Droplet Formation Process Research Articles

Investigation of Rheological Properties of Molten Materials for Dripping Pills Based on Imaging Monitoring.

Rheological properties, as critical material attributes (CMAs) of solid dispersion drugs such as dripping pills, affect the melting, dispersion, and solidification. Therefore, characterization and assessments of rheological properties in the pharmaceutical process are important in enhancing drug stability and bioavailability. The study aimed to develop a method for analyzing the rheology of molten materials, assessing their consistency and how rheological properties affect the dripping process and pills quality. The rheological behavior of molten materials composed of Ginkgo biloba leaf extract (GBE) and polyethylene glycol (PEG) 4000 was characterized. Batch consistency of molten materials was evaluated. Image monitoring technology was utilized to capture and process images of the droplet formation process. We established the relationship between the rheological properties of molten materials and various attributes. The quality consistency of molten materials was evaluated, with 12 batches showing similarity above 0.8. The MLR models showed strong correlations (R2 > 0.80) between rheological properties and evaluation attributes. The rheological properties, including consistency coefficient, flow index, and viscosity at 80°C, were identified as critical rheological properties of the molten materials. Rheological property differences of molten materials have an impact on the morphology of droplet and quality performance. A rheological method was established, enabling quality consistency evaluation of molten materials in dripping pills. This study revealed the influence of rheological properties on droplet formation process and dripping pills quality, providing a reference for researches on material attributes control of other traditional Chinese medicine dripping pills.

High-Speed Imaging of Giant Unilamellar Vesicle Formation in cDICE.

Giant unilamellar vesicles (GUVs) are widely used as in vitro model membranes in biophysics and as cell-sized containers in synthetic biology. Despite their ubiquitous use, there is no one-size-fits-all method for their production. Numerous methods have been developed to meet the demanding requirements of reproducibility, reliability, and high yield while simultaneously achieving robust encapsulation. Emulsion-based methods are often praised for their apparent simplicity and good yields; hence, methods like continuous droplet interface crossing encapsulation (cDICE), which make use of this principle, have gained popularity. However, the underlying physical principles governing the formation of GUVs in cDICE and related methods remain poorly understood. To this end, we have developed a high-speed microscopy setup that allows us to visualize GUV formation in real time. Our experiments reveal a complex droplet formation process occurring at the capillary orifice, generating >30 μm-sized droplets and only in some cases GUV-sized (∼15 μm) satellite droplets. According to existing theoretical models, the oil-water interface should allow for the crossing of all droplets, but based on our observations and scaling arguments on the fluid dynamics within the system, we find a size-selective crossing of GUV-sized droplets only. The origin of these droplets remains partly unclear; we hypothesize that some small GUVs might be formed from large droplets sitting at the second interface. Finally, we demonstrate that proteins in the inner solution affect GUV formation by increasing the viscosity and altering the lipid adsorption kinetics. These results will not only contribute to a better understanding of GUV formation processes in cDICE but ultimately also aid in the development of more reliable and efficient methods for GUV production.

Hydrodynamic characteristics of detachment length and flow mapping in T-junction circular microchannel

This study investigates the droplet formation process in T-junction circular microchannels, emphasizing the influence of wetting properties and contact angles under various operating conditions. A coupled Volume-of-Fluid (VOF)/Level-set multiphase model is employed for Computational Fluid Dynamics (CFD) simulations to accurately capture the microscale phenomena. The effects of contact angles ranging from 125° to 145°, two-phase flow velocities (0.04–0.25 m/s), and interfacial tension (0.035–0.055 N/m) are systematically analyzed. The findings reveal that these parameters significantly impact the detachment length and flow patterns, with four distinct flow regimes identified: dripping, slug, jetting, and annular flow. New dimensionless correlations for slug and detachment lengths are developed and validated through experimental data, offering improved predictive capabilities. The comprehensive flow mapping provided in this study enhances the understanding of multiphase flows in microchannels, offering valuable insights for the optimization of microfluidic device design and practical applications such as extractive desulphurization. These contributions present a novel approach to integrating contact angle effects into microfluidic research, addressing previously overlooked aspects and providing robust guidelines for future studies.

