Fluid Droplets Research Articles

Membrane curvature sensing by model biomolecular condensates.

Biomolecular condensates (BCs) are fluid droplets that form in biological cells by liquid-liquid phase separation. Their major components are intrinsically disordered proteins. Vast attention has been given in recent years to BCs inside the cytosol and nucleus. BCs at the cell membrane have not been studied to the same extent so far. However, recent studies provide increasingly more examples of interfaces between BCs and membranes which function as platforms for diverse biomolecular processes. Galectin-3, for example, is known to mediate clathrin-independent endocytosis and has been recently shown to undergo liquid-liquid phase separation, but the function of BCs of galectin-3 in endocytic pit formation is unknown. Here, we use dissipative particle dynamics simulations to study a generic coarse-grained model for BCs interacting with lipid membranes. In analogy to galectin-3, we consider polymers comprising two segments - one of them mediates multivalent attractive interactions between the polymers, and the other one has affinity for association with specific lipid head groups. When these polymers are brought into contact with a multi-component membrane, they spontaneously assemble into droplets and, simultaneously, induce lateral separation of lipids within the membrane. Interestingly, we find that if the membrane is bent, the polymer droplets localize at membrane regions curved inward. Although the polymers have no particular shape or intrinsic curvature, they appear to sense membrane curvature when clustered at the membrane. Our results indicate toward a generic mechanism of membrane curvature sensing by BCs involved in such processes as endocytosis.

ADVECTION AND DEPOSITION OF MICRODROPLETS IN STAGNATION POINT FLOW

We investigate trajectories of microscale evaporating droplets in a stagnation point flow near a wall of a respiratory airway. The configuration is motivated by the problem of advection and deposition of microscale droplets of respiratory fluids in human airways during transmission of infectious diseases, such as tuberculosis and COVID-19. Laminar boundary layer equations are solved to describe the airflow while the equations of motion of the droplet include contributions from gravity, aerodynamic drag, and Saffman force. Evaporation is accounted for at both the droplet surface and the wall of the respiratory airway and is shown to delay droplet deposition as compared to the predictions of isothermal models. Evaporation at the airway wall has a stronger effect on droplet trajectories than evaporation at the droplet surface, leading to droplets being advected away by the flow and thus avoiding deposition in the stagnation point flow region.

Analysis of the breaking process and the components of the crown structure after a single droplet hits thin liquid film

• Three types of crown breakup: splash breaking, hole breaking, and mixed breaking. • Fluid film viscosity and droplet Weber number affect the crown breakup. • Empirical correlations to classify three types of crown breakup was proposed. The breaking process and the components of the crown formed when the single droplet impact on different liquid films were investigated experimentally and simulatly. Laser-induced fluorescence (LIF) methods were used to study the breakup process of the crown structure. The composition of the incident droplets in this test is ethanol, and the glycerol aqueous solution was used as the liquid film. Results show that at different droplet Weber numbers, liquid film thickness, and liquid film viscosity, it is reasonable to classify the breakup of crown structure into the following three regimes: splash breaking, hole breaking, and mixed breaking. The characteristics of each breaking regime, such as the breaking mechanism, diameter of crown structure and spatial distribution of the secondary droplets formed after breakup, have been qualitatively analyzed. The viscosity of the liquid film and the Weber number of incident droplets have significant influence on the breakup of the crown structure. With decreasing viscosity and increasing Weber number, the crown structure breaking process transits from unbroken to splash breaking, hole breaking, and finally mixed breaking. According to the experimental results, a fitting formula for describing critical parameters of the different breaking regimes of crown structure is established. To further quantify the material components of the crown structure and the mixing of the incident droplets with the liquid film, Volume of Fluid (VOF) model and species transport model were used to establish a model for simulation calculations, and the effects of droplet Weber number, liquid film thickness and liquid film viscosity were clarified.

Spontaneous motion of a passive fluid droplet in an active microchannel.

