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1. Microstructural evolution of droplet phase separation in calcium aluminosilicate glasses

   https://doi.org/10.1111/jace.18050  

   Clark, Nicholas L.; Chuang, Shih‐Yi; Mauro, John C. (August 2021, Journal of the American Ceramic Society)

   Abstract Glasses with nanoscale phase separation have the potential to possess improved hardness and fracture toughness while maintaining their optical transparency. Here we present the results of isothermal heat treatments of phase‐separated calcium aluminosilicate glasses. Our results indicate that a transition from Lifshitz–Slozof–Wagner (LSW)‐type kinetics to a diffusion‐controlled pseudo‐coalescence mechanism occurs at ~17% droplet volume fraction, which results in the droplets becoming increasingly elongated and interconnected. The activation barrier for both mechanisms suggests that calcium diffusion is the underlying means for the coarsening of the silica‐rich domains. Simple approximations show the transition cannot be explained by Brownian motion or Van der Waals attraction between domains, and instead suggest various osmotic forces may be responsible. 

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2. Scaling Law-Based Analysis of Coalescence Dynamics of Colloidal Droplets on Substrates During the Freezing Process

   https://doi.org/10.1115/FEDSM2025-158596  

   Zhang, Haipeng; Kumar, Sandeep; Liu, Yang (July 2025, American Society of Mechanical Engineers)

   Abstract In order to improve the quality of products during additive manufacturing, we developed a novel freezing sublimation-based method for inkjet-based three-dimensional (3D) printing technology, which can significantly improve the uniformity of material distribution in printed products. In our previous studies, we used a laboratory prototype with single droplets of inkjet solution containing colloidal particles to prove the concept of this study. However, understanding the interaction between droplets on the printing substrate surface is also crucial for determining the printing resolution and accuracy of this method, which cannot be fully investigated through single droplet-based experimental studies. To fill this knowledge gap, we conducted a series of experiments on colloidal droplet impingement, freezing, and sublimation on substrates using dual droplets. The experimental setup allowed the release of two droplets in quick succession from a modified nozzle with two needles. These droplets coalesced on the substrate surface due to spreading during their impingement processes. Observations revealed that the coalescence pattern of these two droplets varied depending on the time interval between their release. When the second droplet was released immediately after the first, their coalescence was governed by fluid dynamics. However, when the second droplet was released after the first droplet had frozen on the substrate, it spread above the ice surface of the first droplet in a relatively slower process. This observation provides new insights for the continued study and optimization of the proposed novel freezing sublimation-based 3D printing method. 

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3. Electrowetting-based coalescence of droplets during dropwise condensation of humid air

   Wikramanayake, Enakshi; Bahadur, Vaibhav (July 2019, Summer Heat Transfer Conference)

   Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement. 

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4. On‐Demand Programming of Liquid Metal‐Composite Microstructures through Direct Ink Write 3D Printing

   https://doi.org/10.1002/adma.202200182  

   Haake, Aaron; Tutika, Ravi; Schloer, Gwyneth_M; Bartlett, Michael_D; Markvicka, Eric_J (April 2022, Advanced Materials)

   Abstract Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle‐like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high‐power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response. 

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5. Jet Formation After Droplet Impact on Microholed Hydrophilic Surfaces

   Md Nur E Alam and Hua Tan (November 2022, IMECE2022)

   Droplet impacts on solid surfaces produce a wide variety of phenomena such as spreading, splashing, jetting, receding, and rebounding. In microholed surfaces, downward jets through the hole can be caused by the high impact inertia during the spreading phase of the droplet over the substrate as well as the cavity collapse during recoil phase of the droplet. We investigate the dynamics of the jet formed through the single hole during the impacting phase of the droplet on a micro-holed hydrophilic substrate. The sub-millimeter circular holes are created on the 0.2 mm-thickness hydrophilic plastic films using a 0.5 mm punch. Great care has been taken to ensure that the millimeter-sized droplets of water dispensed by a syringe pump through a micropipette tip can impact directly over the micro-holes. A high-speed video photography camera is employed to capture the full event of impacting and jetting. A MATLAB code has been developed to process the captured videos for data analysis. We study the effect of impact velocity on the jet formation including jet velocity, ejected droplet volume, and breakup process. We find that the Weber number significantly affects outcomes of the drop impact and jetting mechanism. We also examine the dynamic contact angle of the contact line during the spreading and the receding phase. 

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