Properties Of Droplets Research Articles

Biomolecular Condensates: Structure, Functions, Methods of Research.

The term "biomolecular condensates" is used to describe membraneless compartments in eukaryotic cells, accumulating proteins and nucleic acids. Biomolecular condensates are formed as a result of liquid-liquid phase separation (LLPS). Often, they demonstrate properties of liquid-like droplets or gel-like aggregates; however, some of them may appear to have a more complex structure and high-order organization. Membraneless microcompartments are involved in diverse processes both in cytoplasm and in nucleus, among them ribosome biogenesis, regulation of gene expression, cell signaling, and stress response. Condensates properties and structure could be highly dynamic and are affected by various internal and external factors, e.g., concentration and interactions of components, solution temperature, pH, osmolarity, etc. In this review, we discuss variety of biomolecular condensates and their functions in live cells, describe their structure variants, highlight domain and primary sequence organization of the constituent proteins and nucleic acids. Finally, we describe current advances in methods that characterize structure, properties, morphology, and dynamics of biomolecular condensates in vitro and in vivo.

Pioneering artificial cell-like structures with DNA nanotechnology-based liquid-liquid phase separation.

Recent studies have revealed that liquid-liquid phase separation (LLPS) plays crucial roles in various cellular functions. Droplets formed via LLPS within cells, often referred to as membraneless organelles, serve to concentrate specific molecules, thus enhancing biochemical reactions. Artificial LLPS systems have been utilized to construct synthetic cell models, employing a range of synthetic molecules. LLPS systems based on DNA nanotechnology are particularly notable for their designable characteristics in droplet formation, dynamics, properties, and functionalities. This review surveys recent advancements in DNA-based LLPS systems, underscoring the programmability afforded by DNA's base-pair specific interactions. We discuss the fundamentals of DNA droplet formation, including temperature-dependence and physical properties, along with the precise control achievable through sequence design. Attention is given to the phase separation of DNA nanostructures on two-dimensional closed interfaces, which results in spatial pattern formation at the interface. Furthermore, we spotlight the potential of DNA droplet computing for cancer diagnostics through specific microRNA pattern recognition. We envision that DNA-based LLPS presents a versatile platform for the exploration of cellular mimicry and opens innovative ways for the development of functional synthetic cells.

Controlling the size and adhesion of DNA droplets using surface- enriched DNA molecules.

Liquid droplets of biomolecules serve as organizers of the cellular interior and are of interest in biosensing and biomaterials applications. Here, we investigate means to tune the interfacial properties of a model biomolecular liquid consisting of multi-armed DNA 'nanostar' particles. We find that long DNA molecules that have binding affinity for the nanostars are preferentially enriched on the interface of nanostar droplets, thus acting as surfactants. Fluorescent measurements indicate that, in certain conditions, the interfacial density of the surfactant is around 20 per square micron, indicative of a sparse brush-like structure of the long, polymeric DNA. Increasing surfactant concentration leads to decreased droplet size, down to the sub-micron scale, consistent with droplet coalesence being impeded by the disjoining pressure created by the brush-like surfactant layer. Added DNA surfactant also keeps droplets from adhering to both hydrophobic and hydrophilic solid surfaces, apparently due to this same disjoining effect of the surfactant layer. We thus demonstrate control of the size and adhesive properties of droplets of a biomolecular liquid, with implications for basic biophysical understanding of such droplets, as well as for their applied use.

Flow and clogging of capillary droplets.

Capillary droplets form due to surface tension when two immiscible fluids are mixed. We describe the motion of gravity-driven capillary droplets flowing through narrow constrictions and obstacle arrays in both simulations and experiments. Our new capillary deformable particle model recapitulates the shape and velocity of single oil droplets in water as they pass through narrow constrictions in microfluidic chambers. Using this experimentally validated model, we simulate the flow and clogging of single capillary droplets in narrow channels and obstacle arrays and find several important results. First, the capillary droplet speed profile is nonmonotonic as the droplet exits the narrow orifice, and we can tune the droplet properties so that the speed overshoots the terminal speed far from the constriction. Second, in obstacle arrays, we find that extremely deformable droplets can wrap around obstacles, which leads to decreased average droplet speed in the continuous flow regime and increased probability for clogging in the regime where permanent clogs form. Third, the wrapping mechanism causes the clogging probability in obstacle arrays to become nonmonotonic with surface tension Γ. At large Γ, the droplets are nearly rigid and the clogging probability is large since the droplets can not squeeze through the gaps between obstacles. With decreasing Γ, the clogging probability decreases as the droplets become more deformable. However, in the small-Γ limit, the clogging probability increases since the droplets are extremely deformable and wrap around the obstacles. The results from these studies are important for developing a predictive understanding of capillary droplet flows through complex and confined geometries.

Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet.

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Experimental Investigation of Electrospraying Properties Based on Ring Electrode Modification.

Electrospraying uses a high-voltage potential difference to create fine droplets. This study conducts a comparative analysis of the spray pattern and droplet properties using ring electrode parameters. The spray pattern and droplet characteristics are analyzed based on the experimental parameters of the ring electrode. The results show that the cone-jet mode forms quickly for the ring electrode. In addition, as the ring diameter decreases, the ring voltage increases and an increase in the distance between the ring and the nozzle in the bottom direction decreases the Sauter mean diameter and its standard deviation. The optimal conditions for the formation of fine and uniform droplets include a ring diameter of 15 mm, a ring voltage of 7 kV, and a nozzle-to-ring distance of (+) 20 mm.

Ultrafast Droplets Trajectories Modulation of Electrohydrodynamic Jet Printing for Pattern Refinement Using Electrostatic Deflection

AbstractElectrohydrodynamic jet (E‐Jet) printing is a highly promising non‐contact additive manufacturing technology. However, the fast jets generated by E‐Jet may cause droplet accumulation on small curvature features due to the slow motion control achieved by the mechanical stage, leading to a degradation of printed patterns in localized areas. Here, a method is proposed for ultrafast modulation of jet trajectories to achieve finely patterned E‐Jet printing by introducing jet‐deflecting electrodes around the jet, which can continuously transform the jet trajectory with lateral acceleration up to 104 ms−2. This extraordinary acceleration is two orders of magnitude higher than that can be achieved by using mechanical stage motion. Through a combination of numerical calculations and experiments, the effects of electric field parameters on the feature size and kinematic properties of jet droplets are investigated. In addition, deposition errors of <5 µm are fabricated by selecting appropriate working parameters and using UV‐curable adhesives as printing ink. Subsequently, complex micropatterns with feature curvatures <4 µm are successfully printed, demonstrating the consistency and reliability of this electrostatic deflection technique in achieving a uniform droplet deposition pitch. This innovative approach holds significant promise in the field of high‐quality microscale additive manufacturing.

Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 2: Strong surfactants

Abstract. Surfactants have been a focus of investigation in atmospheric sciences for decades due to their ability to modify the water uptake and cloud formation potential of aerosols. Surfactants adsorb at the surface and can decrease the surface tension of aqueous solutions. In microscopic aqueous droplets with finite amounts of solute, surface adsorption may simultaneously deplete the droplet bulk of the surfactant. While this mechanism is now broadly accepted, the representation in atmospheric and cloud droplet models is still not well constrained. We compare the predictions of five bulk–surface partitioning models documented in the literature to represent aerosol surface activity in Köhler calculations of cloud droplet activation. The models are applied to common aerosol systems, consisting of strong atmospheric surfactants (sodium myristate or myristic acid) and sodium chloride in a wide range of relative mixing ratios. For the same particles, the partitioning models predict similar critical droplet properties at small surfactant mass fractions, but differences between the model predictions increase significantly with the surfactant mass fraction in the particles. Furthermore, significantly different surface tensions are predicted for growing droplets at given ambient conditions along the Köhler curves. The inter-model variation for these strong surfactant particles is different than previously observed for moderately surface active atmospheric aerosol components. Our results highlight the importance of establishing bulk–surface partitioning effects in Köhler calculations for a wide range of conditions and aerosol types relevant to the atmosphere. In particular, conclusions made for a single type of surface active aerosol and surface activity model may not be immediately generalized.