Molecular dynamics simulation on de-icing behavior of nanodroplets on amorphous SiO2

Abstract The ice formation and detachment process of nanoscale water droplets on the amorphous SiO2 substrate, which is for simulating the glass of solar panels, is studied in this paper by molecular dynamics simulation methods, considering the effects of environmental temperature, onset of icing time, and parameters of the nanoarray on the ice formation morphology and ice adhesion strength. The results show that the ice adhesion strength is independent of the environmental temperature. The time of icing onset influences the recoil state of droplets, with a larger spreading area leading to higher adhesion strength. The increase in height and decrease in width of the nanocolumns of nanoarray structure leads to an increase in the contact angle of the droplet, causing a reduction in ice adhesion strength, albeit with a slight rise within a certain height range.

Calculation of apparent contact angle for sessile droplets on structured hydrophobic surfaces based on dynamic of contact line during droplet formation

The sessile-droplet method is widely used to characterize surface wettability, however, the measured apparent contact angle (APCA) is usually a non-unique value between the advancing and receding contact angles, which seems hardly to be predicted by the classical Cassie-Baxter model. Thus, a reliable calculation model for APCAs of sessile droplets on structured hydrophobic surfaces is needed to guide the structural design of functional hydrophobic surfaces. Focusing on the force and motion analysis of the contact line during the formation process of sessile droplets, a theoretical model was developed to calculate the APCA of the droplet on hydrophobic surfaces featured with square micropillar arrays. The present model considers the synergistic effect of the droplet shape and the contact-line pinning on the APCA. Compared with the Cassie-Baxter model, the APCAs calculated by the proposed model agree better with the experimental data. This study shows that APCA is generally more susceptible to advancing contact angle, which is not significantly different on structured hydrophobic surfaces with different dimensions, resulting in the inability of APCA to accurately characterize surface wettability. As a complement, receding-contact-angle and adhesion-force measurements, which can reflect the effects of structural dimensions, are recommended to be adopted to characterize surface wettability.

The formation process and mechanism of coaxial electrostatic atomization of nanofluid oil film droplets

Abstract In coaxial electrostatic atomization cutting, the atomization effect of nanofluid oil film droplets directly affects the cooling and lubrication performance. To further understand the formation process of tiny oil film water droplets in coaxial electrostatic atomization, this paper takes LB2000/water, LB2000/0.1 vol.% graphite water-based nanofluids, LB2000/0.3 vol.% graphite water-based nanofluids, LB2000/0.5 vol.% graphite water-based nanofluids as the research object. Coaxial electrostatic atomization simulation is carried out based on the determined common voltage of the conical jet (-6.5 kV). The formation process of nanofluid oil film water droplets was analyzed using electric field force, charge concentration, and volume force, and the droplet states under different fluid types were compared. The results show that when nanoparticles are added to the inner fluid, the charge concentration increases, the electric field force and volume increase, and the formation speed of the cone jet increases significantly.

Effect of solute concentration on the number of NaCl particles generated by millimeter- and micron-sized droplets

Salt particles generated by aerosol generator are commonly used in air filtration test. As a common phenomenon, the number of generated particles increases with the salt concentration in the atomization solution. Thus far, the dependence of the particle number on the concentration of the atomization solution is not fully understood. In this study, two hypotheses were proposed to explain this phenomenon: i) a single droplet generates multiple solid particles, and the generated particle number is influenced by the salt concentration, and ii) the original droplet number increases with an increase in the salt concentration of the atomization solution. By observing the particle formation process of the levitated millimeter-scale NaCl droplet, one droplet was found to generate one particle in free air. However, whether these results are applicable to micrometer-scale droplets remains to be determined. In situ measurements were performed to detect the size distributions of the original droplets generated by the atomization solutions with various salt concentrations. A more concentrated atomization solution induced a higher number of original droplets. Particle formation patterns of droplets on glass substrates and filaments were demonstrated. Traditional observation methods for droplet drying are unsuitable for studying the particle formation of atomized droplets because of the large difference in droplet size and the existence of external forces.

