Fast-freezing kinetics inside a droplet impacting on a cold surface

Frozen Patterns in Viscoelastic Droplets Impacting on a Subcooled Surface

The impact of droplets on a cold surface is ubiquitous in nature and various industrial applications, ranging from the icing of supercooled droplets on aircraft to the solidification of ink droplets in 3D printing. However, our understanding of the impact dynamics of droplets of complex fluids on cold surfaces is still very limited. Here, we experimentally study the spreading and frozen patterns of viscoelastic polymer droplets falling onto a subcooled substrate. We observe that the maximum spreading diameter of post-impact droplets decreases with increasing the subcooling temperature and the polymer concentration. Remarkably, all experimental data for spreading collapse into a universal curve, following the classic theory that accounts for inertial, capillary, and viscous forces. Unexpectedly, we find that, in contrast to the case of pure fluids, which exhibits three frozen modes, only two distinct modes, namely, freezing and hierarchical cracking, can be observed for polymer droplets. Finally, based on the undercooling temperature and polymer concentration, we construct a phase diagram for characterizing the morphologies of all frozen patterns. We expect that our findings may have implications in understanding the solidification of complex fluids on cold surfaces, for instance, in the fields of spray coating, inkjet printing, and additive manufacturing.

Droplet Impact on Superhydrophobic Mesh Surfaces

Reducing the contact time of droplet impacts on surfaces is crucial for various applications including corrosion prevention and anti-icing. This study aims to explore a novel strategy that greatly reduces contact time using a superhydrophobic mesh surface with multiple sets of mutually perpendicular ridges while minimizing the influence of the impacting location. The effects of the impact Weber numbers and ridge spacing on the characteristics of the impact dynamics and contact time are studied experimentally. The experimental results reveal that, for the droplet impact on mesh surfaces, ridges can segment the liquid film into independently multiple-retracting liquid subunits. The retracted subunits provide the upward driving force, which may promote the splashing or pancake bouncing of droplets. At this point, the contact time has a negligible sensitivity for the impacting position and is significantly reduced by up to 68%. Furthermore, the time, dynamic pressure, and energy criteria for triggering splashing and pancake bouncing are proposed theoretically. This work provides an understanding of the mechanism and the design guidelines for effectively reducing the contact time of the impacting droplet on superhydrophobic surfaces.

Freezing Delay of a Drop Impacting on a Monolayer of Cold Grains

We investigate a subfreezing droplet impact scenario in a low-humidity environment, where the target is a cold granular monolayer. When the undercooling degree of targets passes a threshold, such a granular layer effectively postpones the bulk freezing time of the droplet in comparison with the impact on the bare substrate underneath. In this case, the retraction of the droplet after impact reduces the contact area with the cold substrate, even though both the grains and the substrate are wettable to the liquid. We find that the significant changes in the dynamic behavior are triggered by freezing the liquid that wets the pores. Owing to the small dimension of the pores, the freezing process is rapid enough to match the dynamics over the droplet dimension. In certain circumstances, the rapid freezing may even stop liquid penetration and shed icing from the surface underneath.

Unraveling the role of vaporization momentum in self-jumping dynamics of freezing supercooled droplets at reduced pressures

Supercooling of water complicates phase change dynamics, the understanding of which remains limited yet vital to energy-related and aerospace processes. Here, we investigate the freezing and jumping dynamics of supercooled water droplets on superhydrophobic surfaces, induced by a remarkable vaporization momentum, in a low-pressure environment. The vaporization momentum arises from the vaporization at droplet's free surface, progressed and intensified by recalescence, subsequently inducing droplet compression and finally self-jumping. By incorporating liquid-gas-solid phase changes involving vaporization, freezing recalescence, and liquid-solid interactions, we resolve the vaporization momentum and droplet dynamics, revealing a size-scaled jumping velocity and a nucleation-governed jumping direction. A droplet-size-defined regime map is established, distinguishing the vaporization-momentum-dominated self-jumping from evaporative drying and overpressure-initiated levitation, all induced by depressurization and vaporization. Our findings illuminate the role of supercooling and low-pressure mediated phase change in shaping fluid transport dynamics, with implications for passive anti-icing, advanced cooling, and climate physics.

Cold granular targets slow the bulk freezing of an impacting droplet

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. The freezing process over the pore dimension occurs rapidly enough to match the characteristic timescales of impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears "hydrophobic," and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification.

Recent progress in understanding the anti-icing behavior of materials

Despite the significant progress in fundamental research in the physics of atmospheric icing or the revolutionary changes in modern materials and coatings achieved due to the recent development of nanotechnology and synthetic chemistry, the problem of reliable protection against atmospheric icing remains a hot topic of surface science. In this paper, we present a brief analysis of the mechanisms of anti-icing behavior that attracted the greatest interest of the scientific community and approaches which realize these mechanisms. We also note the strengths and weaknesses of such approaches and discuss future studies and prospects for the practical application of developed coatings.

