Solution Droplets Research Articles

Numerical simulation of binary droplet collisions using a Front Tracking interface technique

Droplet-droplet interactions of highly viscous liquids or suspensions are relevant in a broad range of industrial applications, such as spray drying, fuel injection or, coating and granulation. The efficiency of these processes is heavily determined by the surface area and volume of the droplets. To operate these processes efficiently, it is critical to accurately describe the physical mechanisms dominating the collision. Nonetheless, a complete description of the forces governing the collision is still missing, particularly when the liquids have a high viscosity. Front tracking techniques represent a promising alternative for studying droplet-droplet interactions as they ensure a sharper representation of the liquid-gas interface. In this work, the Local Front Reconstruction Method (LFRM) is the front tracking technique chosen as it allows merging and break-up. The validation of this method is extended for different Weber numbers, impact parameters, and liquid properties by determining the capabilities of LFRM to reproduce the collision of glycerol solution droplets. The simulation results are validated with dedicated experiments performed at the same Weber and impact parameter conditions. In the majority of the studied collisions, LFRM shows a good correspondence with experiments and literature, which proves the ability of LFRM to capture droplet collisions under the studied conditions.

Simple and Multiplexed Detection of Nucleic Acid Targets Based on Fluorescent Ring Patterns and Deep Learning Analysis.

Simple diagnostic tests for nucleic acid targets can provide great advantages for applications such as rapid pathogen detection. Here, we developed a membrane assay for multiplexed detection of nucleic acid targets based on the visualization of two-dimensional fluorescent ring patterns. A droplet of the assay solution is applied to a cellulose nitrate membrane, and upon radial chromatographic flow and evaporation of the solvent, fluorescent patterns appear under UV irradiation. The target nucleic acid is isothermally amplified and is immediately hybridized with fluorescent oligonucleotide probes in a one-pot reaction. We established the fluorescent ring assay integrated with isothermal amplification (iFluor-RFA = isothermal fluorescent ring-based radial flow assay), and feasibility was tested using nucleic acid targets of the receptor binding domain (RBD) and RNA-dependent RNA polymerase (RdRp) genes of SARS-CoV-2. We demonstrate that the iFluor-RFA method is capable of specific and sensitive detection in the subpicomole range, as well as multiplexed detection even in complex solutions. Furthermore, we applied deep learning analysis of the fluorescence images, showing that patterns could be classified as positive or negative and that quantitative amounts of the target could be predicted. The current technique, which is a membrane pattern-based nucleic acid assay combined with deep learning analysis, provides a novel approach in diagnostic platform development that can be versatilely applied for the rapid detection of infectious pathogens.

Investigation of one-dimensional quantum droplets in a temporally perturbed external harmonic trap

Using analytical methods we investigate the formation of quantum droplets in symmetric Bose-Bose mixture confined in temporally mutable parabolic traps. The dynamics of the droplet is observed to be heavily dependent upon dynamic nature of the trap. We demonstrate a mechanism to obtain exact droplet solution even if there are imperfections in the dynamic trap, provided the imperfections are of a certain nature. This is done through isospectral Hamiltonian technique of supersymmetric Quantum Mechanics, which also provides an intelligent way to control the dynamics of the quantum droplets through Riccati parameter. This method allows judicious tailoring of the trap frequency which in turn controls the width of the droplet, and quantum droplet can be stretched or squeezed by careful choice of this Riccati parameter. It provides a valuable tool for investigating and adapting to unforeseen variations and deviations encountered in experimental scenarios.

Combing of picogram level DNA equivalent to genomic DNA present in single human cell by self propelled droplet motion over a stable gradient surface