Study of droplet formation in parallel flow focusing microchannel under electrostatic field control

To achieve efficient preparation of emulsion droplet, this paper uses a two-dimensional mathematical model coupled with the phase field model and the electrostatic field model to calculate the droplet formation in parallel flow focusing multichannel, and analyzed the effect of electric field strength on the droplet formation process. In addition, the effects of different capillary numbers and dispersed phase viscosity on droplet formation were also analyzed. The results show that the emulsion in the channel always generates droplet by dripping regime, and the droplet have the best monodispersity. Applying appropriate electric field strength can realize droplet formation frequency and size regulation. Increasing the dispersed phase viscosity increases the breakup thread of the dispersed phase fluid, and the droplet size obtained gradually decreases, the difference in droplet size can be slowed down by applying the electric field. Increasing the electrostatic force leads to a higher droplet deformation rate. The droplet shape classification is plotted using the capillary number (Ca) and electrostatic number (CaE). It is found that the droplet flow pattern in the channel remains a circle only when Ca and CaE are sufficiently small.

Understanding droplet formation in T-shaped channels with magnetic field influence: A computational investigation

This study investigates the production of ferrofluid droplets in a T-junction geometry using the level set method and magnetic force manipulation in the three-dimensional. The analysis reveals key insights into droplet formation processes in four stages: entering, blocking, necking, and detachment. The results show that increasing the Capillary number leads to a significant decrease in volume for non-ferrofluid droplets. Application of a magnetic force enhances the balance of forces during droplet formation, directly impacting droplet volume. Moreover, increasing the magnetic Bond number substantially increases droplet volume, with a more pronounced effect at lower Capillary numbers. Modifying magnetic properties influences droplet volume, with doubling the magnetization results in a significant volume increase. Overall, magnetic forces emerge as a crucial control parameter for droplet volume in ferrofluid systems, offering potential applications in droplet-based technologies and microfluidic devices.

Role of elasticity on polymeric droplet generation and morphology in microfluidic cross-junctions

Recently, our direct numerical simulations [Duan et al., Phys. Fluids 36, 033112 (2024)] showed that fluid elasticity affects the extension length and pinch-off time of the droplet formation process, thus changing the flow pattern. However, the effect of fluid elasticity on the morphology and properties of polymeric droplets is not yet fully understood. In this work, by analyzing the stretched state of the polymer macromolecule and the velocity distribution of the flow process, we find that the increase in fluid elasticity (characterized by the relaxation time) inhibits the contraction of the dispersed phase during droplet pinching and resists the effect of surface tension after droplet generation, which significantly affects the droplet geometry, volume, and generation frequency. The results demonstrate that the length and volume of polymeric droplets increase with the relaxation time of the polymer fluid, while the generation frequency decreases. Meanwhile, the effects of polymer viscosity and the superficial velocity ratio of the continuous to the dispersed phase on the droplets' morphology are investigated. The semi-empirical models for the length, volume, and generation frequency of polymeric droplets are developed for the first time by considering the elastic interaction. The purpose of our work is to provide a better understanding and experimental guidance for controlling the parameters of polymeric droplets with viscoelasticity of different shapes and sizes.

Study on jetting droplet formation regime in a low-speed annular shear flow field

This study aims to explore the mechanism of droplet generation and the change rule of liquid bridge in an annular shear flow field. With the help of a two-phase system of continuous phase (deionized water)-dispersed phase (fluorescent oil), the process of droplet generation under the jetting mechanism was captured using a camera. By studying the effect of the dispersed phase flow rate on the length of the liquid bridge, it was found that the volume flow rate of the dispersed phase is positively correlated with the frequency of droplet formation and the droplet size, and that the distance of the liquid bridge is also positively correlated with the droplet size under the jetting mechanism. Besides, a prediction mechanism for the maximum stable droplet under the jetting mechanism is proposed based on the droplet formation process.

Formation of viscoelastic droplets in symmetrical parallelized microchannels

The dynamics of viscoelastic droplets and the distribution of viscoelastic fluids in symmetrical parallelized microchannels are studied by using a high-speed camera. Typical parallel and slug flows are observed. The droplet formation process is divided into four stages, and the pinch-off point of the droplet moves downstream under high dispersed phase flow rates. When the flow rate ratio of the continuous phase to the dispersed phase is very low, the expanding stage disappears and the remaining three stages gradually lose clear boundaries, and the slug flow transforms into parallel flow. The influence of fluid elasticity is manifested in the appearance of unbreakable filaments when the droplet is pinched off, and the duration and maximum length of filaments are positively correlated with the fluid elasticity. Increasing the velocity of the dispersed phase appropriately, the homogeneity of droplet generation can be effectively enhanced. Finally, a prediction model of droplet size is established.

