Cold Substrate Research Articles

Solidification dynamics of an impacted drop

This paper is dedicated to the solidification of a water drop impacting a cold solid surface. In the first part, we establish a one-dimensional (1-D) solidification model, derived from the Stefan problem, that aims at predicting the freezing dynamics of a liquid on a cold substrate, taking into account the thermal properties of this substrate. This model is then experimentally validated through a 1-D solidification set-up, using different liquids and substrates. In the second part, we show that during the actual drop spreading, a thin layer of ice develops between the water and the substrate and pins the contact line at its edge when the drop reaches its maximal diameter. The liquid film then remains still on the ice and keeps freezing. This configuration lasts until the contact line eventually unpins and the liquid film retracts on the ice. We measure and interpret this crucial time of freezing during which the main ice layer is built. Finally, we compare our 1-D model prediction to the thickness of this ice pancake and we find a very good agreement. This allows us to provide a general expression for the frozen drop’s main thickness, using the drop’s impact and liquid parameters.

Optical Characterization of Non-Stoichiometric Silicon Nitride Films Exhibiting Combined Defects

The study was devoted to optical characterization of non-stoichiometric silicon nitride films prepared by reactive magnetron sputtering in argon-nitrogen atmosphere onto cold (unheated) substrates. It was found that these films exhibit the combination of three defects: optical inhomogeneity (refractive index profile across the films), uniaxial anisotropy with the optical axis perpendicular to the boundaries and random roughness of the upper boundaries. The influence of the uniaxial anisotropy was included into the corresponding formulae of the optical quantities using the matrix formalism and the approximation of the inhomogeneous layer by a multilayer system consisting of large number thin homogeneous layers. The random roughness was described using the scalar diffraction theory. The processing of the experimental data was performed using the multi-sample modification of the least-squares method, in which experimental data of several samples differing in thickness were processed simultaneously. The dielectric response of the silicon nitride films was modeled using the modification of the universal dispersion model, which takes into account absorption processes corresponding to valence-to-conduction band electron transitions, excitonic effects and Urbach tail. The spectroscopic reflectometric and ellipsometric measurements were supplemented by measuring the uniformity of the samples using imaging spectroscopic reflectometry.

Formation of complex organic molecules in astrophysical environments: Sugars and derivatives

AbstractCarbonaceous meteorites contain a large variety of complex organic molecules, including amino acids, nucleobases, sugar derivatives, amphiphiles, and other compounds of astrobiological interest. Photoprocessing of ices condensed on cold grains with ultraviolet (UV) photons was proposed as an efficient way to form such complex organics in astrophysical environments. This hypothesis was confirmed by laboratory experiments simulating photo-irradiation of ices containing H2O, CH3OH, CO, CO2, CH4, H2CO, NH3, HCN, etc., condensed on cold (~10–80 K) substrates. These experiments resulted in the formation of amino acids, nucleobases, sugar derivatives, amphiphilic compounds, and other organics comparable to those identified in carbonaceous meteorites. This work presents results for the formation of sugars, sugar alcohols, sugar acids, and their deoxy variants from the UV irradiation of ices containing H2O and CH3OH in relative proportions 2:1, and their comparison with meteoritic data. The formation mechanisms of these compounds and the astrobiological implications are also discussed.

Structural, Magnetic, and Optical Studies of the Polymorphic 9'-Anthracenyl Dithiadiazolyl Radical.

The fluorescent 9'-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid state. Polymorph 3α (monoclinic P21/ c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph), whereas 3β (orthorhombic P21 21 21, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π-π interactions between anthracenyl groups, while 3β exhibits edge-to-face π-π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 °C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P21/ c, Z' = 1), and on further cooling below 165 K, a further transition is observed to 3δ (monoclinic P21/ n, Z' = 2). Both 3β → 3γ and 3γ → 3δ transitions are reversible (single-crystal X-ray diffraction), and the 3γ → 3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 °C generates a fifth polymorph (3ε) which is distinct from 3α-3δ based on powder X-ray diffraction data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = 1/2 paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K, the value of χm T for 3δ approaches 0 emu·K·mol-1 consistent with formation of a gapped state with an S = 0 ground-state configuration. In solution, both paramagnetic 3 and diamagnetic [3][GaCl4] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λem ∼ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl4]) affording a switching ratio [ΦF(3+)/ΦF(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl4] exhibit emission bands at a longer wavelength (490 nm) attributed to excimer emission.

