Drop Volume Research Articles

Influence of transient heat flux on boiling flow pattern in a straight microchannel applied in concentrator photovoltaic systems

Concentrated photovoltaic (CPV) overheating due to the high radiation intensity became the critical challenge to preventing its benefits in advancing the photoelectric efficiency. To promote the application of CPV, efficient heat dissipation becomes the key technology. Meanwhile, microchannel flow boiling, as one of the most promising solution to advance cooling of high-flux equipment, has attracted increasing attentions in recent years. However, the fluctuating and non-uniform heat fluxes of CPVs may lead to challenges to microchannel boiling for cooling CPV. Therefore, the transient flow and phase change phenomenon in a microchannel with deionized (DI) water was studied through a numerical modeling on flow boiling. The effects of different sudden heat flux increases on flow boiling two-phase flow pattern were analyzed. It was found that different heat flux increase resulted to different flow pattern evolutions, a much higher sudden increase in heat flux leaded to serious variation of flow pattern, especially the fluctuation of elongated restricted bubbles, which may evolve to annular flow or even dry-out. The vapor phase volume fraction showed a much higher growth rate. When the heat flux increase was raised from 97.96 kW/m2 to 580 kW/m2, the maximum vapor phase volume fraction was increased from 0.104 to 0.8064, and the bubble length increased from smaller than 0.3 mm to 1.5 mm in the dry zone, indicating rapid phase change. Based on analysis on pressure drop and vapor volume fraction variation, the period after receiving sudden heat flux was divided into three stages: initial period, rapid development period, and quasi-stable period. With the rising of heat flux increase from 97.96 to 580 kW/m2, the duration of rapid development period was increased from 0.03 s to 0.066 s, the pressure drop increase was obviously enlarged, from 2.43 kPa to 14.51 kPa, and the vapor phase volume fraction increase in the microchannel was raised by 3.3 times to 25.6 times, indicating more intensive boiling, and deeper effect of higher heat flux increase on the phase change. In addition, the fluctuation intensity of pressure drop in the quasi-stable period was obviously increased from 0.025 to 0.193, demonstrating the advancement in flow boiling instability under higher heat flux increase, which should be controlled.

Waterproof Cellulose-Based Substrates for In-Drop Plasmonic Colorimetric Sensing of Volatiles: Application to Acid-Labile Sulfide Determination in Waters.

The present work reports on the assessment of widely available waterproof cellulose-based substrates for the development of sensitive in-drop plasmonic sensing approaches. The applicability of three inexpensive substrates, namely, Whatman 1PS, polyethylene-coated filter paper, and tracing paper, as holders for microvolumes of colloidal solutions was evaluated. Waterproof cellulose-based substrates demonstrated to be highly convenient platforms for analytical purposes, as they enabled in situ generation of volatiles and syringeless drop exposure unlike conventional single-drop microextraction approaches and can behave as sample compartments for smartphone-based colorimetric sensing in an integrated way. Remarkably, large drop volumes (≥20 μL) of colloidal solutions can be employed for enrichment processes when using Whatman 1PS as holder. In addition, the stability and potential applicability of spherical, rod-shaped, and core–shell metallic NPs onto waterproof cellulose-based substrates was evaluated. In particular, Au@AgNPs showed potential for the colorimetric detection of in situ generated H2S, I2, and Br2, whereas AuNRs hold promise for I2, Br2, and Hg0 colorimetric sensing. As a proof of concept, a smartphone-based colorimetric assay for determination of acid-labile sulfide in environmental water samples was developed with the proposed approach taking advantage of the ability of Au@AgNPs for H2S sensing. The assay showed a limit of detection of 0.46 μM and a repeatability of 4.4% (N = 8), yielding satisfactory recoveries (91–107%) when applied to the analysis of environmental waters.

Anomalous interfacial dynamics of pendant droplets of N,N-dimethylformamide containing Silwet

BackgroundSilwet L-77 has the unique ability to spread aqueous solutions across surfaces to a greater extent than conventional surfactants. Moreover, the Silwet-laden N,N-dimethylformamide (DMF) droplet can exhibit self-propulsion on substrates. Although the Marangoni effect is responsible, the origin of the surface tension gradient along the droplet remains unclear. MethodsThe time evolution of a hanging Silwet-laden DMF droplet below a substrate is observed. Then, the dynamic surface tensions of pendant drops are determined for both high and low Silwet concentrations in ambient and saturated DMF vapor conditions. Significant FindingsFor pendant drops with maximum volumes, the dynamic surface tensions are found to decrease slowly for more than 10 min at low Silwet concentrations (e.g., 1 wt%). However, at high Silwet concentrations (e.g., 10 wt%), the drop falls off around 1 min. As the initial drop volume is slightly less than its maximum value, surface tensions continue decaying at low Silwet concentrations but reach equilibrium values at high Silwet concentrations. Once the equilibrium surface tension is attained, the increment of the drop volume does not change the result. The peculiar interfacial dynamics is attributed to the weakly solvophobic nature of few siloxane groups of Silwet in DMF.