Coupled Eulerian Wall Film–Discrete Phase model for predicting respiratory droplet generation during a coughing event

Infectious respiratory diseases have long been a serious public health issue, with airborne transmission via close person-to-person contact being the main infection route. Coughing episodes are an eruptive source of virus-laden droplets that increase the infection risk of susceptible individuals. In this study, the droplet generation process during a coughing event was reproduced using the Eulerian wall film (EWF) model, and the absorption/expulsion of droplets was tracked using the discrete phase model (DPM). A realistic numerical model that included the oral cavity with teeth features and the respiratory system from the throat to the first bifurcation was developed. A coughing flow profile simulated the flow patterns of a single coughing episode. The EWF and DPM models were coupled to predict the droplet formation, generation, absorption, and exhalation processes. The results showed that a large droplet number concentration was generated at the beginning of the coughing event, with the peak concentration coinciding with the peak cough rate. Analysis of the droplet site of origin showed that large amounts of droplets were generated in the oral cavity and teeth surface, followed by the caudal region of the respiratory system. The size of the expelled droplets was 0.25–24 μm, with the peak concentration at 4–8 μm. This study significantly contributes to the realm on the site of origin and localized number concentration of droplets after a coughing episode. It can facilitate studies on infection risk assessment, droplet dispersion, and droplet generation mechanisms from other sneezing or phonation activities.

Numerical investigation of highly viscous droplet generation based on level set method

Piezo-driven needle valves are widely used in electronic packaging due to their fast response, high resolution and good dispensing consistency. However, the stable generation of high-viscosity droplets is one of the key issues to its packaging quality. To investigate the formation mechanism of the high-viscosity droplet. In this paper, a 2D finite element model of the drop-on-demand injection process of the high-viscosity droplet is established based on the level set method, the droplet formation and separation processes are numerically simulated, and the reliability of the simulation results is checked by comparing the outcomes with published data. Specifically, the detailed evolution of the high-viscosity droplet formation and separation process is gained by coupling the two-phase flow-level set method and the dynamic grid technique, and the pressure distribution in the injection chamber is further discussed and the effects of operating parameters on the droplet formation volume are examined. The results of the study show that the needle motion is the main factor of pressure fluctuations in the injection chamber. Moreover, we also found that among the parameters of needle stroke, nozzle diameter, supply pressure, fluid viscosity, and surface tension, the nozzle diameter has the most significant effect on droplet volume, while surface tension has the least effect on droplet formation.

Characterization of Sprays Generated by the Expansion of Emulsions with Liquid Carbon Dioxide

AbstractExpanding emulsions with liquid CO2 facilitates the creation of aerosols with an average droplet diameter in the low micrometer size range, which is challenging with conventional atomizers. The droplet formation process of the expansion of high‐pressure emulsions was investigated using a plain‐orifice atomizer and different swirl nozzles. The local droplet size and droplet velocities were measured and used to estimate the local Weber number and thus infer the droplet size reduction. Measurements of the local mass concentration in the aerosol showed that, for the swirl nozzle, the highest concentration was found outside of the central axis, indicating radial momentum generated by the swirl nozzle. Furthermore, it was shown that the type of expansion nozzle used has an influence on the resulting median droplet size in the aerosol. For a water mass load of 0.01, the median droplet diameter was reduced from 8 to 3 μm by increasing the swirl number from 0.01 to 0.1.