DNA combing is a powerful technique for studying replication profile, fork-directionality and fork velocity. At present, there is requirement of a methodology to comb DNA present in a single human cell for studying replication dynamics at early embryonic stage. In our study, a surface having dual characteristics i.e., affinity towards negatively charged single DNA molecules and a hydrophobic gradient for self propelled droplet motion of combing solution was developed. The surface was made by coating of TCOS (trichloro-octylsilane) by vapor diffusion on APTES (Aminopropyl-triethoxysilane) coated glass slides. A gradient surface having high deposition efficiency (DE) was developed on which 5 picogram DNA equivalent to genomic DNA present in one single human cell can be combed. The gradient surface was thermostable in nature having the ability to sustain boiling temperature for two hours and sustain anisotropy in 70 % ethanol for 80 h. Applicability for multiple runs was enhanced such that the surface can be used for 13–14 times. Factors associated with gradient surface are unidirectional movement of combing solution droplet over the gradient surface for combing straight DNA molecules and a longer gradient surface of more than 1 cm such that long size DNA molecules can be combed. Ellipsometry and contact angle hysteresis confirmed the presence of hydrophobic gradient. XPS (X-ray photoelectron spectroscopy) and FTIR (Fourier Transform Infrared Spectroscopy) confirmed the presence of characteristic affinity towards negatively charged DNA molecules on the gradient surface. Combing solution was optimized for increasing deposition efficiency and for increasing the applicability of gradient surface for multiple runs. High temperature of combing solution was found to increase Deposition Efficiency. Combing solution was also optimized for combing single DNA molecules over the gradient surface. Single DNA molecules were combed by reducing pH and lowering concentration of triton-X in the combing solution. Dye: bp ratio was optimized for high fluorescent intensity and low surface background.

The viscosity and surface tension of supercooled levitated droplets determined by excitation of shape oscillations

Abstract. We report a new method for determining the viscosity and surface tension of supercooled liquid droplets using electrodynamic levitation and phase analysis of shape oscillations. The method uses a high-frequency alternating electrical potential to excite shape oscillations in a levitated droplet, and the phase shift in the oscillations is used to simultaneously determine droplet viscosity and surface tension. The advantages over existing contactless methods include its applicability to atmospherically relevant temperatures and the possibility of continuously monitoring changes in real time. We demonstrate proof-of-concept measurement for supercooled water droplets and dilute sucrose solution droplets, and we anticipate that the technique could be used to measure viscosity values for droplets containing dilute organics. The technique is especially well-suited for investigation of the role of atmospheric processing in the viscosity and surface tension of solution droplets in equilibrium with a given or changing relative humidity.

Taming the coffee-ring effect on solid surface by localized heating upon the suspension droplet

Coffee-ring effect is a very common phenomenon in our daily life, especially in the evaporation process of colloidal solutions on solid surfaces. However, in processes such as inkjet printing, this effect can change the uniformity of the deposited film, thus affecting the quality of the printed products. In this paper, a facile and convenient method of localized heating of liquid droplets by using a tiny steel wire is proposed. The results indicated that this method can effectively suppress the generation of coffee-ring effect, and the deposition morphology of the suspended particles can be adjusted by controlling the applied temperature, wire diameter and the distance from the wire to the droplet. And it was found that the heated wire-induced inward flows caused by the temperature gradient of the droplet are the main reason for adjusting the deposition morphology. The coffee-ring effect is mainly controlled by the free convection from the edge to the center caused by the evaporation loss of the solution, so as to adjust the distribution of suspended particles in the evaporating droplet and thus achieve the goal of suppressing the coffee-ring effect. This method has no special requirements for the substrate and droplet solution, and it is expected to be universally applicable to uniform deposition of colloidal solutions.

Mechanics of assembling two-dimensional materials on a solid substrate by droplet drying

Assembly of two-dimensional (2D) nanomaterials by droplet drying offers a straightforward and low-cost route to obtain their bulk forms for widespread applications in manufacturing and printing of functional structures and devices. However, unlike rigid nanoparticles that usually do not experience mechanical deformation, 2D nanomaterials are easily deformed and folded during assembly by evaporative drying, and traditional assembly theory that can address these fundamental deformation mechanisms is currently lacking. In the present study, we have developed an energy-based rotational spring-mechanical slider mechanics model to describe the mechanical deformation and assembly of 2D material graphene on a solid substrate during the evaporation of its droplet solution. In the development of theory, the mechanical folding deformation of 2D material graphene itself is modeled by the rotational spring, and the folding-induced interior interactions of graphene itself and its assembly interactions with neighboring ones and solid substrate all due to van der Waal force are modeled by the mechanical sliders. The surface wettability of substrate and the evaporative modes of droplet on substrate including constant contact angle (CCA), constant contact radius (CCR), and their combination are also incorporated into the mechanics model. In parallel, large-scale molecular dynamics (MD) simulations with the development of coarse-grained model of 2D graphene and its virtual force field interaction with liquid is performed and show remarkable agreement with theoretical predictions on both assembly patterns and dimensional sizes. The effect of graphene size and its interaction strength with substrate on assembly is also elucidated. This work uncovers fundamental assembly mechansim of mechanically deformable nanomaterials by solution drying, and also provides immediate application guidance to ink-based printing techniques for manufacturing deformable nanomaterials-enabled devices with controlled patterns on substrates.

