Droplet Volume Research Articles

Droplet temperature measurement using a fiber Bragg grating

The need to measure droplets temperature with high precision in moderate or extreme environments has driven the development of advanced methods. Here, we report, for the first time, the measurement of droplet temperature using optical fiber-based temperature sensors. Specifically, a fiber Bragg grating sensor was used as a non-invasive technique to determine the temperature behavior of single micro-volumetric pendant droplets. The fiber sensor method, which provides indirect measurements, was validated by experimental and simulation data from the literature, showing good agreement. The temperature measurements rely on monitoring the vapor layer temperature near the droplet at sub-millimeter fiber-to-droplet distances and on determining a coefficient that characterizes the temperature difference between the droplet and the surrounding vapor layer at the liquid–gas interface. We calibrated the method by conducting measurements under various relative humidities and fiber-droplet contact conditions. We found that the coefficient remains constant for a specific distance regardless of the relative humidity or the droplet volume. Furthermore, we observed the minor influences that the environmental relative humidity and droplet content has on the behavior of the recorded temperatures during the short period after droplet generation.

Study on the hydrate growth characteristics and rheology of silica-containing sand water-in-oil emulsion system

Understanding the transportability of silica-sand-containing hydrate slurries is crucial for production safety of non-consolidated natural gas hydrate reservoirs, especially when solid-state fluidization technology is utilized. Despite some research on silica sand and marine sediments’ influence on hydrate formation kinetics, rheological studies are scarce for systems with silica sands. The formation process of cyclopentane hydrates under the coexistence of hydrophilic silica particles and surfactants were observed. The effects of the concentration and particle size of silica sands as well as the water cut and surfactant concentration of emulsion on the rheological properties of hydrate slurries were investigated using a rheometer. The results indicated: (1) Hydrophilic sand aggregates were entrapped in water droplets, indirectly affecting the oil–water contact area due to the increase in water droplet volume, which was conducive to the combination of alkanes and water molecules to form a hydrate shell; silica particles provided nucleation sites at the oil–water interface to promote the adsorption and aggregation of cyclopentane molecules. (2) The synergistic effect of silica sand and surfactants significantly shortened the induction time of hydrates and increased the viscosity of the slurry. As the concentration of silica sand (1 wt%, 5 wt%, 10 wt%) increased, the induction time of hydrate formation decreased by 44.8 min. Within the concentration range below 5 wt%, the viscosity of the slurry increased as the concentration increased, with an amplitude of 13830.7 mPa·s. The induction time of hydrate formation decreased with increasing silica sand particle size (2 μm, 6.5 μm, 12 μm), but the size of silica sand particles had no significant effect on the viscosity of the slurry. (3) Hydrate slurries in all systems exhibited shear-thinning behavior. There was no obvious relationship between the yield stress of slurry under different silica sand concentrations. However, the yield stress of the slurry was inversely proportional to the size of silica sand particles. The yield stress decreased by 141.13 Pa when the size of silica sand particles increased from 2 μm to 12 μm. Overall, this study highlighted the influence of silica sand presence on flow safety assurance in solid-state fluidization technology.

Manipulating the duration of picoinjection controls the injected volume of individual droplets.

The ability to add reagents into droplets is required in many microfluidic workflows. Picoinjection can address this need; however, it is unable to control the injection volume for each individual droplet. Here, we present an improved picoinjection method that can inject controlled volumes into individual droplets. We achieve this by adjusting the injection duration for each picoinjection event. This improved picoinjection method can be used to create complex microfluidic workflows that are able to control the biochemical composition of individual droplets.

Enhanced insights into paired droplet evaporation dynamics on heated substrates: Unveiling the role of convection and diffusion

This study aims to examine the evaporation characteristics of single and multiple droplets on a heated substrate. By utilizing a multi-syringe pump, deionized water droplets were precisely deposited on a copper substrate, ensuring uniformity and accuracy in the experimental setup. The shadowgraph technique was instrumental in determining the droplet contact angle and volume with exceptional clarity and precision. This work numerically predicted the vapor distribution and local evaporation flux across the liquid-air interface. A critical assessment of the role of natural convection at varying substrate temperatures was performed by contrasting diffusion-only cases with those incorporating both diffusion and convection. The findings reveal that the droplet pinning motion remains unchanged across different distances between droplets and various substrate temperatures, indicating that neither vapor accumulation nor substrate temperature significantly influences the behavior of the contact line. Notably, the study identifies a reduction in the evaporation rate of closely positioned paired droplets, related to a shielding effect. However, with increasing substrate temperature, the role of natural convection was found to become more pronounced, effectively reducing the overall evaporation time for both single and paired droplets, thus facilitating a quicker evaporation process.