Controlled generation of high-frequency liquid metal microdroplets

Liquid metal (LM) microdroplets with high conductivity and fluidity have drawn extensive attention in the field of electronics, biomedicine, and catalysis. The properties of LM droplets, e.g., melting points, electrical actuation, and stability, are affected by their sizes. Continuous generation of LM microdroplets with precisely controlled size will benefit their applications, which however, remains an outstanding challenge. In this study, we have designed a scalable high frequency droplet generator for the liquid metal (EGaIn) that works on the principle of manipulating the electrohydrodynamic behaviour of EGaIn. By applying electric potentials < − 0.75 V vs Ag/ AgCl, EGaIn droplets with uniform sizes can be rapidly and continuously generated. The electric potential effectively tunes the thickness of the oxide layer on the EGaIn surface which in turn modulates the surface tension of the EGaIn and induces the generation of consistent sized EGaIn droplets. The size of the EGaIn microdroplets can be varied between 40 and 200 µm, depending on the electric potential applied and the diameter of the exit orifice. Droplet generation using this method requires relatively low energies, i.e., 2.77 × 10-7 kWh, and is coupled with high frequency droplet generation ∼ 60 droplet/min. The scalability of this droplet generation methodology can be further increased via design of an array of exit orifice. With the presence of surfactants, these EGaIn droplets can remain well-dispersed without coalescing. This considerably widens their capability for further applications.

How tank-mix adjuvant type and concentration influence the contact angle on wheat leaf surface.

Currently, the utilization of unmanned aerial vehicles (UAVs) for spraying pesticides is a prevalent issue in Asian countries. Improving the pesticide efficiency of UAV spraying is a major challenge for researchers. One of the factors that affect the efficiency is the wetting property of the spraying solutions on crop leaves. Tank-mix adjuvants, which can modify the wetting ability of the solutions, are often used for foliar application. However, different types and concentrations of tank-mix adjuvants may have different impacts on the wetting properties of droplets. In this article, we investigated the effects of four tank-mix adjuvants, Beidatong (BDT), Velezia Pro (VP), Nongjianfei (NJF), and Lieying (LY), on the dynamic contact angle (CA) values of droplets on the adaxial surface of wheat leaves. We measured the dynamic CA values of various concentrations of each adjuvant solution and determined the optimal concentrations based on the CA values, droplet spreading time, and cost. The results showed that adding any of the four adjuvants decreased the CA values, but the patterns of decrease varied among them. The CAs of BDT and VP solutions decreased slowly during the observation time (0-8.13 s), while those of NJF and LY solutions decreased rapidly throughout the observation period. According to the dynamic CA values of different concentrations, the optimal concentrations of BDT, VP, NJF, and LY for wheat field application were 12%, 16%, 6‰, and 0.3‰, respectively. Alkoxy-modified polytrisiloxane adjuvant (LY) could be recommended as an appropriate tank-mix adjuvant for wheat field application, considering spreading efficiency and cost. This study provides theoretical and practical guidance for selecting and optimizing tank-mix adjuvants for UAV spraying.

Parameterization of optical properties for liquid cloud droplets containing black carbon based on neural network.

This paper introduces a novel back propagation (BP) neural network method to accurately characterize optical properties of liquid cloud droplets, including black carbon. The model establishes relationships between black carbon volume fraction, wavelength, cloud effective radius, and optical properties. Evaluated on a test set, the value of the root mean square error (RMSE) of the asymmetry factor, extinction coefficient, single-scattering albedo, and the first 4 moments of the Legendre expansion of the phase function are less than 0.003, with the maximum mean relative error (MRE) reaching 0.2%, which are all better than the traditional method that only uses polynomials to fit the relationship between the effective radius and optical properties. Notably, the BP neural network significantly compresses the optical property database size by 37,800 times. Radiative transfer simulations indicate that mixing black carbon particles in water clouds reduces the top-of-atmosphere (TOA) reflectance and heats the atmosphere. However, if the volume fraction of black carbon is less than 10-6, the black carbon mixed in the water cloud has a tiny effect on the simulated TOA reflectance.