Generator of Monodisperse Droplets and Some Applications

A device for generation of monodisperse droplets with adjustable size and generation frequency is presented. The solid particles are formed by usage of drying techniques of droplets of different solutions in the glass tube. Experimental investigation are performed in order to create periodical structures (particularly photonic crystals) using these solid particles. For focusing and “trapping” of particles thermo-phoretic force is applied. For fabrication of opal photonic crystals self-assembly techniques is used.

Effect of hydroxyl position in reagent molecule on coal dust dedusting

Using the high-speed motion acquisition system, we studied the adhesion ability and wetting ability of coal dust under the effect of four octanol solutions. The adhesion ability followed the order: 2-Octanol > 4-Octanol > 1-Octanol > 3-Octanol. The wrap angle of coal dust on the octanol surface exhibited an increasing trend with the rise in coal proportion of dust. The observed behavior was substantiated by the interaction force, ascertained through molecular dynamic simulations. The interaction force between coal dust and 3-Octanol was the smallest leading to the weakest adhesion ability by the 3-Octanol solution. The contact angle of octanol solution droplet at the coal dust surface under the same octanol concentration was investigated to follow the order: 2-Octanol > 4-Octanol > 1-Octanol > 3-Octanol. The wetting velocity decreased as the coal proportion of dust increased. The wetting ability followed the order: 2-Octanol < 4-Octanol < 1-Octanol < 3-Octanol. The surface energy of dust increased as the coal proportion decreased. The dedusting efficient under the effect of octanols depended on the difference in surface energy between the coal dust and the octanol solution. The greater the Δγ (difference between the surface energy of the dust and solution), the weaker the adhesion ability, and the stronger the wetting ability. The findings of our study can offer valuable insights for the development of dedusting reagent design.

Simple Protein Analysis by Droplet Paper Spray Ionization Mass Spectrometry with Polyolefin Silica-Based Paper.

Paper spray ionization mass spectrometry (PSI MS) has emerged as a notable method for the rapid analysis of biological samples. However, the typical cellulose-based paper tip is incompatible with protein detection due to the strong interaction between cellulose hydroxyl groups and proteins. In this study, we utilized a commercially available polyolefin-based synthetic paper, Teslin®, as an alternative PSI substrate for simple protein analysis. We have named this method "droplet PSI" MS, as the aqueous protein solution droplet retains its shape on the Teslin® paper tip. For droplet PSI, no further chemical pretreatment was necessary for the Teslin® substrate; the only required preparation was shaping the Teslin® paper into a triangular tip. In droplet PSI MS, protein ion signals were instantly detected from a protein solution droplet upon applying a spray solvent in situ along with high voltage (HV). When compared with conventional PSI MS, our method demonstrated superior sensitivity. The droplet PSI MS utilizing Teslin® also showcased flexibility in real-time observation of protein alterations induced by an acid additive. Additionally, the effects of spray solvent composition and the application method were discussed.

Sensing the Positions of Multi Droplets on a Piezoelectric Substrate Using a Smartphone Based on CIELab Color Model

A method for simultaneously sensing positions of multi droplets on a piezoelectric substrate was presented. A double D-value algorithm was presented to distinguish target droplets using CIELab color model. An application soft (app) was developed to sense positions of target droplets. In order to verify the presented method, black ink solution droplets on a 128°-YX LiNbO3 substrate were used to demonstrate for sensing droplets position. Results show that different number of target droplets can be distinguished and their positions can be simultaneously sensed. As an application, a 5 micro-liters black ink solution droplet was driven on the piezoelectric substrate by surface acoustic wave. Its position was sensed and its movement path was analyzed using developed app. In the meantime, the transporting velocity of the droplet was obtained, of which value is 0.33 mm/s from 6th second to 7th second at 27.5 dBm electric signal power.