We numerically study the dynamics of a passive fluid droplet confined within a microchannel whose walls are covered with a thin layer of active gel. The latter represents a fluid of extensile material modelling, for example, a suspension of cytoskeletal filaments and molecular motors. Our results show that the layer is capable of producing a spontaneous flow triggering a rectilinear motion of the passive droplet. For a hybrid design (a single wall covered by the active layer), at the steady state the droplet attains an elliptical shape, resulting from an asymmetric saw-toothed structure of the velocity field. In contrast, if the active gel covers both walls, the velocity field exhibits a fully symmetric pattern considerably mitigating morphological deformations. We further show that the structure of the spontaneous flow in the microchannel can be controlled by the anchoring conditions of the active gel at the wall. These findings are also confirmed by selected 3D simulations. Our results may stimulate further research addressed to design novel microfludic devices whose functioning relies on the collective properties of active gels.

Lithographic Microneedle-Motors from Multimodal Microfluidics for Cargo Delivery.

Micromotors have led to an unprecedented revolution in the field of cargo delivery. Attempts in this area trend toward enriching their structures and improving their functions to promote their further applications. Herein, novel microneedle-motors (MNMs) for active drug delivery through a flexible multimodal microfluidic lithographic approach are presented. The multimodal microfluidics is composed of a co-flow geometry-derived droplet fluid and an active cargo mixed laminar flow in a triangular microchannel. The MNMs with sharp tips and spherical fuel-loading cavities are obtained continuously from microfluidics with the assistance of flow lithography. The structural parameters of the MNMs could be precisely tailored by simply choosing the flow speed or the shape of the photomask. As the actives are mixed into the phase solution during the generation, the resultant MNMs are loaded with cargoes for direct applications without any extra complex operation. Based on these features, it is demonstrated that with sharp tips and autonomous movement, the MNMs can efficiently penetrate the tissue-like substrates, indicating the potential in overcoming physiological barriers for cargo release. These results indicate that the proposed multimodal microfluidic lithographic MNMs are valuable for practical active cargo delivery in biomedical and other relative areas.

On shape forming by contractile filaments in the surface of growing tissues.

Growing tissues are highly dynamic, and flow on sufficiently long timescales due to cell proliferation, migration, and tissue remodeling. As a consequence, growing tissues can often be approximated as viscous fluids. This means that the shape of microtissues growing in vitro is governed by their surface stress state, as in fluid droplets. Recent work showed that cells in the near-surface region of fibroblastic or osteoblastic microtissues contract with highly oriented actin filaments, thus making the surface properties highly anisotropic, in contrast to what is expected for an isotropic fluid. Here, we develop a model that includes mechanical anisotropy of the surface generated by contractile fibers and we show that mechanical equilibrium requires contractile filaments to follow geodesic lines on the surface. Constant pressure in the fluid forces these contractile filaments to be along geodesics with a constant normal curvature. We then take this into account to determine equilibrium shapes of rotationally symmetric bodies subjected to anisotropic surface stress states and derive a family of surfaces of revolution. A comparison with recently published shapes of microtissues shows that this theory accurately predicts both the surface shape and the direction of the actin filaments on the surface.

Augmenting the Leidenfrost Temperature of Droplets via Nanobubble Dispersion.

Droplets may rebound/levitate when deposited over a hot substrate (beyond a critical temperature) due to the formation of a stable vapor microcushion between the droplet and the substrate. This is known as the Leidenfrost phenomenon. In this article, we experimentally allow droplets to impact the hot surface with a certain velocity, and the temperature at which droplets show the onset of rebound with minimal spraying is known as the dynamic Leidenfrost temperature (TDL). Here we propose and validate a novel paradigm of augmenting the TDL by employing droplets with stable nanobubbles dispersed in the fluid. In this first-of-its-kind report, we show that the TDL can be delayed significantly by the aid of nanobubble-dispersed droplets. We explore the influence of the impact Weber number (We), the Ohnesorge number (Oh), and the role of nanobubble concentration on the TDL. At a fixed impact velocity, the TDL was noted to increase with the increase in nanobubble concentration and decrease with an increase in impact velocity for a particular nanobubble concentration. Finally, we elucidated the overall boiling behaviors of nanobubble-dispersed fluid droplets with the substrate temperature in the range of 150-400 °C against varied impact We through a detailed phase map. These findings may be useful for further exploration of the use of nanobubble-dispersed fluids in high heat flux and high-temperature-related problems and devices.