Towards more predictive correlations for conjugated mass transfer across a droplet interface

Most studies of heat or mass transfer in dispersed, two-phase flows are either analytical or empirical, and therefore limited to simplified or specific operating conditions. In this frame, Direct Numerical Simulations (DNS) can be a powerful tool to address the needs arising from the development of new liquid–liquid solvent extraction processes, involving e.g., more viscous solvents or different hydrodynamic conditions. In this study, a large parametric study, in the range: Reynolds number Re∈[1,500], Péclet number Pee∈[10,1000], viscosity ratio μ∗∈[0.05,20] and a thermodynamic equilibrium coefficient k∈[0.1,10] , of mass transfer from an isolated droplet is achieved by DNS to propose a physically based correlation of the apparent Sherwood number, Sh, that is sufficiently generic to apply to emerging and future applications. The updated correlations derive first from a solid analysis of the features, in terms of concentration fields inside and outside the droplet and interface properties, to challenge the validity of the assumptions of mass transfer correlations. Then, we propose proper modifications in the formulation of existing models. In a second step, the accuracy and the robustness of the new models have been assessed by comparison with Sh obtained from DNS. The proposed correlations are applicable for mass transfer as well as for heat transfer problems, in most configurations encountered in chemical plants.

The interaction of a droplet with an immiscible deep liquid pool for density ratio greater than unity

Droplet interaction with liquid pools has been widely studied. The focus of these studies has primarily been on like-fluids, the droplet being miscible with the pool. Such miscible droplet–pool interactions have been extensively studied for their regimes of Rayleigh jet formation, crater formation, splashing, and coalescence. However, the case of immiscible droplet fluid has received considerably less attention from researchers. The immiscible case is relatively complex to understand owing to the involvement of three interfacial tensions. In the current study, we investigate the regimes of droplet–pool interaction for the case of immiscible droplet fluid having higher density and surface tension than pool fluid. Droplet properties are characterized through Weber number while Ohnesorge number is used to characterize pool fluid. Weber number is controlled through velocity of droplet and viscosity of pool is used to predict Ohnesorge number. Validating the numerical methods with experimental data, extensive numerical simulations are performed to gain insight into droplet–pool interaction. Current investigation reveals that for the case of immiscible fluids, regime formation is observed due to Rayleigh–Plateau instability as well as droplet fluid interacting with the pool fluid. This enables the pool fluid jet separation at high Ohnesorge numbers too, in contrast to miscible fluids case. The regimes of droplet–pool interaction for current case are described in detail and classified over wide range of Weber and Ohnesorge numbers.

Study on Effects of Different Concentration Adjuvants on the Properties of Prochloraz Emulsion in Water Solution Droplets and Deposition

Adjuvants are frequently incorporated into crop protection operations to modulate the droplet characteristics by diminishing the surface tension (ST) and contact angle (CA), thereby positively influencing the wetting and spreading behavior of the droplets. However, there are no quantitative conclusions on the extent to which the amount of adjuvant added affects droplet properties. Therefore, the decision to add spraying adjuvants in actual pesticide spraying operations relies on the operator’s experience. In this study, we investigated the effect of a surfactant additive (KAO A-200) on the droplet properties and deposition of prochloraz emulsion in water (PEW) solution for crop protection in unmanned aerial vehicle (UAV) aerial spraying. Three KAO A-200 additive concentrations of 0.42%, 0.83% and 1.67% and four solution concentrations of 2.5%, 3.33%, 4.17% and 5% of PEW were set to evaluate the droplet properties of PEW solution with ST and CA as assessment indicators. The results show that the average STs of adjuvant solution droplets tended to decrease as the concentration of KAO A-200 increased. According to the optimal concentration, the KAO A-200 addition concentration of 0.83% was therefore determined to be the most appropriate dosage. With the appropriate KAO A-200 dosage condition, the results show that the average STs increased as PEW solution concentration increased, while the average CAs of PEW solution droplets showed a first decreasing and then increasing trend. The “4.17% concentration PEW (22.5 g a.i./1.2 L) + 0.83% concentration KAO A-200” condition was selected as the optimized combination for crop protection UAV field aerial spraying tests. The test showed that the coverage rate of PEW solution droplets on the upper and lower layers of oilseed rape canopy increased by 71.47% and 41.55%, the deposition density increased by 71.91% and 98.45%, and the coefficient of variation in droplet deposition decreased by 44.41% and 48.13%, respectively. These results are significantly better than those obtained without the adjuvant addition.