Highly Parallel Droplet Dispensing Approach to Provide Homogeneous and Controllable Droplet Arrays for Diagnostic Test Manufacturing.

We introduce a novel approach for highly parallel droplet dispensing with precise control over the droplet parameters such as droplet volume, droplet velocity, etc. This approach facilitates the fabrication of homogeneous and precise thin layers with uniform coverage on defined small areas (e.g., a specific area of 1 × 1.4 mm2 in microfluidic channels or microwells). The presented approach ensures layer uniformity and high precision in X/Y extent and edge resolution, making it well suited for achieving precise and controlled coating for a variety of applications such as homogeneous coatings for lateral flow tests, ELISA plates, and biosensors for continuous glucose monitoring (CGM) devices. Our approach is based on direct liquid displacement employing a piston that is in direct contact with the liquid and an array of nozzles. Considering a variety of nozzle chip designs (i.e., varying nozzle diameter and pitch), we evaluated a multitude of parameters to derive general design rules for the nozzle chip design. Thus, we achieved a tunable droplet volume from 200 to 800 pL and droplet velocities from 0.5 to 2.5 m/s, applying a nozzle diameter of 50 μm and a nozzle pitch of 165 μm. The presented results showcase the versatility of the approach, offering precise dispensing capabilities.

Internal flow in sessile droplets induced by substrate oscillation: towards enhanced mixing and mass transfer in microfluidic systems

The introduction of flows within sessile droplets is highly effective for many lab-on-a-chip chemical and biomedical applications. However, generating such flows is difficult due to the typically small droplet volumes. Here, we present a simple, non-contact strategy to generate internal flows in sessile droplets for enhancing mixing and mass transport. The flows are driven by actuating a rigid substrate into oscillation with certain amplitude distributions without relying on the resonance of the droplet itself. Substrate oscillation characteristics and corresponding flow patterns are documented herein. Mixing indices and mass transfer coefficients of sessile droplets on the substrate surface are measured using optical and electrochemical methods. They demonstrate complete mixing within the droplets in 1.35 s and increases in mass transfer rates of more than seven times static values. Proof of concept was conducted with experiments of silver nanoparticle synthesis and with heavy metal ion sensing employing the sessile droplet as a microreactor for synthesis and an electrochemical cell for sensing. The degrees of enhancement of synthesis efficiency and detection sensitivity attributed to the internal flows are experimentally documented.

A droplet-intermixing printing method for high-uniformity pixel ink volume control based on integer programming

Inkjet printing is a promising technology for new display mass production. However, a major challenge in inkjet printing applications is pixel ink volume nonuniformity caused by the volume error of droplets ejected from multiple nozzles. This nonuniformity can lead to nonuniform pixel film thickness and Mura defects in a display panel. To address this issue, an integer programming-based droplet-intermixing printing method for high-uniformity pixel ink volume control is proposed in this study. The printing method is used to homogenize the volume of ink deposited in each pixel by intermixing droplets from different nozzles to improve the whole-panel uniformity. To ensure that the droplet intermixing results meet the printing process requirements, a specific scheduling and printing planning model for the method is established. According to the droplet volume from each nozzle and the printing pattern, the most efficient printing path and nozzle ejection action are obtained by the established model while meeting the volume uniformity requirement of pixel ink. Experiments show that the proposed printing method can realize pixel film thickness error control under ±6 nm on a 400 cm2 display substrate, and the uniformity of different pixels is less than ±4.62 %. Compared with traditional method, the achieved uniformity is improved at 45.4 % under the same conditions. This method has been applied in printed display manufacturing equipment and achieved good results.