Buoyancy-Marangoni Fingering of a Miscible Spreading Drop

We experimentally investigate the interfacial instability that emerges when a water droplet is deposited on a bath of glycerol-water solution. Despite the absence of surface tension to stabilize short-wavelength undulations, we observe finite-size fingers that grow and pinch off from the drop. We show that the fingering patterns formed in the experiments resultes from a balance between the outward buoyancy effect and inward Marangoni flow. This induced Marangoni flow inhibits small perturbations and acts as an effective surface tension on the miscible interface of the spreading drop. To characterize the final size and shape of the drop, we perform systematic experiments by varying the drop volume and the glycerol-water volume fraction. In addition, we have developed scaling arguments for the drop’s final radius using key physical forces, and show that the final wavelength is inversely proportional to the Bond number.

Mass, momentum and energy partitioning in unsteady fragmentation

Upon drop impact on a surface of comparable size to that of the drop, a sheet is produced that evolves freely in the air, bounded by a rim from which ligaments and droplets are continuously shed. This process is a canonical example of unsteady sheet fragmentation. The sheet dynamics is coupled with continuous ligament and drop shedding (Wang & Bourouiba,J. Fluid Mech., vol. 848, 2018b, 946–967; Wang & Bourouiba,J. Fluid Mech., vol. 910, 2021b, A39) and is governed by a nonlinear non-Galilean Taylor–Culick law (Wang & Bourouiba, 2022 (in press)). Here, we report the results of a combined theoretical and experimental study of the partition and temporal evolution of mass, momentum and energy in each part of the system composed of sheet, rim, ligaments and drops. We elucidate and derive analytical predictions, without fitting parameters, of the temporal evolution of the fractions of volume/mass, momentum and energy in each sub-part of the system: from sheet, to rim to fluid shed. We show that their temporal evolution and partitioning are independent of impact conditions. Interestingly this implies, for example, that the fraction of initial drop volume shed from an impacting drop is independent of the initial energy (or Weber number) of impact. We validate our predictions against precise measurements. Finally, we show that the partition laws for this unsteady fragmentation system are robust to changes of fluid properties (viscosity, surface tension and density). We provide the ranges of validity of our partition law on a Weber–Reynolds numbers regime map.

Comparison of ablation index versus time-guided radiofrequency energy dosing using normal and half-normal saline irrigation in a porcine left ventricular model.

Ablation Index (AI) is a multiparametric quality marker to assess the durability of radiofrequency (RF) lesions. The comparative effectiveness and safety of AI versus time-based energy dosing for ablation of ventricular arrhythmias are unknown. We compared AI and time-based RF dosing strategies in the left ventricles (LVs) of freshly harvested porcine hearts. Ablation was performed in vitro with an open-irrigated ablation catheter (Thermocool ST/SF), 40 W, contact force 10-15 g. Tissue samples were stained in triphenyltetrazolium chloride for the measurement of lesion dimensions. A total of 560 lesions were performed (AI-group: [n = 360]; time-group: [n = 200]). Using normal saline (NS) (n = 280), growth in lesion depth slowed after 30 s and AI > 550 in comparison to width, volume, and magnitude of impedance drops which continued to increase with longer RF duration. Risk of steam pop (SP) was higher for RF > 30 s (RF < 30 s:1 SP [2.5%] vs. RF > 30 s: 15 SP [25%];p = .002) or AI targets >550 (AI: 350-550: 2 SP [2%] vs. AI 600-750: 15 SP [19%];p = .001). Using half-normal saline (HNS) (n = 280), lesion dimension and impedance drops were larger and growth in lesion depth slowed earlier (AI: 500). Risk of SPs was higher above AI 550 (AI: 350-550: 7 [7%] SPs vs. AI 600-750: 28 [35%] SPs; p < .00001). While codependent variables, correlation between AI and time was modest-to-strong but decreased with longer RF duration. In this ex vivo study, AI was a better predictor of lesion dimensions than ablation time and magnitude of impedance drop in the LV using NS and HNS irrigation. AI targets above 550 led to a higher risk of SPs. Future trials are required to verify these findings.

