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1. Spreading dynamics of a droplet impacting a sphere

   https://doi.org/10.1063/5.0120642  

   Long, Ming; Hasanyan, Jalil; Jung, Sunghwan (October 2022, Physics of Fluids)

   In nature, high-speed rain drops often impact and spread on curved surfaces, e.g., leaves and animal bodies. Although a drop's impact on a surface is a traditional topic for industrial applications, drop-impact dynamics on curved surfaces are less known. In the present study, we examine the time-dependent spreading dynamics of a drop onto a curved hydrophobic surface. We also observed that a drop on a curved surface spreads farther than one on a flat surface. To further understand the spreading dynamics, a new analytical model is developed based on volume conservation and temporal energy balance. This model converges to previous models at the early stage and the final stage of droplet impact. We compared the new model with measured spreading lengths on various curved surfaces and impact speeds, which resulted in good agreement. 

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2. Drop impact on solids: contact-angle hysteresis filters impact energy into modal vibrations

   https://doi.org/10.1017/jfm.2021.547  

   Kern, Vanessa R.; Bostwick, Joshua B.; Steen, Paul H. (September 2021, Journal of Fluid Mechanics)

   The energetics of drop deposition are considered in the capillary-ballistic regime characterized by high Reynolds number and moderate Weber number. Experiments are performed impacting water/glycol drops onto substrates with varying wettability and contact-angle hysteresis. The impacting event is decomposed into three regimes: (i) pre-impact, (ii) inertial spreading and (iii) post contact-line (CL) pinning, conveniently framed using the theory of Dussan & Davis ( J. Fluid Mech. , vol. 173, 1986, pp. 115–130). During fast-time-scale inertial spreading, the only form of dissipation is CL dissipation ( $$\mathcal {D}_{CL}$$ ). High-speed imaging is used to resolve the stick-slip dynamics of the CL with $$\mathcal {D}_{CL}$$ measured directly from experiment using the $$\Delta \alpha$$ - $$R$$ cyclic diagram of Xia & Steen ( J. Fluid Mech. , vol. 841, 2018, pp. 767–783), representing the contact-angle deviation against the CL radius. Energy loss occurs on slip legs, and this observation is used to derive a closed-form expression for the kinetic K and interfacial $$\mathcal{A}$$ post-pinning energy $$\{K+\mathcal {A}\}_p/\mathcal {A}_o$$ independent of viscosity, only depending on the rest angle $$\alpha _p$$ , equilibrium angle $$\bar {\alpha }$$ and hysteresis $$\Delta \alpha$$ , which agrees well with experimental observation over a large range of parameters, and can be used to evaluate contact-line dissipation during inertial spreading. The post-pinning energy is found to be independent of the pre-impact energy, and it is broken into modal components with corresponding energy partitioning approximately constant for low-hysteresis surfaces with fixed pinning angle $$\alpha _p$$ . During slow-time-scale post-pinning, the liquid/gas ( $lg$ ) interface is found to vibrate with the frequencies and mode shapes predicted by Bostwick & Steen ( J. Fluid Mech. , vol. 760, 2014, pp. 5–38), irrespective of the pre-impact energy. Resonant mode decay rates are determined experimentally from fast Fourier transforms of the interface dynamics. 

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3. Predictive modeling of drop impact force on concave targets

   https://doi.org/10.1063/5.0116795  

   Dickerson, Andrew K.; Alam, MD Erfanul; Buckelew, Jacob; Boyum, Nicholas; Turgut, Damla (October 2022, Physics of Fluids)

   Impacting drops are ubiquitous and the corresponding impact force is their most studied dynamic quantity. However, impact forces arising from collisions with curved surfaces are understudied. In this study, we impact small cups with falling drops across drop Reynolds number 2975–12 800, isolating five dominant parameters influencing impact force: drop height and diameter, surface curvature and wettability, and impact eccentricity. These parameters are effectively continuous in their domain and have stochastic variability. The unpredictable dynamics of the system incentivize the implementation of tools that can unearth relationships between parameters and make predictions about impact force for parameter values for which there is not explicit experimental data. We predict force due to the impacting drop in a concave target using an ensemble learning algorithm comprised of four base algorithms: a random forest regressor, k-nearest neighbor, a gradient boosting regressor, and a multi-layer perceptron. We train and test our algorithm with original experimental data comprising 387 total trials using four cup radii with two wetting conditions each. Our approach permits the determination of relative importance of the input features in producing impact force and force predictions which can be compared to scaling relations modified from those for flat targets. Algorithmic predictions indicate that deformation of the drop and surface wettability, often neglected in scaling for impact force on flat surfaces, are important for concave targets. Finally, our approach provides another opportunity for the application of machine learning to characterize complex systems' fluid mechanics for which experimental variables are numerous and vary independently. 