Overhanging shapes of freezing sessile drops under continuous liquid supply

Predicting the shape of solidifying material on a substrate is currently motivating an increasing number of studies related to additive manufacturing. Here we consider the solidification of a liquid drop on a cold substrate, below the freezing temperature, under continuous feeding at a constant flow-rate through a thin needle placed above. We compare experimental shapes to those predicted by a theoretical model where heat transfer is mainly due to conduction within the solid and where the inner flow is neglected. Hence, this is an analogous of Stefan problem for a sessile drop, with additional feeding of liquid. We show that, depending on the control parameters, namely the initial angle, substrate temperature and flow-rate, different shapes are observed, in particular original overhanging ones. We provide a simple model, which is in very good agreement with experiments. We then discuss the limitations of the model.

Liquid carbon quenching on a cold diamond layer: Multiparticle molecular dynamics simulation

In the present paper, in order to create the model structure of amorphous carbon, we have performed classical molecular-dynamic calculations of liquid carbon quenching on a cold diamond substrate. For this purpose a double-layer simulation cell diamond–liquid carbon containing 90 000 atoms of carbon at a pressure of 1.2 GPa has been used. We have exploited a classical potential ReaxFFC-2013. Characteristic quenching rates versus the distance from the diamond substrate and the velocity of thermal front of quenching have been obtained. The influence of the number of particles in a simulation cell on the results has been investigated as well.

A comprehensive model for predicting droplet freezing features on a cold substrate

Water droplet freezing affects many aspects of our daily lives, although there is no comprehensive model which retrieves all of the experimentally observed features when a liquid water droplet deposited on a cold substrate turns to ice. In this paper, we present general governing equations to describe water droplet freezing on a solid substrate by accounting for the physical properties of each phase present, namely the liquid and ice, in addition to the solid substrate. The approach, which takes advantage of the full mean curvature expression of both the droplet–air and liquid–ice interfaces, disjoining pressure, the Gibbs–Thomson effect, natural convection and the substrate thermal and physico-chemical properties, enables us to model a more realistic frozen droplet shape, without a prior assumption for the freezing growth angle. In addition to correctly predicting the freezing time, we capture both qualitatively and quantitatively the key experimentally observed features during water droplet freezing such as volume expansion, concave ice front and the cusp singularity. Furthermore, the proposed equation for the tip angle seems to explain its experimentally observed variability.

Assessing the formation of valley networks on a cold early Mars: Predictions for erosion rates and channel morphology