How Swelling, Cross-Linking, and Aging Affect Drop Pinning on Lubricant-Infused, Low Modulus Elastomers

Soft, slippery surfaces have gained increasing attention because of their wide range of potential applications, for example in biomaterials, self-cleaning, antifouling, liquid collection, and more. One approach to prepare a soft, slippery surface is by swelling a cross-linked polymer network with a lubricant. However, an understanding of how swelling and cross-linking relate to slippery properties is still underdeveloped for low modulus elastomers. We study when a water drop sticks or slides on a vertical, silicone oil-swollen polydimethylsiloxane (PDMS) elastomer as a function of the degree of cross-linking and the degree of swelling. Our results indicate that the critical water drop volume required for sliding is strongly controlled by the degree of swelling; higher swelling leads to lower critical drop volumes. In addition, we demonstrate that highly swollen surfaces, and not lightly swollen surfaces, recover their slippery behavior during an aging process after they are rinsed with water. This is likely associated with regeneration of an oil-layer on the surface coming from the bulk substrate, illustrating the durability of lubricant-swollen elastomers for practical uses.

Methodological aspects of dried blood spot sampling for the determination of isoprostanoids and prostanoids

Dried Blood Spot (DBS) analysis is a well-established practice for monitoring newborns in neonatal units because of the non-invasive collection of a very limited blood volume. Unfortunately, the haematocrit level and the drop volume can adversely affect the quality of the analytical data. For this reason, we performed an in-depth comparison of non-volumetric and volumetric sampling (e.g. Minivette® POCT microcapillary, and the microfluidic-based systems HemaXisTM DB and Capitainer-B) for the DBS analysis of the isoprostanoids 8-iso prostaglandin F2α and 8-iso prostaglandin E2, and prostaglandin E2, which represent well recognized indexes of oxidative stress and inflammation.Results highlighted a significant impact of drop volume on the analytical performances for the non-volumetric approach due to the inhomogeneous distribution of the analytes across the DBS. The innovative addition of labelled internal standards (ISs) on the filter paper before sampling did not circumvent the problem, as ISs did not accurately mimic the spreading of analytes in the DBS. Differently from HemaXisTM DB and Capitainer-B, the Minivette® POCT device showed an almost quantitative recovery with an overall variability of about 15%, and resulted the best option for an easy and reliable DBS collection in the clinic. Our procedure was used for the determination of the three metabolites in preterm newborns suffering from Patent Ductus Arteriosus (PDA). During our longitudinal monitoring, most of the preterm newborns with PDA showed a marked decrease (40–60%) of target analytes upon ibuprofen therapy, suggesting the potential involvement of the isoprostanoids and prostanoids in promoting drug responsiveness and ductal closure.

Experimental study on the diffusion characteristics of the miscible ScCO2/oil system based on the droplet interface shrinkage behavior

Determination of the diffusion coefficient of a miscible Supercritical CO2 (ScCO2) and oil system is substantial for guiding the CO2 EOR and greenhouse gas geological storage practices. Based on the oil droplet interface shrinkage behavoir after reaching miscibility with the surrounding CO2, the diffusion coefficients of a miscible ScCO2/paraffin system were measured with the Dynamic Pendant Drop Volume Analysis (DPDVA) technique. A refined diffusion coefficient derivation model was proposed and validated for the CO2/oil system at the Minimum Miscibility Pressure (MMP) points. The measurements were performed under pressure range of 9–21 MPa and temperature range of 40.6–81.2 ℃. Exponential relationships have been obtaind between the diffusion coefficient and the pressure and the intrinsic oil viscosity, revealing the fact that the diffusion rate increases significantly with the system pressure after reaching miscibility. On the other hand, it is found higher temperature could retard the diffusion process between the ScCO2 and the oil phases under miscible conditions.

Control of Drop Volume and Drop Jetting Velocity in Inkjet Printing

Consistent dosages with high deposition accuracy are critical for drop-on-demand(DoD) inkjet printing to be used in additive manufacturing and device characterization. Practically, drop volume and drop jetting velocity are subject to process uncertainties, such as applied pressure fluctuation and variation in printheads, for which open-loop approaches are unable to compensate. In this work, a stochastic system model of the relation between two control parameters of the firing waveform and two outputs, drop volume and drop jetting velocity, is developed from experimental data. An image-based control strategy which comprises a one-step look ahead controller combined with a Kalman estimator for model parameters estimation is proposed to control the drop volume and drop jetting velocity. The effectiveness of the proposed control strategy is validated experimentally.