Highly Reproducible Synthesis of Hollow Zirconia Particles via Atmospheric-Pressure Plasma Processing with Inkjet Droplets

Hollow particles have attracted considerable attention owing to their unique properties. In this study, hollow monoclinic zirconia particles were directly synthesized from inkjet droplets of a zirconyl hydroxychloride aqueous solution via atmospheric-pressure plasma processing. Hollow structures with craggy surfaces were obtained in the plasma at gas temperatures above 1000 K. The steep solvent evaporation rate induced by the localized high-energy reaction field of the atmospheric-pressure plasma may have induced solute condensation near the droplet surface and contributed to the formation of hollow particles. The average diameter of the synthesized particles was ~ 3 μm, while their size distribution was narrow (coefficient of variation: 0.06–0.10). The high reproducibility of the synthesized particles was attributed to the small variations in inkjet droplet size. The proposed method enables the rapid synthesis of hollow particles of various inorganic materials, while controlling their number and composition.

Novel bubble shrinkage phenomenon in n-butanol/PODE4 binary droplet combustion

Atomization of multi-component solution droplets in combustion is usually controlled by bubble dynamics such as bubble growth and breakup. In this communication, a novel bubble shrinkage phenomenon was observed in n-butanol/polyoxymethylene dimethyl ether 4 (PODE4) binary droplet combustion, which leads to the weak micro-explosion in the atomization of droplet rather than the strong micro-explosion caused by monotonic bubble growth. A simple evaporation and content evolution model for the liquid film of the bubble is proposed to explain the observed bubble shrinkage phenomenon. Vapor condensation on the interface between the bubble and the liquid film caused by the vapor-liquid nonequilibrium is proposed to induce the bubble shrinkage, and a relatively small bubble growth velocity is found to be associated with the nonequilibrium.

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.

Surface Passivation Method for the Super-repellence of Aqueous Macromolecular Condensates.

Solutions of macromolecules can undergo liquid-liquid phase separation to form droplets with ultralow surface tension. Droplets with such low surface tension wet and spread over common surfaces such as test tubes and microscope slides, complicating in vitro experiments. The development of a universal super-repellent surface for macromolecular droplets has remained elusive because their ultralow surface tension requires low surface energies. Furthermore, the nonwetting of droplets containing proteins poses additional challenges because the surface must remain inert to a wide range of chemistries presented by the various amino acid side chains at the droplet surface. Here, we present a method to coat microscope slides with a thin transparent hydrogel that exhibits complete dewetting (contact angles θ ≈ 180°) and minimal pinning of phase-separated droplets in aqueous solution. The hydrogel is based on a swollen matrix of chemically cross-linked polyethylene glycol diacrylate of molecular weight 12 kDa (PEGDA), and can be prepared with basic chemistry laboratory equipment. The PEGDA hydrogel is a powerful tool for in vitro studies of weak interactions, dynamics, and the internal organization of phase-separated droplets in aqueous solutions.

Nanoparticle-releasing xerogel films prepared by a one-step method

Aggregation remains a challenge in the development of solid dosage forms of nanoparticles. Herein, we investigated whether xerogel films can allow sustained release of nanoparticles encapsulating curcumin (as a model drug) while preventing their aggregation. Such films can be fabricated by casting nano-oil droplets in an aqueous solution. In this study, curcumin and poly (lactic-co-glycolic acid) (PLGA) were dissolved in methylene chloride and emulsified in a polyvinyl alcohol (PVA)-propylene glycol (PG) solution. While drying the cast, the nano-oil droplets solidified to form nanoparticles. Concurrently, the PVA-PG solution was gelatinated and further dried to form a xerogel. The xerogel film containing curcumin-loaded nanoparticles disintegrated in vitro within 10 min, releasing the nanoparticles without aggregation. The emitted nanoparticles in turn released curcumin at a first-order rate over 90 min. Thus, xerogel films containing nanoparticles could be useful sustained-release solid preparations for delivering nanoparticles at a controlled rate without aggregation.

Construction and mechanism study of clean N2 foam fracturing fluid stabilized by viscoelastic surfactant in concentrated brines for unconventional oil and gas