The ancient Noachian highlands of Mars host an extensive population of valley networks which formed predominantly during the early geologic history of the planet. Morphologic characteristics of the valley networks have been interpreted to indicate the formation of these features through precipitation-derived fluvial activity and therefore as evidence for a relatively warm and wet Noachian Mars climate. However, these interpretations conflict with the results of sophisticated global climate modeling studies, which suggest that early Mars was dominated by a cold and icy climate with conditions characterized by adiabatic cooling and regional ice sheets within the highlands. Difficulties in replicating the warm and wet early climate conditions interpreted from the valley networks has served as the basis for alternative suggestions for the formation of valley networks by transient heating and snowmelt in an otherwise cold and icy climate. Here we test a conceptual model for valley network formation and incision under cold and icy conditions with a substrate characterized by the presence of an ice-free, desiccated surface regolith and subjacent ice-cemented regolith, similar to that found in the Antarctic McMurdo Dry Valleys on Earth. We implement numerical thermal models, quantitative erosion and transport estimates, and morphometric analyses in order to outline and test predictions for: (1) the nature and structure of the cold and icy Noachian substrate, (2) valley network fluvial erosion and incision rates, and (3) resulting channel/valley morphology. Morphologic predictions are compared against observational data to determine if the valley networks characteristics are consistent with formation in a cold and icy climate. Through this approach we present and develop the underlying conceptual model, identify and generate the fundamental data inputs required to evaluate it, and perform a preliminary first-order assessment to serve as a basis for further investigation. While these analyses have been performed over a broad parameter space to address relative uncertainties, we report findings with a specific emphasis on the nominal results.We find that under cold conditions, the substrate is characterized by a kilometers-thick, globally-continuous cryosphere with a ∼ 50–100 m thick ice-free surface layer in diffusive equilibrium with the atmosphere and underlying ice-cemented regolith, the top of which forms the ice-table. Estimates of the potential infiltration capacity of the ice-free surface regolith layer indicate that surface runoff generation does not require excessively large precipitation rates, and thus does not preclude fluvial activity sourced by rainfall. Additionally, due to the predicted thicknesses of the ice-free surface regolith, any transient period of warming would need to be sustained in duration (∼ 2 kyr) to induce melting of ground ice to initiate solifluction/gelifluction activity. The predicted range of ice-table depths is exceeded by the incised depths of a majority of the valley network population, therefore suggesting interactions were possible. Evaluation of the rates of mechanical substrate erosion and thermal ice-cement erosion by valley network fluvial activity are subject to large uncertainties due to several poorly constrained parameters (i.e. the substrate erodibility factor and the valley network fluvial water temperature) but generally indicate that at low water temperatures, consistent with a cold and icy surface environment, thermal erosion of the ice-cement can be outpaced by mechanical erosion of the substrate. In this scenario, the relative efficiency of lateral erosion is predicted to be enhanced during incision below the ice-table, causing preferential channel/valley widening and increased width-to-depth ratios. Assessment of this prediction through a morphometric analysis of valley network width-to-depth ratios and occurrence of U-shaped cross sections indicates no significant correlation to ice-table depths. This result suggests that either: (1) permafrost was not present in the Noachian Mars substrate, indicating a warmer climate, (2) the permafrost ice-table did not significantly influence valley network width-to-depth ratios, possibly owing to more rapid thermal erosion rates than mechanical, or (3) additional factors not accounted for in this analysis are involved. Potential additional factors which could influence the results of this analysis include variable surface geothermal heat flux, substrate thermal conductivity, substrate composition and physical state as well as valley network preservation state, and morphometric sampling biases (due to data availability constraints). These factors represent areas for future investigation to refine assessments of valley network formation under cold early Mars conditions.

Effect of nucleation time on freezing morphology and type of a water droplet impacting onto cold substrate

The freezing morphologies and types for droplets at supercooled (−15 °C) and room temperature (20 °C) to impact onto a cold and polished aluminum substrate are investigated experimentally in this paper. It is found that the nucleation in different dynamic processes has a significant effect on the final freezing morphology and there are multiple freezing morphologies under constant conditions due to the uncertain nucleation time. Thus, based on the final freezing shapes, a basic classification for the different freezing morphologies is presented in terms of basin, pancake and semisphere; and three corresponding freezing types, including instantaneous freezing, non-instantaneous freezing and quasi-static freezing, are proposed based on the nucleation in distinct dynamic phases. Furthermore, the relative number of nucleation drops is taken as a function of time, and then the probability of various freezing morphologies and types in each experimental condition is provided. As a result, for both of the supercooled and room temperature water droplets, although the relative number of nucleation drops have roughly the same variational tendency for impact freezing, the nucleation time and the probability of various freezing morphologies remain different, for example, no basin shapes were attained in the case of room temperature water droplets under the experimental conditions of the current study. Meanwhile, the experimental results show that there are obvious diversities on freezing process. Three kinds of freezing processes, including traditional supercooled water freezing process, are presented for the impact freezing of room temperature droplets, which is mainly determined by the nucleation time. Therefore, for further studies of ice formation, the present results suggest that not only the effect of kinetics and heat transfer but also the influence of the nucleation time should be considered, particularly for the large droplets supercooled deeply.