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4. Hydrophobicity versus pore size: polymer coatings to improve membrane wetting resistance for membrane distillation

   https://doi.org/10.1021/acsapm.9b01133  

   McGaughey, Allyson L.; Karandikar, Prathamesh; Gupta, Malancha; Childress, Amy E. (February 2020, ACS applied polymer materials)

   null (Ed.)

   Initiated chemical vapor deposition (iCVD) was used to coat two porous substrates (i.e., hydrophilic cellulose acetate (CA) and hydrophobic polytetrafluoroethylene (PTFE)) with a crosslinked fluoropolymer to improve membrane wetting resistance. The coated CA membrane was superhydrophobic and symmetric. The coated PTFE membrane was hydrophobic and asymmetric, with smaller pore size and lower porosity on the top surface than on the bottom surface. Membrane performance was tested in membrane distillation experiments with (1) a high-salinity feed solution and (2) a surfactant-containing feed solution. In both cases, the coated membranes had higher wetting resistance than the uncoated membranes. Notably, wetting resistances were better predicted by LEP distributions than by minimum LEP values. When LEP distributions were skewed towards high LEP values (i.e., when small pores with high LEP were greater in number), significant (measurable) salt passage did not occur. For the high-salinity feed solution, the coated PTFE membrane had greater wetting resistance than the coated CA membrane; thus, reduced surface pore size/porosity (which may reduce intrapore scaling) was more effective than increased surface hydrophobicity (which may reduce surface nucleation) in preventing scaling-induced wetting. Reduced pore size/porosity was equally as effective as increased hydrophobicity in resisting surfactant-induced wetting. However, reduced porosity can negatively impact water flux; this represents a permeability/wetting resistance tradeoff in membrane distillation – especially for high-salinity applications. Membrane and/or membrane coating properties must be optimized to overcome this permeability/wetting resistance tradeoff and make MD viable for the treatment of challenging streams. Then, increasing hydrophobicity may not be necessary to impart high wetting resistance to porous membranes. These results are important for future membrane design, especially as manufacturers seek to replace perfluorinated materials with environmentally friendly alternatives. 

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5. Effects of surfactant properties on pore wetting of membrane distillation

   https://doi.org/10.1016/j.memlet.2024.100077  

   Coolidge, Connor; Alhadidi, Azal_Mohammed Hassan; Wang, Wei; Tong, Tiezheng (December 2024, Journal of Membrane Science Letters)

   Pore wetting is a major constraint to the performance of membrane distillation (MD) for hypersaline brine treatment. Despite the existence of surfactants with diverse properties, an explicit relationship between the properties of surfactants and their capabilities of inducing pore wetting has yet to be established. In this study, we perform a comparative analysis of the wetting behaviors of various surfactants with different charges and molecular weights in MD desalination. The induction time of surfactants to initiate pore wetting was correlated to the apparent contact angle and surface tension of the feedwater. Our results show that different surfactants resulting in similar feedwater surface tensions can lead to drastically different wetting potential, suggesting that both charge of the head group and molecular weight of surfactants have a significant influence on membrane pore wetting. Further, we demonstrate that parameters that have been commonly used to indicate wetting potential, including apparent contact angle and solution surface tension, are not reliable in predicting the wetting behavior of MD membranes, which is intricately linked with surfactant properties such as charge and molecular size. We envision that our results not only improve our fundamental understanding of surfactant-induced wetting but also provide valuable insights that necessitate thorough consideration of surfactant properties in evaluating wetting potential and membrane wetting resistance for MD desalination. 

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