Numerical study on the effect of actuation parameters on the formation characteristics of metal droplets in magnetohydrodynamic drop-on-demand (DOD) jetting

Numerical simulations have investigated the effects of electromagnetic drive waveform amplitude and pulse width on the state of metal droplet generation during drop-on-demand printing of molten metal aluminum by electromagnetic drive. It indicates that positive and negative pulses of magnetohydrodynamic(MHD) actuation applied to the molten metal in the crucible can form a so-called “push-pull” effect, which is the main mechanism affecting droplet generation and breakup. The positive pressure pulse causes the liquid metal to be extruded outward, and the negative pressure pulse promotes the fluid retraction at the nozzle to promoting the thinning of the liquid bridge and the generation of liquid droplets. With the increase in the amplitude of the applied current, the ejected molten aluminum from the crucible experiences three states: no droplet, single droplet, and droplet with satellite droplets. In the single drop state, the droplet formation time is shortened as the current peak increases and the droplet volume increases with the increase of pulse width. The energy conversion and force changes during droplet formation and falling are analyzed. When the peak kinetic energy of the ejected molten metal exceeds its surface energy, the jet breaks up and generates a single droplet. For the actuation with constant amplitude and duration for positive pulses, extending the duration of negative pulses weakens the "pull" effect, leading to a transition from the generation of a single droplet to multiple droplets. The numerical simulation results provide information on the current amplitude, pulse width, and Weber number range necessary for stable single droplet generation.

Droplet freeze desalination high fidelity modeling for sustainable water purification and diverse applications

Freeze desalination (FD) is an environmentally clean water purification technique that holds a great promise for low-energy consumption and with unlimited brine's salinity levels. The brine cooling induces freezing, rendering separation and removing contaminants. A high-fidelity molding using CFD is employed to explain the purification of ice from saline droplets in a sub-cold environment under natural free-convection. Comparisons were made with analytical predictions derived from experimentally acquired data through brine cooling in bulk and droplet scales. The procedural sequence employed facilitated the model's ability to accurately represent the ion diffusion and phase-change phenomena across various temperatures (−15 °C and − 25 °C), concentrations, partition & heat-transfer coefficients, and droplet volumes (3 μL to 9 μL). The salt concentration ranged from seawater (35 g/L) to RO (70 g/L) salinity levels. The model successfully accommodates the experimental temperatures observed during the equilibrium between the solid and liquid phases. The achieved desalination efficiency ranges from 20 % to above 40 %. Lastly, machine learning is implemented to identify the principal experimental parameters influencing desalination efficiency. In the suggested system, dynamic salinity/impurity and temperature profiles enable natural free convection brine cooling research. The current framework can study the purification/concentration efficiency of diverse range of mixtures subjected to freeze crystallization.

Dynamic behavior of floating ferrofluid droplet through an orifice with a magnetic field

In this study, we utilize the simplified lattice Boltzmann method (SLBM) to investigate numerically the motion of buoyancy-driven deformable ferrofluid droplets through the orifice of varying widths and depths in two-dimensional (2D) space. Positioned directly beneath a plate with a central hole, the magnetic fluid droplets undergo acceleration to meet the plate under the influence of buoyancy and magnetic forces. We investigate the impact of magnetic field strength (Bom), pore ratio (PR), plate thickness ratio (WR), droplet viscosity (Re), and the plate's wettability (contact angle) on the dynamic behavior of ferrofluid droplets ascending through the orifice. Our results reveal significant effects on the efficiency and morphology of ferrofluid droplets passing through the hole. The introduction of a magnetic field facilitates a larger volume of liquid droplets passing through the hole at PR = 0.25. Moreover, increasing magnetic field intensity leads to the generation of secondary droplets during passage through the orifice. In practical applications, to prevent the generation of secondary droplets, we recommend Bom < 3 when the pore ratio falls within 0.35 < PR < 0.45 and plate thickness ratio WR = 1. Additionally, with increasing obstacle thickness, ferrofluid droplets on the hydrophobic wall can pass through the orifice more easily. Furthermore, when the magnetic field strength exceeds a certain threshold (Bom = 6.08), the droplets can pass through the orifice regardless of the wall's hydrophilicity or hydrophobicity. For practical applications with the pore ratio PR = 0.25 and plate thickness ratio WR > 1, we suggest Bom > 3.