HSS coating with keyholes in penetration produced by laser cladding process

This paper is devoted to the assurance of higher durability of hard coatings by using irregular method of laser cladding (LC). The purpose of this study was to demonstrate that by producing of keyhole mode LC coatings enables to create functionally graded structure in interface. This can contribute to decrease the gradient of trough-depth residual stresses of the coating-substrate system formed by dissimilar steels. Experimental work was carried out using precise LC system, which consists of industrial robot (KUKA) and 1kW Yb-fiber laser (IPG Laser). Experimental coatings were created on cold work tool steel substrate AISI D2 using high-speed steel (HSS) powder AISI M2. The properties of coatings produced by two different methods have been evaluated in the course of a comparative analysis by examination of morphology of transverse cross-sections, elemental composition, microhardness distribution inside coatings and tribological tests. Results showed that coating with keyhole mode in penetration had lower hardness due to a larger amount of dilution however the smooth transition of properties in the interface was obtained. Despite the significant difference in hardness both coatings demonstrated very similar tribological behaviour.

Mesoscale simulation of a molten droplet impacting and solidifying on a cold rough substrate

A molten droplet impacting and solidifying on a cold substrate with micro-pillars on the top is studied numerically in this paper using a newly developed multi-component, triple-phase solidification 3D lattice Boltzmann method (LBM). Simulated results confirm that droplets on a rough surface can exist in three states, depending on the static contact angle of the substrate θ: (i) Hemi-wicking Cassie-Baxter state for θ < θc1, (ii) Wenzel state for θc1 < θ < θc2, and (iii) Cassie-Baxter state for θ > θc2, where θc1 and θc2 are two critical contact angles whose values depending on the geometry of the micro-pillars. It is shown that the air between the droplet and micro-pillars is compressed by the falling droplet before impact, and the compressed air is pushed into grooves of the surface microstructure after impact. As a result, no air bubble is formed under the droplet bottom surface after its impact and solidification on the rough surface. The solidified droplet's bottom surface is smooth for a substrate with contact angle θ > θc2 and the bottom surface becomes rougher for θ < θc2. Two rough substrates with wetting and non-wetting interlaced wettabilities are presented for different applications. Simulated results show that fingers are formed at the triple-phase contact line along the rim of the droplet on the rough surface with non-wetting interlaced base (with θ > θc2).

Barium Titanate Dielectrics Sprayed by a High Feed-Rate Water-Stabilized Plasma Torch

Barium titanate coatings were first time sprayed by a high feed-rate plasma torch with water stabilization. Two power levels of the torch were applied. Two substrate materials were used—stainless steel and carbon steel. Various substrate preheating levels from 125 to 450 °C were applied to change the cooling conditions. Microstructure, phase composition including crystallinity quantification, surface roughness and microhardness were evaluated. Dielectric measurements proved that coatings with medium-level relative permittivity and with stable loss factor course versus frequency of the AC electric field were fabricated by spraying. The cold substrates provided coatings with low reflectivity and visibly darker surface compare to the hot substrates. The coatings exhibited good dielectric properties—on the top of an expectable range for plasma-sprayed BaTiO3. This study is targeted to coatings prospective for electrical industry, namely via optimization of conditions at the high feed-rate spraying.

Influence of Salinity on the Mechanism of Surface Icing: Implication to the Disappearing Freezing Singularity.