As a much-anticipated clean thickener, viscoelastic surfactants (VESs) have drawn much attention in the development of foam fracturing fluids for unconventional oil and gas stimulation. Herein, the mechanism of foam stabilization by VESs and wormlike micelles (WLMs) under a high-salinity environment was investigated by evaluating three kinds of salt-tolerant VESs with different molecular structures through a series of experiments, including drainage tests, foam microscopic characteristic observations and interface rheology tests. The drainage test proved that the liquid film drainage was effectively inhibited under the proper salinity range for WLM construction. Based on microscopic observation, although WLMs are formed, gas diffusion inevitably occurs in each foam fluid as bubble size diversity exists, while the foam stabilized by VES-BC, the Gemini structure with steric hindrance on its spacer, exhibits comparably high uniformity of bubble size. Relying on the Gibbs criterion, this further proves that the foam stabilized by VES-BC resists gas diffusion better than the other two foams, as the critical shrinking speed of the VES-BC solution droplet to resist gas diffusion is lowest, which explains why the foam stabilized by VES-BC exhibits the best anti-coarsening capacity under static conditions. The oscillation droplet test proves that the VES-BC molecules and WLMs they construct impart the interface with the best resistance to external disturbance and deformation under the proper salinity range for WLM construction. Meanwhile, the interface shear modulus measurement indicates that the WLMs construed by VES-BC molecules and the interaction between WLMs and the VESs on the interface play crucial roles in anti-coalescence between bubbles. Under high-temperature and high-pressure conditions, the VES-C and VES-BC foam fracturing fluids prepared by brines, with 80% foam quality, both exhibit excellent thickening and proppant carrying capacity, and the VES-BC foam fracturing fluid can withstand 140 °C to sustain a low settling rate of proppant, which potentially decreases the cost of cleaning fracturing fluid by displacing 80% of the fluid into N2 and causes little damage to unconventional reservoirs. All experiments prove that foam coarsening is the key factor causing foam decay regardless of whether WLMs exist, yet WLMs can efficiently inhibit the drainage of liquid film and resist bubble coalescence by interacting with the molecules on the interface.

High-Performance Pressure Sensors Based on Shaped Gel Droplet Arrays.

Polymer gel-based pressure sensors offer numerous advantages over traditional sensing technologies, including excellent conformability and integration into wearable devices. However, challenges persist in terms of their performance and manufacturing technology. In this study, a method for fabricating gel pressure sensors using a hydrophobic/hydrophilic patterned surface is introduced. By shaping and fine-tuning the droplets of the polymer gel prepolymerization solution on the patterned surface, remarkable sensitivity improvements compared to unshaped hydrogels have been achieved. This also showcased the potential for tailoring gel pressure sensors to different applications. By optimizing the configuration of the sensor array, an uneven conductive gel array is fabricated, which exhibited a high sensitivity of 0.29 kPa-1 in the pressure range of 0-30kPa, while maintaining a sensitivity of 0.13 kPa-1 from 30 kPa up to 100kPa. Furthermore, the feasibility of using these sensors for human motion monitoring is explored and a conductive gel array for 2D force detection is successfully developed. This efficient and scalable fabrication method holds promise for advancing pressure sensor technology and offers exciting prospects for various industries and research fields.

Osmosis-induced hydrodynamic centering of W/O/W double emulsion droplets for quasi-concentric microcapsule/microsphere fabrication

Water-in-oil-in-water (W/O/W) double emulsion droplets are a typical colloidal system applied in various practical fields. However, certain high-specification applications necessitate exact geometric homogeneity in terms of both shell diameter and thickness. Despite achieving uniformity in shell diameter, on-chip microfluidic emulsification always leads to an eccentric phenomenon due to innate density differences of the droplet solution. At the same time, the current regulation methods have multiple limitations, including their applicability, regulatory scope, and objective restrictions. Herein, we propose an off-chip osmosis-induced hydrodynamic method to accurately regulate the eccentricity of W/O/W double emulsion droplets. The rational selection of hypertonic osmosis can energize the droplets and induce inward transmembrane water flux. This flow helps to reposition the droplet core to align concentrically with the shell, regardless of the initial dimension and eccentricity of the droplets. As the osmotic induction reduces due to the dilution of the inner solute molar concentration, the optimal droplet configuration can be achieved when osmosis-induced hydrodynamic regulation sufficiently counteracts the adverse effects of droplet solution density difference. The microcapsules can thus be produced by instant UV polymerization of the regulated double emulsion droplets, and the silica microspheres with fine geometric homogeneity can be prepared through further heat treatment, which is expected to meet the stringent morphological requirements in practical applications.

Controlling the Size of C70 Fullerene Whiskers by Evaporation of Solution Droplets

"The formation of one-dimensional nanostructures (nanowhiskers) during the evaporation of a drop of C70 fullerene solution in ortho-xylene on a smooth surface of a glass substrate was studied. The growth mechanisms of fullerene nanostructures were analyzed, as well as their morphological and size characteristics were determined. "