Response model between nanobubble preparation parameters and properties: Controllable preparation & its application example

Nanobubbles have attracted much attention in recent years due to their excellent properties. However, the uncontrollable preparation of nanobubble properties limits its application. In this study, the relationship between the preparation parameters (including water pressure, gas flow, and shearing time) of nanobubbles and their properties was systematically investigated. The preparation parameters with different mechanisms have different effects on nanobubble properties, including density (1.93 × 107–1.52 × 108/mL), zeta potential (−33.6 to −13.5 mV), dissolved oxygen (6.8–10.1 mg/L), oxidation-reduction potential (194–262 mV), viscosity (0.88–0.95 mPa·s), and surface tension (49–57 mN/m). Then, the controllable preparation of nanobubble properties was achieved through constructing a mathematical model between preparation parameters and nanobubble properties. According to the multi-objective optimization prediction of the model (maximizing bubble density and minimizing surface tension), the optimal preparation parameters were determined as follows: shearing time (9.5 min), air flow (0.1 L/min), and water pressure (0.1 MPa). And nanobubbles with a bubble density of 1.95 × 108/mL and a surface tension of 55 mN/m were prepared. The nanobubbles were tried to be applied to boost the leaching efficiency of organic pollutants, which are difficult to migrate in groundwater and soil environments. At 300 min, the total elution volume of oil droplets in sample with nanobubbles was 91 % higher than that of the control. The response model constructed in this study can support the targeted preparation of nanobubbles to meet the need of specific application scenarios and achieve quality and efficiency improvement in different application scenarios.

Butyrate attenuates cold-induced hypertension via gut microbiota and activation of brown adipose tissue

ObjectiveChronic exposure to cold temperature is known to elevate blood pressure, leading to a condition known as cold-induced hypertension (CIH). Our previous research suggested correlations between alterations in gut microbiota, decrease in butyrate level, and the onset and progression of CIH. However, the role of butyrate in CIH and the underlying mechanisms need further investigation. MethodsWe exposed Specific Pathogen Free (SPF) rats to continuous cold temperature (4 ± 1 °C) for 6 weeks to establish a CIH rat model. Rats were divided into different groups by dose and duration, and the rats under cold were administered with butyrate (0.5 or 1 g/kg/day) daily. We assessed hypertension-associated phenotypes, pathological morphological changes, and endocrine-related phenotypes of brown adipose tissue (BAT). The effects of butyrate on gut microbiota and intestinal content metabolism were evaluated by 16s RNA sequencing and non-targeted metabolomics, respectively. ResultsThe systolic blood pressure (SBP) of rats exposed to cold after supplemented with butyrate were significantly lower than that of the Cold group. Butyrate may increase the species, abundance, and diversity of gut microbiota in rats. Specifically, butyrate intervention enriched beneficial bacterial genera, such as Lactobacillaceae, and decreased the levels of harmful bacteria genera, such as Actinobacteriota and Erysipeiotrichaceae. Cold exposure significantly increased BAT cells and the number of mitochondria. After butyrate supplementation, the levels of peroxisome proliferator-activated receptor gamma coactivator 1a and fibroblast growth factor 21 in BAT were significantly elevated (P < 0.05), and the volume and number of lipid droplets increased. The levels of ANG II and high-density lipoprotein were elevated in the Cold group but decreased after butyrate supplementation. ConclusionButyrate may attenuate blood pressure in CIH by promoting the growth of beneficial bacteria and the secretion of beneficial derived factors produced by BAT, thus alleviating the elevation of blood pressure induced by cold. This study demonstrates the anti-hypertensive effects of butyrate and its potential therapeutic mechanisms, offering novel insights to the prevention and treatment of CIH in populations living or working in cold environments.