Quasi-Stable, Non-Magnetic, Toroidal Fluid Droplets in a Ferrofluid with Annular Magnetic Field

A relatively stable, non-magnetic, torus-shaped fluid droplet within a linearly magnetizable surrounding ferrofluid medium, and subject to the annular magnetic field induced by an electric current in a wire passing perpendicularly through its centre, has been found through the use of coupled finite element/boundary element computer simulations.

A Multi-Scale Approach for Modelling Airborne Transport of Mucosalivary Fluid

COVID-19 pandemic promoted a lot of research activities in relation to mucosalivary fluid airborne transport. Indeed, infection mechanisms are the result of mucosalivary fluid droplets exchange and the knowledge in this area is still largely inadequate. One of the main challenges concerns the modelling of mucosalivary fluid complex nature. Specifically, this is a key element to predict small diameters dry nuclei formation which are highly relevant from the transmission risk point of view. For this reason, in this paper we present and discuss the development of a new multi-scale modelling technique which incorporates the Population Balance Equation into a standard particle-source-in-cell method. Thus, the effectiveness of the aforementioned technique in droplet nuclei generation modelling is showed and discussed. Also the impact of velocity boundary conditions at the mouth print is assessed as well as the effect of the correlations for mass transfer showing that their neglect causes an underestimation in distance reached by the droplets.

Impingement and mixing dynamics of micro-droplets on a solid surface

• Impingement on sessile micro-droplet with different impinging angles was simulated. • Convective time, diffusion time and total mixing time were determined quantitatively. • Influence factors on the dimensionless droplet mixing time were discussed. • Mixing time can be shortened by reducing the impinging angle. The hydrodynamics and mixing during the nonaxisymmetry impingement of a micro-droplet and a sessile droplet of the same fluid are investigated by many-body dissipative particle dynamics (MDPD) simulation. In this work, the range of the impingement angle ( θ i ) between the impinging droplet and the sessile droplet is 0°–60° and the contact angle is set as 45° or 124°. The droplets impingement and mixing behavior is analyzed based on the droplet internal flow field, the concentration distribution and the time scale of the decay of the kinetic energy of the impinging droplet. The dimensionless total mixing time ( τ m ) is calculated by a modified mixing function. With the Weber number ( We ) ranging from 5.65 to 22.7 and the Ohnesorge number ( Oh ) ranging from 0.136 to 0.214, we find τ m hardly changes with We and Oh . Whereas, θ i and surface wettability are found to have a significant effect on τ m . We find that θ i has no clear effect on τ m on a hydrophobic surface, while on the hydrophilic surface, τ m increase with the θ i . Thus, reducing the impinging angle is a valid method to shorten the τ m .

Mesoscale Polymer Surfactants: Photolithographic Production and Localization at Droplet Interfaces.

Stabilization of fluid droplets, classically as oil-in-water or water-in-oil emulsions, is typically conducted using molecular surfactants or small particulates that localize at oil-water interfaces. In this paper, we describe a method whereby thin polymer films are converted photolithographically to ribbon-like mesoscale objects, which, in turn, adsorb to fluid interfaces where they extend as appendages, or arms, from the droplet surface. These "mesoscale polymer surfactants", or MPSs, were prepared from thin polymer films containing reactive functional moieties, including coumarin for photo-cross-linking, triphenylsulfonium for photoacid generation, and tert-butyl ester for solubility switching. The resultant MPSs, prepared initially on Si substrates, were released into water to reveal an exquisite shape sensitivity (forming straight, bent, or helical structures) and affinity for droplet interfaces based on their preparation conditions and the properties of the surrounding liquid. Notably, the lithographic techniques employed were amenable to differentiating the wettability of MPS segments, affording access to diblock-like MPSs which adhered to dispersed droplets via their hydrophobic segments, allowing their hydrophilic segments to extend into the continuous phase.