Arrested Dynamics of Droplet Spreading on Ice

We investigate the arrested spreading of room temperature droplets impacting flat ice. The use of an icy substrate eliminates the nucleation energy barrier, such that a freeze front can initiate as soon as the droplet's temperature cools down to 0 °C. We employ scaling analysis to rationalize distinct regimes of arrested hydrodynamics. For gently deposited droplets, capillary-inertial spreading is halted at the onset of contact line freezing, yielding a 1/7 scaling law for the arrested diameter. At low impact velocities (We≲100), inertial effects result in a 1/2 scaling law. At higher impact velocities (We>100), inertio-viscous spreading can spill over the frozen base of the droplet until its velocity matches that of a kinetic freeze front caused by local undercooling, resulting in a 1/5 scaling law.

Contact Line Catch Up by Growing Ice Crystals

The effect of freezing on contact line motion is a scientific challenge in the understanding of the solidification of capillary flows. In this Letter, we experimentally investigate the spreading and freezing of a water droplet on a cold substrate. We demonstrate that solidification stops the spreading because the ice crystals catch up with the advancing contact line. Indeed, we observe the formation and growth of ice crystals along the substrate during the drop spreading, and show that their velocity equals the contact line velocity when the drop stops. Modeling the growth of the crystals, we predict the shape of the crystal front and show that the substrate thermal properties play a major role on the frozen drop radius.

Modulating the Pore Architecture of Ice-Templated Dextran Microparticles Using Molecular Weight and Concentration

Spray freeze drying (SFD) is an ice templating method used to produce highly porous particles with complex pore architectures governed by ice nucleation and growth. SFD particles have been advanced as drug carrier systems, but the quantitative description of the morphology formation in the SFD process is still challenging. Here, the pore space dimensions of SFD particles prepared from aqueous dextran solutions of varying molecular weights (40-200 kDa) and concentrations (5-20%) are analyzed using scanning electron microscopy. Coexisting morphologies composed of cellular and dendritic motifs are obtained, which are attributed to variations in the ice growth mechanism determined by the SFD system and modulation of these mechanisms by given precursor solution properties leading to changes in their pore dimensions. Particles with low-aspect ratio cellular pores showing variation of around 0.5-1 μm in diameter with precursor composition but roughly constant with particle diameter are ascribed to a rapid growth regime with high nucleation site density. Image analysis suggests that the pore volume decreases with dextran solid content. Dendritic pores (≈2-20 μm in diameter) with often a central cellular region are identified with surface nucleation and growth followed by a slower growth regime, leading to the overall dendrite surface area scaling approximately linearly with the particle diameter. The dendrite lamellar spacing depends on the concentration according to an inverse power law but is not significantly influenced by molecular weight. Particles with highly elongated cellular pores without lamellar formation show intermediate pore dimensions between the above two limiting morphological types. Analysis of variance and post hoc tests indicate that dextran concentration is the most significant factor in affecting the pore dimensions. The SFD dextran particles herein described could find use in pulmonary drug delivery due to their high porosity and biocompatibility of the matrix material.

Pancake Jumping of Sessile Droplets

Rapid droplet shedding from surfaces is fundamentally interesting and important in numerous applications such as anti-icing, anti-fouling, dropwise condensation, and electricity generation. Recent efforts have demonstrated the complete rebound or pancake bouncing of impinging droplets by tuning the physicochemical properties of surfaces and applying external control, however, enabling sessile droplets to jump off surfaces in a bottom-to-up manner is challenging. Here, the rapid jumping of sessile droplets, even cold droplets, in a pancake shape is reported by engineering superhydrophobic magnetically responsive blades arrays. This largely unexplored droplet behavior, termed as pancake jumping, exhibits many advantages such as short interaction time and high energy conversion efficiency. The critical conditions for the occurrence of this new phenomenon are also identified. This work provides a new toolkit for the attainment of well-controlled and active steering of both sessile and impacting droplets for a wide range of applications.

Improving Compactness of 3D Metallic Microstructures Printed by Laser-Induced Forward Transfer

Laser-induced forward transfer (LIFT) has been shown to be a useful technique for the manufacturing of micron-scale metal structures. LIFT is a high-resolution, non-contact digital printing method that can support the fabrication of complex shapes and multi-material structures in a single step under ambient conditions. However, LIFT printed metal structures often suffer from inferior mechanical, electrical, and thermal properties when compared to their bulk metal counterparts, and often are prone to enhanced chemical corrosion. This is due mostly to their non-compact structures, which have voids and inter-droplet delamination. In this paper, a theoretical framework together with experimental results of achievable compactness limits is presented for a variety of metals. It is demonstrated that compactness limits depend on material properties and jetting conditions. It is also shown how a specific choice of materials can yield compact structures, for example, when special alloys are chosen along with a suitable donor construct. The example of printed amorphous ZrPd is detailed. This study contributes to a better understanding of the limits of implementing LIFT for the fabrication of metal structures, and how to possibly overcome some of these limitations.