Size prediction of drug-loaded Polymeric (PLGA) microparticles prepared by microfluidics

Drug-loaded poly (lactic-co-glycolic acid) (PLGA) microparticles with ex-act control over size and homogeneity were created using a glass microfluidic cross-junction droplet generator. A numerical simulation based on the volume-of-fluid (VOF) method is used to study the behavior of two immiscible fluids that are injected into the microdevice: hydrophobic drug and (PLGA) in dichloromethane (DCM) serving as the dispersed phase and aqueous solution of poly (vinyl alcohol) (PVA) as the continuous phase. The simulation predicts the created emulsion droplet size regulation by adjusting the phase's flow rate. In a procedure known as droplet shrinking, dichloromethane extraction, and evaporation took place to further transform the monodisperse emulsion into PLGA microparticles. A numerical relationship is also given for the variations in the volume of droplets during shrinkage. The numerical results were consistent with the experiments. To further illustrate this technique's ability to manufacture designed microspheres with the least amount of repeated experiments, as well as the importance of the droplet microfluidic approach in terms of regulating diameter and monodispersity, opioid antagonist naltrexone is loaded in PLGA microspheres. According to the findings, integrating a versatile microfluidic approach with numerical methodologies enables the development of more reliable and reproducible drug delivery systems.

P‐102: Measurement of Inkjet Droplet Volume based on Fraunhofer Diffraction

This paper presents the feasibility of Fraunhofer diffraction analysis as a measure of the size of fast‐moving inkjet droplets. This measurement method showed that an inkjet droplet measurement tool was possible with a good balance of real‐time measurement and accuracy.

Experimental study of liquid–liquid dispersion patterns in T-inlet microchannels with different junction angles

Liquid-liquid two-phase flow in T-inlet microchannels with different junction angles (θ = 30°, 60°, 90°, 120° and 150°) was investigated experimentally. Four flow regimes of the dispersed phase were identified, i.e., parallel flow, jetting, dripping and squeezing, and the distribution of flow regimes for the dispersed phase corresponding to variations in the junction angle was plotted. The consequences of varying junction angle and flow conditions in the squeezing regime on the generated droplet size were analyzed. The results indicate that low capillary number and large flow rate ratio are conducive to the formation of large-size droplets. For constant flow conditions, junction angle θ = 90° is detrimental to the formation of squeezing microdroplets. The increase in microchannel junction angle causes the droplet size to decrease until θ > 90°, where the droplet size increases with the junction angle. On the basis of experimental results, the scaling law correlation equations containing the junction angle for predicting the droplet length and droplet volume are proposed, respectively. The predicted values match well with the experimental data. The results of this work contribute to the enhancement of the monodispersity of microdroplets and the precise control over a wide range of the generated droplet size by adjusting the junction angle.

Quantitative liquid storage by billiards‐like droplet collision on surfaces with patterned wettability

AbstractThere has been significant interest in researching droplet transport behavior on composite wetting surfaces. However, current research is primarily focused on modifying individual droplets and lacks an in‐depth investigation into high‐precision droplet storage. This study introduces a “billiard ball” droplet transport and storage platform (TSP) with differentiated areas. Within this platform, the volume of droplets stored in the area reaches a consistent threshold through droplet “scrambling,” inspired by the water‐gathering behavior of spiders. The TSP involves connecting two regions of different sizes using a three‐dimensional stepped wedge angle structure. However, this connection is not seamless, leaving a 2‐mm gap between the regions. This gap is intentionally designed to enable continuous droplet transfer while preventing any static migration. Through systematic experimental and simulation analysis, we investigated the influence of superhydrophilic pattern structures and parameters on quantitative droplet storage. We established a functional relationship between the pattern area and the stored volume, and analyzed the intrinsic mechanism of droplet collision separation. This enabled us to achieve on‐demand quantitative droplet storage and autonomize the storage process. The “billiard ball” droplet transport–storage platform proposed in this study holds promising applications in the fields of biomedical and organic chemistry.