INFUSION MONITORING SYSTEM FOR PATIENTS BASED ON THE INTERNET OF THINGS (IOT) WITH ANDROID NOTIFICATION SYSTEM

Infusion is one of the medical needs that are needed by many patients in various hospitals or health centers. Infusion is a way in which a patient can introduce fluids or drugs into the body quickly or slowly and continuously. Therefore, nurses always monitor the infusion manually and periodically to ensure the condition of the infusion fluid. The experimental method used in this study was by adding several features to make it easier for nurses to monitor patient infusions. The Infusion Monitoring System device used is based on the Internet of Things (IoT) with the Android Notification System. This research is expected to be able to monitor infusions more practically and can be monitored mobile using cellphones anywhere and anytime. To present this tool in this study, the author uses Nodemcu used to process data and also serves to connect to the internet. Then the sensors used are the Load cell sensor as a measuring tool for infusion volume and the Photodiode sensor as a detection tool for infusion fluid droplets. Then the calculation data from the sensor is sent to Firebase which acts as a database, and later it will be accessible and monitored via mobile phones through an android application created by the researcher. From the test results of the tools that the researcher made and the testing of comparison tools. The average of these tests gets an accuracy difference of approximately 2%. (Abstract)

COMBINED METHOD OF WASTEWATER TREATMENT OF A SMALL-CAPACITY BREWERY

With the help of the proposed circuit of EMT, the method of simultaneous measurements of the static characteristics of the magnetic fluid: specific electrical conductivity χt, magnetic susceptibility kt and temperature t of the magnetic fluid, which is recommended to be used for wastewater treatment of breweries, was investigated. A combined method of cleaning has been developed and the appropriate technological scheme has been improved, which provides, together with the use of mechanical cleaning devices, the use of an additional filter for magnetizing wastewater samples, which includes a special zone for the introduction of a pre-calculated magnetic liquid. Thus, in the process of wastewater treatment, mini-breweries using the proposed combined method, after applying methods of pre mechanical cleaning, use the wastewater interaction and the magnetic fluid: after intensive stirring of the mixture, the droplets of magnetic fluid dissolve in impurities and a weakly magnetic fluid is formed, which enters the magnetic separator for further cleaning. A rational ratio of wastewater: magnetic fluid (WW:MF), WW:MF = 8,7‧10-3: 1 was found. A comparison of the data obtained using informative methods with the data given in the regulatory documents witness the effectiveness of the proposed combined method of beer wastewater treatment in relation to the norms of water discharge into the city sewer, namely, the hydrogen pH indicator decreases as a result of the treatment by 34.6%, COD by 7.14 times, BOD5 by 1.52 times, suspended substances by 1.52 times, calcium cations Ca2+ by 1.53 times, magnesium cations Mg2+ by 1.51 times, total hardness dH by 1.64 times, carbonate hardness KH by 1, 86 times.

Fluid droplet spreading and adhesion studied with a microbalance: a review

A contact angle observed for a liquid–solid system is not necessarily a unique value, and a few contact angles need to be considered carefully in relation to liquid spreading, adhesion and phase separation. Understanding of the significance of different contact angles has improved in the last few years through direct measurements of interactive forces between droplets/bubbles and solids together with the simultaneous visualization of the changes in their shapes. A microelectronic balance system is employed to measure the force of spreading after either liquid droplet or gas bubble attachment to a substrate surface and the droplet/bubble–substrate adhesion forces after droplet/bubble compression, retraction and detachment. Equipped with a camera in flank and data-acquisition software, the instrument measures directly the forces, monitors droplet/bubble–surface separation with respect to distances over which the droplet/bubble stretches and collects optical images simultaneously. The images are used to analyze capillary pressure and surface tension forces based on the measured droplet/bubble dimensions, shapes of surfaces and values of contact angles. These measurements allow researchers to correlate the advancing, receding and most stable contact angles with liquid–solid interactive forces and analyze their scientific meaning. This review summarizes the very recent literature reports on measurements and interpretation of liquid droplet/gas bubble interactive forces and associated contact angles.

Spontaneous separation and evaporation mechanism of self-rewetting fluid droplets on chemically stripe-patterned surfaces: A lattice Boltzmann study.