Machine learning enhanced droplet microfluidics

Machine learning has recently been introduced in the context of droplet microfluidics to simplify the process of droplet formation, which is usually controlled by a variety of parameters. However, the studies introduced so far have mainly focused on droplet size control using water and mineral oil in microfluidic devices fabricated using soft lithography or rapid prototyping. This approach negated the applicability of machine learning results to other types of fluids more relevant to biomedical applications, while also preventing users that do not have access to microfluidic fabrication facilities to take advantage of previous findings. There are a number of different algorithms that could be used as part of a data driven approach, and no clear comparison has been previously offered among multiple machine learning architectures with respect to the predictions of flow rate values and generation rate. We here employed machine learning to predict the experimental parameters required for droplet generation in three commercialized microfluidic flow-focusing devices using phosphate buffer saline and biocompatible fluorinated oil as dispersed and continuous liquid phases, respectively. We compared three different machine learning architectures and established the one leading to more accurate predictions. We also compared the predictions with a new set of experiments performed at a different day to account for experimental variability. Finally, we provided a proof of concept related to algae encapsulation and designed a simple app that can be used to generate accurate predictions for a given droplet size and generation rate across the three commercial devices.

Numerical simulation of the droplet formation involving fluids with high viscosity ratio by lattice Boltzmann method

A multiple-relaxation-time color gradient lattice Boltzmann model is established for simulating the flow mechanism of viscous fluids or fluids with high viscosity ratios in the microchannel. The regularized method is incorporated in this MRT framework to deal with the high viscosity ratio problems involving practical inlet–outlet boundaries. By taking several static and dynamic cases, we prove that this model could accurately describe interfacial tension, wettability, and flow problems in two-phase flows with a low spurious velocity at the range of viscosity ratio up to O(103). Using this model, we successfully simulate the droplet formation process of fluids with a high viscosity ratio in the common T-junction channel. The results are in good agreement with the experiments in the literature. We further investigate the effect of high viscosity ratios on the dispersion process, revealing that the substantial increase in terms of the viscosity ratio of fluids leads to the enhancement of continuous phase viscous shear and dispersed phase inertia effect, which would bring the deviation of the operating range from mostly reported flow systems.

Understanding droplet jetting on varying substrate for biological applications.

In the inkjet printing process, the droplet experience two phases, namely the jetting and the impacting phases. In this review article, we aim to understand the physics of a jetted ink, which begins during the droplet formation process. Following which, we highlight the different impacts during which the droplet lands on varying substrates such as solid, liquid, and less commonly known viscoelastic material. Next, the article states important process-specific considerations in determining the success of inkjet bioprinted constructs. Techniques to reduce cell deformation throughout the inkjet printing process are highlighted. Modifying postimpact events, such as spreading, evaporation, and absorption, improves cell viability of printed droplet. Last, applications that leverage on the advantage of pixelation in inkjet printing technology have been shown for drug screening and cell-material interaction studies. It is noteworthy that inkjet bioprinting technology has been integrated with other processing technologies to improve the structural integrity and biofunctionality of bioprinted construct.

Research on the Effect of Geometric Parameters of a Six-Way Junction Microchannel on the Formation of a Double Emulsion Droplet

As a typical microdroplet, double emulsion droplet, has received much attention and been widely used in recent years. For a simplified double-cross-shaped microchannel, the process of preparing double emulsion droplets is numerically simulated in this paper. The mechanism of droplet forming was analyzed, and the effects of the angles of the inner-, middle-, and outer-phase channels of the microchip, length of the focusing hole, and expansion angle on the process and quality of the double emulsion droplet formation were investigated. The variation in angles between each inlet channel affects the droplet area; the change in expansion angle affects the flow pattern of droplets; the change in each geometric parameter affects the monodispersity of droplets. The droplet area is fitted to the microchannel geometric parameters and the functional expressions that represent their relationship are derived. The work in this paper provides a reference for the practical production and research of double emulsion droplets.

Waveform Design Method for Piezoelectric Print-Head Based on Iterative Learning and Equivalent Circuit Model

Piezoelectric print-heads (PPHs) are used with a variety of fluid materials with specific functions. Thus, the volume flow rate of the fluid at the nozzle determines the formation process of droplets, which is used to design the drive waveform of the PPH, control the volume flow rate at the nozzle, and effectively improve droplet deposition quality. In this study, based on the iterative learning and the equivalent circuit model of the PPHs, we proposed a waveform design method to control the volume flow rate at the nozzle. Experimental results show that the proposed method can accurately control the volume flow of the fluid at the nozzle. To verify the practical application value of the proposed method, we designed two drive waveforms to suppress residual vibration and produce smaller droplets. The results are exceptional, indicating that the proposed method has good practical application value.