Analysis of the impact dynamics of water-in-oil composite droplets on metal plates

Composite droplets composed of immiscible liquids often have unique applications in many situations, where splashing and recoil of the composite droplet upon impact with plane surfaces are important. Due to the different properties of the constituent liquids, composite droplets will have different flow morphology than single-phase droplets. This research employs an injection method to generate composite droplets and investigates the impact dynamics of these droplets on a horizontal copper plate at room temperature using high-speed cameras. Four impact heights (H = 10 cm, 15 cm, 20 cm, 25 cm) and seven water volume fractions (α = 0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.375, 0.4375) were considered. The results show that the double squeezing of the oil phase by the water phase and the copper plate is the main reason for the droplet splashing phenomenon. The degree of droplet splashing and the probability of the droplet generating a satellite droplet are directly proportional to the impact velocity. When the impact height is constant, the extent of splashing decreases with an increase in α. Additionally, the maximum value of the jetting height for composite droplets is mainly influenced by the volume of satellite droplets during the recoil process. Overall, the maximum relative jetting height of composite droplets shows an initial increasing trend followed by a decreasing trend with an increase in α. At α = 0.1875 and 0.25, there is a negative correlation between the relative jetting height and impact height. This implies that the maximum value of the relative jetting height is primarily controlled by the volume of the satellite droplets during the recoil process.

Longitudinal transducer based membrane resonant ejector for high-viscosity and high-frequency micro-droplet jetting

Aiming at the requirement for high-precision on-demand droplet jetting of high-viscosity industrial inks, a longitudinal-transducer-based membrane resonant droplet ejector (LT-MRDE) is designed and fabricated in this work. The jetting and observing system of LT-MRDE is set up to quantitatively explore the variations of droplet volume and velocity with driving voltage. Moreover, the three-dimensional (3D) multi-relaxation-time (MRT) lattice Boltzmann (LB) model coupled immersed boundary (IB) is employed to elucidate the effects of driving voltage and liquid viscosity on droplet characteristics. Experimental results demonstrate that the LT-MRDE could stably eject the silicone oil with a viscosity of 100 mPa·s from ∼50 μm vibrating orifice. Furthermore, both the volume and velocity of droplet exhibit a proportionality to voltage and an inverse proportionality to viscosity. The optimal droplet volume and velocity of 100 mPa·s silicon oil are 67.27 pL and 9.07 m/s with the driving parameters of 75 Vpp/14.754 kHz. With high jetting frequency and controllable droplet volume, LT-MRDE provides a new alternative for on-demand precision jetting of high-viscosity industrial inks.

(Digital Presentation) The Effect of Coating Quality on Water Transport in the Gas Diffusion Layer of PEM Fuel Cells

Present study examines the impact of the quality of the GDL non-wetting coating on water transport. It is found that there are two distinct regions defined by the coating quality which present fundamentally distinct behaviours, one region characterized by few coating defects with the second marked by significant coating degradation. Simulations consider the influence of heterogeneous wetting and pinning conditions on protruding water droplets in contact with the GDL fibrous matrix. The mixed-non-wetting behaviour for a uniform non-wetting coating having less than 25% defects is characterized by a high breakthrough pressure, yielding improved transport properties in the stable displacement regime. The coating degrades under normal operating conditions and it changes the water transport characteristics unfavorably through a significant decrease in breakthrough pressure, and an up to six-fold increase in droplet volume. Various degrees of coating degradation are considered and the corresponding Laplace pressure is determined as a function of droplet volumes at breakthrough.

Inkjet Printing Photoresist with Ultralow Viscosity on Silicon Wafers for Uniform Coating.

Inkjet printing is introduced into the photoresist coating process for uniform photoresist film formation on silicon wafers with the in-house inkjet experimental prototype. The optimization of a dual negative voltage waveform is proposed to achieve stable droplet jetting for the ultralow viscosity (0.71 mPa·s) photoresist with a 1:10 dilution ratio employed in the semiconductor packaging processes. Moreover, the maximum droplet jetting velocity can reach 9.51 m/s, and the droplet volume is controlled at ∼6.5 pL with excellent droplet concentration. The uniform film of the AZ P4620 photoresist is coated on silicon wafers by quantitatively exploring and optimizing the printhead driving frequency and movement velocity utilizing the droplet deposition model and experimental analysis. Results show that the optimal inkjet parameters with 5 kHz in jetting frequency and 6 mm/s in motion velocity can obtain a film evenness index of 4.81% with the thickness of 0.945 μm, which exhibits a more uniform photoresist film than the spray coating method. The study not only expands the application of the inkjet printing technique but also offers an alternative for photoresist coating in the photolithography process.