The effect of salinity on surface icing has been investigated experimentally. Water droplets with a variable salinity are deposited on a cold polished copper substrate. Distinctive two-stage freezing, which can be seen in case of pure water, is not observed in heterogeneous freezing of saltwater droplets. Interestingly, the final freezing stage commences before the initial freezing front completely traverses the saline droplet. A considerable increase in delay for heterogeneous ice nucleation is observed with the increasing salinity. The reduction in the associated degree of metastability due to the depression in the freezing point of the bulk solution and the increase in the nucleation barrier due to the appearance of the solvation shells that are formed around the ions are two possible causes of this nucleation delay. Moreover, the solidification time associated with surface icing increases considerably with the increasing salinity. Because of the insolubility of salt in ice, the salt ions are rejected to the entrapped water in the ice scaffold locally and to the bulk unfrozen water explicitly. This collective implicit and explicit modes of brine rejection contributes to the overall slowdown of freezing of the saline water droplets. From the phase diagram, it can be found that the complete solidification of water within the saline droplet is not possible when the substrate temperature is in between the eutectic temperature and the equilibrium freezing temperature. As a result, the relative magnitude of tip singularity during freezing reduces considerably with the increasing salinity due to the increase in unfrozen water content within the droplet.

Modelling the dynamics of the flow within freezing water droplets

The flow within freezing water droplets is here numerically modelled assuming fixed shape throughout freezing. Three droplets are studied with equal volume but different contact angles and two cases are considered, one including internal natural convection and one where it is excluded, i.e. a case where the effects of density differences is not considered. The shape of the freezing front is similar to experimental observations in the literature and the freezing time is well predicted for colder substrate temperatures. The latter is found to be clearly dependent on the plate temperature and contact angle. Including density differences has only a minor influence on the freezing time, but it has a considerable effect on the dynamics of the internal flow. To exemplify, in the vicinity of the density maximum for water (4 ∘C) the velocities are about 100 times higher when internal natural convection is considered for as compared to when it is not.

Formation mechanisms of single-crystalline InN quantum dots fabricated via droplet epitaxy

This work presents an experimental and theoretical insight into formation mechanisms of single crystalline wurtzite InN quantum dots (QDs) fabricated via metal droplet epitaxy (DE) by employing plasma assisted molecular beam epitaxy. The applied procedure consists of two fabrication stages. During the first stage, the cold substrate (T ≈ 15 °C) is exposed to an impinging In flux, resulting in formation of metallic In droplets on the substrate surface, and then to an impinging active nitrogen flux, resulting in In conversion into polycrystalline InN islands. During the second stage, the substrate, which is still kept exposed to active nitrogen, is heated up to T ≈ 300 °C, to allow for the reorganization of extended polycrystalline InN islands into groups of independent single-crystalline wurtzite InN QDs. This work provides a detailed experimental insight into both fabrication stages and their qualitative explanations within the scopes of adatom surface kinetics (stage I) and total energy per unit crystal volume minimization (stage II). Finally, the formation mechanisms of InN QDs on the three different substrates (Si(1 1 1), Si(0 0 1) and In0.3Ga0.7N/Si(1 1 1)) are compared, and also linked to the formation mechanisms of other more studied nanostructures, such as self-assembled GaN/AlN QDs and self-assembled and selective-area-grown GaN nanowires.

Numerical investigation of air entrapment in a molten droplet impacting and solidifying on a cold smooth substrate by 3D lattice Boltzmann method

In this paper, a novel 3D lattice Boltzmann method (LBM) is proposed for simulating a molten droplet impacting and solidifying on a cold smooth substrate surrounded by air. The numerical simulation shows that a small pocket of air is entrapped in the molten droplet adjacent to the cold surface after its impact on the surface. And this trapped air creates a thermal resistance between the droplet and the substrate. It is demonstrated that the no-slip velocity of the air on the solid surface prevents the air being squeezed out completely, and the air gap between the droplet and the substrate is compressed by the falling droplet. The compressed air results in the first contact away from the impact center and an air film is trapped within the droplet. This trapped air pocket eventually forms multiple air bubbles or a single air bubble depending on the surface wettability. Although droplet solidification has an important effect on the number of multiple air bubbles trapped on a surface having a small contact angle, it does not affect the size of the single air bubble trapped on a surface having a large contact angle. Maximum spread factors and dimensionless bubble heights obtained from simulation match well with theoretical values in the literature, validating the accuracy of this numerical model.