The evaporation characteristics of self-rewetting fluids have attracted much attention in recent years. However, the evaporation dynamics as well as the underlying evaporation mechanism of self-rewetting fluid droplets has not been well understood. In this paper, we numerically investigate the evaporation performance and the dynamic behavior of self-rewetting fluid droplets on chemically patterned surfaces using a thermal multiphase lattice Boltzmann model with liquid-vapor phase change. First, it is shown that a self-rewetting fluid droplet can spontaneously separate into two droplets during its evaporation on a hydrophilic surface with a hydrophobic stripe, while no separation occurs during the evaporation of a conventional fluid droplet. The positive surface tension gradient of the self-rewetting fluid is found to play an important role in the spontaneous separation of the self-rewetting fluid droplet during the evaporation. Meanwhile, the separation behavior of the self-rewetting fluid droplet can effectively increase the length of the triple-phase contact line, which leads to a significant increase in the evaporation rate as compared with that of a conventional fluid droplet. Moreover, by investigating the evaporation performance of self-rewetting fluid droplets on chemically stripe-patterned surfaces with different values of the widths of the hydrophilic and hydrophobic stripes, it is found that the stripe width and the initial location of the droplet significantly affect the dynamic behavior and the evaporation efficiency of the self-rewetting fluid droplet. For different relative positions between the droplet and the stripes, the droplet may spontaneously separate into two or three droplets and achieve much better evaporation efficiency when the stripe width is within an optimal range.

Spreading of droplets under various gravitational accelerations.

We describe a setup to perform systematic studies on the spreading of droplets of complex fluids under microgravity conditions. Tweaking the gravitational acceleration under which droplets are deposited provides access to different regimes of the spreading dynamics, as quantified through the Bond number. In particular, microgravity allows us to form large droplets while remaining in the regime where surface tension effects and internal driving stresses are predominant over hydrostatic forces. The vip-drop2 (visco-plastic droplets on the drop tower) experimental module provides a versatile platform to study a wide range of complex fluids through the deposition of axisymmetric droplets. The module offers the possibility to deposit droplets on a precursor layer, which can be composed of the same or a different fluid. Furthermore, it allows us to deposit four droplets simultaneously while conducting shadowgraphy on all of them and observing either the flow field (through particle image velocimetry) or the stress distribution inside the droplet in the case of stress birefringent fluids. It was developed for a drop tower catapult system, is designed to withstand a vertical acceleration of up to 30 times the Earth's gravitational acceleration in the downward direction, and is capable of operating remotely under microgravity conditions. We provide a detailed description of the module and an exemplary data analysis for droplets spreading on-ground and in microgravity.

Equilibrium shapes of two- and three-dimensional two-phase rotating fluid drops with surface tension: Effects of inner drop displacement

The shapes of rotating fluid drops held together by surface tension are an important field of study in fluid mechanics. Recently, experiments with micrometer-scale droplets of liquid helium have been undertaken and it has proven useful to compare the shapes of the resultant superfluid droplets with classical analogs. If the helium is a mixture of He3 and He4, two phases are present. In a recent paper, the shapes of rotating two-phase fluid droplets were calculated where the inner drop was constrained to stay at the drop center. The outer shapes and dimensionless rotation rate–angular momentum relationships were shown to be similar to single-phase drops, providing that the density and surface tension scales were chosen appropriately. In the current paper, I investigate models in which the inner drop can displace from the center. In order to simplify the analyses, two-dimensional drops are first investigated. I show that the inner drop is unstable in the center position if its density is greater than the outer density and that the inner drop will move toward the outer boundary of the drop in these cases. When the inner drop has a higher density than the outer drop, the moment of inertia of displaced inner drops is increased relative to centered drops, and hence, the kinetic energy is decreased. Shapes of two- and three-dimensional drops, rotation rate–angular momentum, and kinetic and surface energy relationships are investigated for off-axis inner drops with parameters relevant to recent liquid He experiments.