Investigation on the mechanism of air/condensate bubble flotation of emulsified oil droplet

Crude oil droplets in oilfield-produced water are highly emulsified, and the poor attachment between gas bubbles and oil droplets limits the effectiveness of oil-water separation during flotation processes. This paper proposes a new surface-functionalized bubble called a condensate bubble and evaluates its flotation performance based on surface properties and adhesion theory mechanism to address this issue. The surface chemical properties of the oil droplets were characterized using X-ray photoelectron spectroscopy (XPS), contact angle measurement, and zeta potential. Induction time and flotation tests were conducted to compare the attachment effects between air or condensate bubbles and emulsified oil droplets. The interaction forces were calculated using DLVO and extended DLVO theory. The experimental results showed that crude oil contains functional compounds containing oxygen that make it tough to float. However, the contact angle between the oil surface and condensate bubble was greater than 150°, demonstrating that the condensate liquid bubbles have better hydrophobicity than air bubbles. The micro-flotation experiments indicated that condensate bubbles have a strong collection capacity for emulsified oil droplets, which was confirmed by induction time tests. In 70 mM NaCl brine, the oil removal efficiency of condensate bubble flotation could reach 82%, which was a 30% improvement compared to air bubble flotation. However, DLVO theory failed to predict the flotation with a concentration higher than 10 mM NaCl. The extended DLVO theory accounted for hydrophobic interactions, which were observed due to similar solvent extraction and hydrophobicity of the condensate liquid film. As a result, the condensate bubble-oil droplet system exhibited stronger hydrophobic attraction, successfully confirming and illustrating the superiority of the condensate bubble in flotation.

Modulation of physicochemical properties of lipid droplets using soy protein isolate and lactoferrin interfacial coatings.

In order to improve the physicochemical stability of soy protein isolate (SPI) emulsion, lactoferrin (LF) was used to modify the interface layer. The stable multilayer emulsion can be formed when the content of lactoferrin is 0.5% at pH 5. The emulsion with good stability was at pH 3-7, and it was also stable to temperature change. The FFAs release of SPI emulsion and LF-SPI emulsion was 103.9% and 103.7%, respectively. The results showed that the lactoferrin layer did not hinder the digestion of oil and the bioaccessibility of carotenoids, but lactoferrin layer improved the physicochemical stability of SPI emulsions. This work provides information valuable in the design of emulsion formulations for applications in the food, pharmaceutical, and personal care industries.

Uncovering the Properties of Dicationic Ionic Liquid Nanodroplets through Ab Initio Molecular Dynamics Simulations.

The behavior of nanodroplets of an imidazolium-based dicationic ionic liquid, i.e., [C1(mim)2][PF6]2, was investigated in this study using ab initio molecular dynamics simulations. The vibrational features as well as the structural, interfacial, and dynamical properties of different sized droplets were analyzed and compared to the bulk phase system. Structural properties of the droplets, such as π-π stacking, radial distribution functions, structure factors, combined distribution functions, and angular distribution functions were analyzed to understand the interactions and orientations of their ions. The vibrational features and hydrogen bonding strength of droplets were studied by calculating their infrared (IR) and power spectra, determining the contribution of different types of hydrogen bonding to each vibrational mode. The calculated spectra showed good overall agreement with the experimental results. The interfacial properties of the droplets and the orientation of their ions were analyzed using density profiles and an exposed surface. The results showed that, in all systems studied, cations and anions were equally likely to exist in both inner and outer layers, and the cations tended to be oriented toward the center of droplets with obtuse angles. Additionally, the droplet densities were extrapolated to predict the bulk phase density with less than 2% deviation. The dynamical properties of hydrogen bonds, mean square displacement, and van Hove correlations of cations and anions were also analyzed. The results indicated that there was no regular trend in the dynamic properties of droplets with an increasing system size.