Structural transformations of 3-fluoro and 3-fluoro-4-methoxy benzaldehydes under cryogenic conditions: A computational and low temperature infrared spectroscopy investigation

Structural transformations of 3-fluorobenzaldehyde (C7H5FO; 3FBA) and 3-fluoro-4-methoxybenzaldehyde (C8H7FO2; 3F4MBA), taking place in different solid phase environments and at low temperature, were investigated by infrared spectroscopy, complemented by quantum chemistry calculations undertaken at the DFT(B3LYP)/6–311++G(d,p) level of approximation. The studied compounds were isolated from gas phase into cryogenic inert matrices (Ar, Xe), allowing to characterize their equilibrium conformational composition in gas-phase at room temperature. In both cases, two conformers differing by the orientation of the aldehyde moiety (with the carbonyl aldehyde bond cis or trans in relation to the aromatic ring fluorine substituent) were found to coexist, with the cis conformer being slightly more populated than the trans form. In situ narrowband UV irradiation of the as-deposited matrices led either to preferential isomerization of the cis conformer into the trans form or decarbonylation of both conformers, depending on the used excitation wavelength. Deposition of the vapours of 3F4MBA only, onto the cold (15 K) substrate, produced an amorphous solid containing also both the cis and trans conformers of the compound. Subsequent heating of the amorphous phase up to 268 K led to crystallization of the compound, which is accompanied by conformational selection, the cis form being the single species present in the crystal. The experimentally observed transformations of the studied compounds, together with the structural and vibrational results obtained from the performed quantum chemical calculations, allowed a detailed structural and vibrational characterization of the individual conformers.

Ultrastable metallic glasses formed on cold substrates

Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, Tg. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43Tg. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of Tg of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics.

Experimental and numerical study on the impact and freezing process of a water droplet on a cold surface

Ice accretion on cold solid surfaces presents hazards to a broad range of applications. It is quite essential to understand the dynamic and freezing mechanism of water droplets on a cold solid substrate for the prevention of ice accretion. In this study, the impact and freezing process of water droplets was studied experimentally and numerically. The effects of substrate temperature on the whole processes were investigated through changing the surface temperature from room temperature 25 °C to −30 °C. The results showed that the decrease of surface temperature led to the decrease of retraction rate, equilibrium droplet height after oscillation, freezing delay time and freezing time, while increased the final contact area due to the lower viscous dissipation energy. Also, a numerical model was developed to investigate the impact and freezing process of water droplets on an ultra-cold surface (−100 °C to −40 °C). The predicted droplet impact morphology, dynamic characteristics (spreading factor) and the propagation of freezing front were compared with experimental results, which exhibited good agreements. The numerical results suggested that the final contact area decreased slightly with the drastic decrease in surface temperature when the surface temperature is below −40 °C. Moreover, the influence of surface temperature on freezing front propagation and freezing time were analyzed in detail from the numerical results.

Organic ice resists for 3D electron-beam processing: Instrumentation and operation

Organic vapors condensed into thin layers of ice on the surface of a cold substrate are exposed with an electron beam to create resist patterns for lithography applications. The entire spin- and development-free lithography process requires a single custom instrument. We report the design, material choice, implementation and operation of this apparatus. It is based on a scanning electron microscope fitted with an electron beam control system that is normally used for electron beam lithography in a multi-user open-access laboratory. The microscope was also equipped with a gas injection system, a liquid nitrogen cooled cryostage, a temperature control system, and a load-lock. Three steps are required to initialize the apparatus for organic ice resist processing, and two steps are required to restore the apparatus for routine multi-user operations. Five steps are needed to create organic ice resist patterns. Finally, by stacking nanoscale patterns made in organic ice we created 3D structures using two complementary cyclic condense, expose and sublimate processes.