Transient Prediction of Nanoparticle-Laden Droplet Drying Patterns through Dynamic Mode Decomposition

Nanoparticle-laden sessile droplet drying has a wide impact on applications. However, the complexity affected by the droplet evaporation dynamics and particle self-assembly behavior leads to challenges in the accurate prediction of the drying patterns. We initiate a data-driven machine learning algorithm by using a single data collection point via a top-view camera to predict the transient drying patterns of aluminum oxide (Al₂O₃) nanoparticle-laden sessile droplets with three cases according to particle sizes of 5 and 40 nm and Al₂O₃ concentrations of 0.1 and 0.2 wt %. Dynamic mode decomposition is used as the data-driven learning model to recognize each nanoparticle-laden droplet as an individual system and then apply the transfer learning procedure. Along 270 s of droplet drying experiments, the training period of the first 100 s is selected, and then the rest of the 170 s is predicted with less than a 10% error between the predicted and the actual droplet images. The developed data-driven approach has also achieved the acceptable prediction for the droplet diameter with less than 0.13% error and a coffee-ring thickness over a range of 2.0 to 6.7 μm. Moreover, the proposed machine learning algorithm can recognize the volume of the droplet liquid and the transition of the drying regime from one to another according to the predicted contact line and the droplet height.

How different freezing morphologies of impacting droplets form

HYPOTHESIS

Freezing morphologies of impacting water droplets depend on the interaction between droplet spreading and solidification. The existing studies showed that the shape of frozen droplets mostly is of spherical cap with a singular tip, because of much shorter timescale of the droplet spreading than that of the solidification. Here, we create the experimental conditions of extended droplet spreading and greatly enhanced heat transfer for fast solidification, thereby allowing to study such droplet freezing process under the strong coupling of the droplet spreading and solidification.

EXPERIMENTS

We design experiments that a room-temperature water droplet impacts on a subcooled superhydrophilic surface in an enclosure chamber filled with nitrogen gas. We thoroughly investigate the freezing processes of impacting droplets under the effects of impact velocity and substrate temperature. Both the droplet impact dynamics and solidification are studied with a high-speed camera.

FINDINGS

We observed five different freezing morphologies which depend on the droplet impact velocity and substrate temperature. We found that the formation of diverse morphologies results from the competitive timescales related to droplet solidification and impact hydrodynamics. We also develop a phase diagram based on scaling analysis and show how freezing morphologies are controlled by droplet impact and freezing related timescales.

Pattern Formation during the Impact of a Partially Frozen Binary Droplet on a Cold Surface

The impact of a droplet on an undercooled surface is a complex phenomenon as it simultaneously instigates several physical processes that cover a broad spectrum of transport phenomena and phase transition. Here, we report and explain an unexpected but highly relevant phenomenon of fingered growth of the solid phase. It emerges during the impact of a binary droplet that freezes from the outside prior to the impact on the undercooled surface. We establish that the presence of presolidified material at the advancing contact line fundamentally changes the resulting dynamics, namely, by modifying the local flow mobility that leads to an instability analogous to viscous fingering. Moreover, we delineate the interplay between the interfacial deformations of the impacting droplet and patterned growth of the solid phase as disconnected patterns emerge at faster impacts.

Drop impact on hot plates: contact times, lift-off and the lamella rupture

When a liquid drop impacts on a heated substrate, it can remain deposited, or violently boil in contact, or lift off with or without ever touching the surface. The latter is known as the Leidenfrost effect. The duration and area of the liquid-substrate contact are highly relevant for the heat transfer, as well as other effects such as corrosion. However, most experimental studies rely on side view imaging to determine contact times, and those are often mixed with the time until the drop lifts off from the substrate. Here, we develop and validate a reliable method of contact time determination using high-speed X-ray imaging and total internal reflection imaging. We exemplarily compare contact and lift-off times on flat silicon and sapphire substrates. We show that drops can rebound even without formation of a complete vapor layer, with a wide range of lift-off times. On sapphire, we find a local minimum of lift-off times that is much shorter than expected from capillary rebound in the comparatively low-temperature regime of transition boiling/thermal atomization. We elucidate the underlying mechanism related to spontaneous rupture of the lamella and receding of the contact area.