Snoring may transmit infectious aerosols from the upper to the lower respiratory tract

Migration to the lungs of an initial upper airway infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or other respiratory pathogens can lead to pneumonia, associated with progression from mild to severe symptoms. Chemical pneumonitis or bacterial pneumonia may be caused by the ‘macroaspiration’ of large volumes of oropharyngeal or gastroesophageal secretions into the lower respiratory tract. ‘Microaspiration’, i.e., a similar mechanism but involving much smaller amounts of oropharyngeal secretions, is considered the pathogenetic mechanism for most pneumonias, including that associated with COVID-19. Here, we hypothesize an alternative mechanism: Rather than by microaspiration, these fluids enter the lungs as microdroplets that are generated by snoring and then carried by the inspired airstream. Laboratory measurements indicate that snoring generates (a) comparable numbers and sizes of oral fluid droplets as loud speaking and (b) total fluid quantities that are similar to those reported for microaspiration. Snoring propensity is strongly correlated to known risk factors for severe COVID-19, including male gender, age, obesity, diabetes, obstructive sleep apnea, and pregnancy. Therefore, more research is urgently needed to determine if various methods that decrease snoring can prevent progression to pneumonia after initial infection of the upper airways.

Research on laser preparation and grinding performance of hydrophilic structured grinding wheels

In order to solve the problems of workpiece damage and grinding wheel clogging when grinding difficult-to-cut materials such as alumina ceramics, organisms with regular hexagonal structures distributed on the body surface and with strong hydrophilicity and high anti-wear functions were used as biomimetic objects for the first time, and the preparation process optimization, structure size design and grinding performance evaluation of hydrophilic structured bronze-bonded diamond grinding wheels were explored in this paper. The influence of the preparation process parameters on the micro-topography and the dimensional accuracy of the structure on the surface of the grinding wheel was revealed, and a new laser structuring process based on the coordinated control of focus position and scanning times was proposed, which could efficiently prepare regular hexagonal structures with a small wall inclination angle and a depth of several millimeters on the surface of the grinding wheel. It was the first to clarify the influence of sub-millimeter-scale structure size on the contact angle of grinding fluid droplet on the surface of the grinding wheel and the surface hydrophilicity of the grinding wheel. Compared with that of the non-structured grinding wheel, the hydrophilicity of the structured grinding wheel was significantly improved, and its surface hydrophilicity increased with the increase of the structure spacing and depth, but had little correlation with the structure side length. The grinding performance of hydrophilic structured grinding wheels and non-structured grinding wheels was evaluated under extreme working conditions. Under the condition of grinding depth of 50 μm, 100 μm and 200 μm, compared with that of the non-structured grinding wheel, the peak grinding temperature of the structured grinding wheel was reduced by 18.0%, 30.4% and 15.2%, respectively, and the surface damage depth of the alumina ceramic after grinding by the structured grinding wheel was reduced by 53.7%, 46.8% and 24.3%, respectively. The hydrophilic structured grinding wheel can enhance the storage and transportation capacity of grinding fluid/chips, effectively relieve the clogging and dullness of the grinding wheel, and significantly reduce the high temperature and damage of grinding. In the next step, we will try to apply this type of grinding wheel to form grinding, in order to provide a reliable solution for suppressing form grinding damage of difficult-to-cut materials.

Electrophoresis of Dielectric and Hydrophobic Spherical Fluid Droplets Possessing Uniform Surface Charge Density.

The present article deals with the theoretical study on electrophoresis of hydrophobic and dielectric spherical fluid droplets possessing uniform surface charge density. Unlike the ideally polarizable liquid droplet bearing constant surface ζ-potential, the tangential component of the Maxwell stress is nonzero for dielectric fluid droplets with uniform surface charge density. We consider the continuity of the tangential component of total stress (sum of the hydrodynamic and Maxwell stresses) and jump in dielectric displacement along the droplet-to-electrolyte interface. The typical situation is considered here for which the interfacial tension of the fluid droplet is sufficiently high so that the droplet retains its spherical shape during its motion. The present theory can be applied to nanoemulsions, hydrophobic oil droplets, gas bubbles, droplets of immiscible liquid suspended in aqueous medium, etc. Based on weak field and low charge assumptions and neglecting the Marangoni effect, the resultant electrokinetic equations are solved using linear perturbation analysis to derive the closed form expression for electrophoretic mobility applicable for the entire range of Debye-Hückel parameter. We further deduced an alternate approximate expression for electrophoretic mobility without involving exponential integrals. Besides, we have derived analytical results for mobility pertaining to various limiting cases. The results are further illustrated to show the impact of pertinent parameters on the overall electrophoretic mobility.