates the development of a novel continuum framework that captures all modes of motion, allows the dominant physical mechanisms to be understood, and permits the study of larger droplets.
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Affiliation(s)
- Sreehari Perumanath
- Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Mykyta V Chubynsky
- Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom
- Centre for Fluid and Complex Systems, Coventry University, Coventry, CV1 5FB, United Kingdom
| | - Rohit Pillai
- School of Engineering, University of Edinburgh, Edinburgh EH9 3FB, United Kingdom
| | - Matthew K Borg
- School of Engineering, University of Edinburgh, Edinburgh EH9 3FB, United Kingdom
| | - James E Sprittles
- Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom
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Binysh J, Chakraborty I, Chubynsky MV, Melian VLD, Waitukaitis SR, Sprittles JE, Souslov A. Modeling Leidenfrost Levitation of Soft Elastic Solids. PHYSICAL REVIEW LETTERS 2023; 131:168201. [PMID: 37925690 DOI: 10.1103/physrevlett.131.168201] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2022] [Revised: 07/14/2023] [Accepted: 09/05/2023] [Indexed: 11/07/2023]
Abstract
The elastic Leidenfrost effect occurs when a vaporizable soft solid is lowered onto a hot surface. Evaporative flow couples to elastic deformation, giving spontaneous bouncing or steady-state floating. The effect embodies an unexplored interplay between thermodynamics, elasticity, and lubrication: despite being observed, its basic theoretical description remains a challenge. Here, we provide a theory of elastic Leidenfrost floating. As weight increases, a rigid solid sits closer to the hot surface. By contrast, we discover an elasticity-dominated regime where the heavier the solid, the higher it floats. This geometry-governed behavior is reminiscent of the dynamics of large liquid Leidenfrost drops. We show that this elastic regime is characterized by Hertzian behavior of the solid's underbelly and derive how the float height scales with materials parameters. Introducing a dimensionless elastic Leidenfrost number, we capture the crossover between rigid and Hertzian behavior. Our results provide theoretical underpinning for recent experiments, and point to the design of novel soft machines.
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Affiliation(s)
- Jack Binysh
- Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
| | | | - Mykyta V Chubynsky
- Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Vicente Luis Díaz Melian
- Institute of Science and Technology Austria (ISTA), Lab Building West, Am Campus 1, 3400 Klosterneuburg, Austria
| | - Scott R Waitukaitis
- Institute of Science and Technology Austria (ISTA), Lab Building West, Am Campus 1, 3400 Klosterneuburg, Austria
| | - James E Sprittles
- Mathematics Institute, University of Warwick, Coventry CV4 7AL, United Kingdom
| | - Anton Souslov
- Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
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Lin M, Vo Q, Mitra S, Tran T. Viscous droplet impingement on soft substrates. SOFT MATTER 2022; 18:5474-5482. [PMID: 35833825 DOI: 10.1039/d2sm00709f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Viscous droplets impinging on soft substrates may exhibit several distinct behaviours including repeated bouncing, wetting, and hovering, i.e., spreading and retracting after impact without bouncing back or wetting. We experimentally study the conditions enabling these characteristic behaviours by systematically varying substrate elasticity, impact velocity and liquid viscosity. For each substrate elasticity, the transition to wetting is determined as the dependence of the Weber number We, which measures the droplet's kinetic energy against its surface energy, on the Ohnesorge number Oh, which compares viscosity to inertia and capillarity. We find that while We at the wetting transition monotonically decreases with Oh for relatively rigid substrates, it exhibits a counter-intuitive behaviour in which it first increases and then gradually decreases for softer substrates. We experimentally determine the dependence of the maximum Weber number allowing non-wetting impacts on substrate elasticity and show that it provides an excellent quantitative measure of liquid repellency for a wide range of surfaces, from liquid to soft surfaces and non-deformable surfaces.
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Affiliation(s)
- Marcus Lin
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.
| | - Quoc Vo
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.
| | - Surjyasish Mitra
- Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
| | - Tuan Tran
- School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.
- Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
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Chen Z, Hu Y, He X, Xu Y, Liu X, Zhou Y, Hao L, Ruan Y. One-step fabrication of soft calcium superhydrophobic surfaces by a simple electrodeposition process. RSC Adv 2021; 12:297-308. [PMID: 35424497 PMCID: PMC8978675 DOI: 10.1039/d1ra06019h] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2021] [Accepted: 11/24/2021] [Indexed: 11/30/2022] Open
Abstract
A simple, one-step electrodeposition process was rapidly performed on a metal substrate to fabricate calcium superhydrophobic surfaces in an electrolyte containing calcium chloride (CaCl2), myristic acid (CH3(CH2)12COOH), and ethanol, which can avoid the intricate post-processing of surface treatment. The morphology and surface chemical compositions of the fabricated superhydrophobic surfaces were systematically examined by means of SEM, XRD, and FTIR, respectively. The results indicate that the deposited surfaces were mainly composed of calcium myristate, which can dramatically lower surface free energy. The shortest process for constructing a superhydrophobic surface is about 0.5 min, and the maximum contact angle of the as-prepared surfaces can reach as high as 166°, showing excellent superhydrophobicity. By adjusting the electrodeposition time, the structure of the cathodic surface transforms from the turfgrass structure, loose flower structures, larger and dense flower structures, secondary flower structures, and then into tertiary or more flower structures. The superhydrophobic surfaces showed excellent rebound performance with a high-speed camera. After a pressing force, their hardness increases, but the superhydrophobic performance is not weakened. Inversely, the bouncing performance is enhanced. This electrodeposition process offers a promising approach for large areas of superhydrophobic surfaces on conductive metals and strongly impacts the dynamics of water droplets. We investigated a one-step method for calcium superhydrophobic surface preparation and researched the formation process of loose, flower-like microstructures. Also, we found that the pressing force strongly impacts the dynamics of water droplets.![]()
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Affiliation(s)
- Zhi Chen
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Yongbo Hu
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Xu He
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Yihao Xu
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Xuesong Liu
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Yizhou Zhou
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
| | - Limei Hao
- Department of Applied Physics, School of Science, Xi'an University of Science & Technology Xi'an 710054 China
| | - Ying Ruan
- School of Physical Science and Technology, Northwestern Polytechnical University Xi'an 710129 China +86-29-88431664 +86-29-88431664
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Hu M, Zhou J, Li Y, Zhuo X, Jing D. Effects of the surface wettability of nanoparticles on the impact dynamics of droplets. Chem Eng Sci 2021. [DOI: 10.1016/j.ces.2021.116977] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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Li T, Wu Y. Impact Dynamics of Nanodroplets on V-Shaped Substrates: Asymmetrical Behavior and Fast-Rebound Dynamics. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2021; 37:13170-13178. [PMID: 34699717 DOI: 10.1021/acs.langmuir.1c02488] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Controlling the drop impact behaviors plays an important role in both designing surface functional materials and improving industrial techniques. Here, the impact dynamics of nanodroplets on "V-shaped" substrates is studied, which is manifested as asymmetrical spreading and retraction behaviors that could be weakened by increasing the angles of the "V" shape, accompanied by the evolution of the shape from leaf-like, stripe-like, to dumbbell-like at v = 20 Å/ps. When v increases to 40 Å/ps, the previously deposited nanodroplets could be transformed to a rebound one at α = 60 and 90°, which is ascribed to the combined effect of the confinement conditions and the extrusion force. Furthermore, an impact behavior map as a function of impact velocity and angle is depicted, which suggests that a decrease in angles or an increase in the impact velocity is likely to cause the nanodroplets to rebound. More importantly, the results give visible evidence that compared to the flat substrates, the V-shaped structure is advantageous for achieving a fast-rebound behavior even at a low speed of impact, which should be good news for practical applications in many multidisciplinary fields, such as self-cleaning, anti-icing/fogging, pollution prevention, energy storage, and so forth.
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Affiliation(s)
- Tao Li
- Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China
- Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, China
| | - Yan Wu
- Department of Chemistry, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China
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Abstract
The impact and splash of liquid drops on solid substrates are ubiquitous in many important fields. However, previous studies have mainly focused on spherical drops while the non-spherical situations, such as raindrops, charged drops, oscillating drops, and drops affected by electromagnetic field, remain largely unexplored. Using ferrofluid, we realize various drop shapes and illustrate the fundamental role of shape in impact and splash. Experiments show that different drop shapes produce large variations in spreading dynamics, splash onset, and splash amount. However, underlying all these variations we discover universal mechanisms across various drop shapes: the impact dynamics is governed by the superellipse model, the splash onset is triggered by the Kelvin-Helmholtz instability, and the amount of splash is determined by the energy dissipation before liquid taking off. Our study generalizes the drop impact research beyond the spherical geometry, and reveals the potential of using drop shape to control impact and splash. The dynamics of droplet impact and splash is important in many applications, yet its analysis involves difficult intertwined aspects. Here Liu et al. make shape parametrically accessible to experiment, with ferrofluidic drops passing a magnetic field in a defined way.
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