Molecular dynamics simulations of droplet coalescence and impact dynamics on the modified surfaces: A review

The effective control of droplet coalescence and impact behaviors on solid surfaces plays a crucial role in various fields, such as material science, environmental science, energy, biomedicine, agriculture, and aerospace. This review aims to summarize recent advancements in the understanding of coalescence and impact dynamics of droplets under different conditions using molecular dynamics (MD) simulations. We highlight diverse approaches that enable the achievement of controllable coalescence and impact behaviors through the manipulation of surface properties, droplet properties, and coalescence and impact conditions. These approaches encompass adjustments in surface structures, surface morphology, and surface wettability, as well as the manipulation of droplet size, viscosity, and components, and the control of initial shape setting and impact velocity. Furthermore, we also introduce the dynamics associated with the coexistence of these two behaviors. Importantly, this review is expected to provide valuable insights for both industrial and technological applications, particularly in the areas of self-cleaning surfaces and microfluidics.

Hypergolic ignition response to oxidizer droplet properties

The hypergolic ignition response to oxidizer droplet properties is an important consideration for oxidizer injector design in hybrid rocket motors. Hence, the present work focused on hypergolic ignition characterization as a function of droplet properties and impact dynamics for a promising propellant. The propellant for the present work consisted of rocket grade hydrogen peroxide (RGHP) as oxidizer, high-density polyethylene (HDPE) as fuel and sodium borohydride (NaBH4) as the hypergolic additive embedded in the HDPE matrix. This propellant was chosen for this detailed study since it showed promising results in prior studies, exhibiting low (<10 ms) ignition delays. For this work, in a drop-test setup, RGHP droplet diameters between 1.90±0.11 mm and 4.35±0.26 mm were dropped on the HDPE + NaBH4 fuel samples at velocities between 0.3 m/s to 1.93 m/s. The ensuing ignition event was studied using both visible and infrared high-speed imaging. Post-impact droplet parameters were determined to enable a deeper analysis of observed phenomena. While ignition delay was the focus of this work, flame spread areas and heat release were also studied. Higher impact velocities generally led to lower ignition delays while splashing oxidizer drops increased probability of larger flame areas and higher heat release. Additionally, the initial ignition/heat release event was found to originate in a small region at the thinnest area of the post-impact droplet. Surface temperature measurements were also performed using infrared imaging. The data suggested that the initial heat release was found to be likely due to RGHP-NaBH4 reaction with no contribution from RGHP decomposition. Regression and random forest machine learning analysis found ignition delay depended strongly on the minimum post-impact drop layer thickness. Based on the data analysis, a simple mathematical model was developed whose results matched experimentally observed trends. The study is expected to further understanding of the processes that control hypergolic ignition in addition to aiding rocket engineers design better oxidizer feed systems. Novelty and SignificanceThis work constitutes a novel in-depth study on the influence of drop impact properties on hypergolic ignition. Using innovative techniques, this work is the first to show the overwhelming importance of residual droplet layer thickness on ignition delay. Additionally, a simple mathematical model based on that finding performed well against experimental data which is of substantial significance, given the complex and multi-parametric nature of hypergolic ignition. Finally, the work establishes that operating in the splashing regime enhances heat release and flame spread. The candidate propellant for this work was hydrogen peroxide and high-density polyethylene with embedded NaBH4. However, the findings of this study are likely generalizable over a range of solid-liquid hypergolic propellants involving solid fuels with embedded energetic materials, of which there are many. The findings of this work can enhance understanding of hypergolic ignition in addition to aiding in better rocket injection system design.

Effect of Nanopillars on the Wetting State and Adhesion Characteristics of Molten Aluminum Droplets.

To solve the adhesion problem between molten aluminum and vacuum ladle liner during the electrolytic aluminum production process, the wetting state and adhesion properties of molten aluminum droplets on substrate surfaces with different nanopillars are investigated based on molecular dynamics. The results show that the adhesion strength of molten aluminum droplets in different wetting states has the pattern Young state > Wenzel state > Cassie state. Effects of increasing nanopillar height or interval are poles apart in the wetting state and adhesion characteristics of aluminum molten droplets. The critical height and critical interval of the nanopillar where the wetting state transition occurs are obtained. The increase of the nanopillar width can induce the wetting state transition from the Cassie state to the Wenzel state. In addition, the phantom wall method is applied to study the variation of the separation force. It is found that a peak in the separation force curve occurs when the molten droplet separates from the bottom of the nanopillar interval or the top of the nanopillar. The separation force curves of the droplets in the Young state and the Cassie state have single peaks, while the droplets in the Wenzel state have double peaks.