Surface Wettability Of Boron And Oil-Treated Wood

Background: Nowadays, demands for more environmentally friendly and cost-effective preservatives are increasing, and new non-traditional procedures are being explored in wood protection field. Plant oils improve the dimensional stability, water repellency and equilibrium moisture content of wood, and protect wood against decay fungi by means of its hydrophobic properties. The aim of this study is to investigate the influence of heat treatment and oil impregnation with or without prior treatment with boron compounds on wetting characteristic of Scots pine ( Pinus sylvestris L.) and beech ( Fagus orientalis L.) wood. Wood specimens were impregnated with boric acid, borax and agricultural boron at concentrations of 1%, 2% and 5% followed by oil heat treated with waste and sunflower oil at 160ºC. Wettability was measured by contact angle with the sessile drop technique using water. Results: Water contact angles on oil treated specimens increased while wetting tension decreased, and the wood more poorly wetted by water compared to the controls. A change in the drop volume on the surface of double treated specimens was around 5% based on the initial drop volume. Waste oil treatment resulted in having the greatest water repellent efficiency. High loadings of boron compounds decreased the contact angle and therefore the quantity of water absorbed by the wood increased. Conclusion: Wettability was decreased in specimens pretreated with boron and this confirmed that the hydrophobic surface was created by oil. Wettability is a prerequisite for good adhesion, coating and painting and this feature may be reduced by the less hydrophilic surfaces created after oil heat treatment.

Computational investigation of drop behavior and breakup in peristaltic flow

The behavior of liquid drops in the retropulsive jet produced by a peristaltic wave is investigated computationally. The computational geometry consists of a tube which is closed at one end, with the peristaltic wave that deforms the boundary moving toward it. A modified solver with the capability to couple mesh deformation and adaptive mesh refinement around moving drops was developed and validated with experimental data, and good agreement was found. A parametric study was then performed to determine the effect of interfacial tension, viscosity ratio, relative occlusion, and initial drop position on the drop's behavior and breakup characteristics. In particular, breakup regimes on graphs of capillary number vs viscosity ratio were determined for each initial drop position and relative occlusion. It was found that these breakup regimes were bounded above and below, and an optimal capillary number for breakup was determined. The volume of the parent drop after breakup decreased linearly with capillary number for low capillary numbers and was independent of the viscosity ratio. For higher capillary numbers, this volume generally increased with the viscosity ratio. It was also found that a drop with lower interfacial tension reached the apex plane sooner than a drop with higher interfacial tension, but once there, took longer to pass through this plane and longer to breakup. The viscosity ratio had negligible influence on the drop transit times for viscosity ratios less than one, while the breakup time generally increased with the viscosity ratio.

Enhancing hydrophobic properties of ZnO structures using gold coatings

The effect of enhancement the hydrophobic properties of the ensemble of micro- and nanostructures ZnO as a result of coating with gold was discovered. For the first time, it has been shown that coating the ensemble with a micro- and nanoparticle ZnO layer of gold leads to a sharp increase in the wetting edge angle from 145 to 168o (water drop volume 5 mm3) and a decrease in the hydrophobicity/hydrophilicity transition time under ultraviolet irradiation. Keywords: ZnO, gold, superhydrophobic. Keywords: ZnO, gold, superhydrophobic

Усиление гидрофобных свойств структур ZnO золотым покрытием

The effect of enhancement the hydrophobic properties of the ensemble of micro- and nanostructures ZnO as a result of coating with gold was discovered. For the first time, it has been shown that coating the ensemble with a micro- and nanoparticle ZnO layer of gold leads to a sharp increase in the wetting edge angle from 145 to 168 ° (water drop volume 5 mm3) and a decrease in the hydrophobicity/hydrophilicity transition time under ultraviolet irradiation. Keywords: ZnO, gold, superhydrophobic.

Evaluation of the evaporation route of a liquid droplet on Au coated and non-coated glass surfaces

The contact angle was used to estimate the rate of evaporation of liquid droplets on bare glass or gold (Au) sputtered glass surfaces. The rate of evaporation of water (a pure liquid) was higher than non-pure liquid composed of a 3 wt% solution of silica nanoparticles (SNP) on these two solid supports. Despite using the same initial drop volume (1 µL) throughout the experiment, the base diameter of the liquid droplet after evaporation on the different surfaces interestingly showed variations. While the liquid–solid interface displayed slip-length and contact angle variations throughout the evaporation time, the slip-length variations were more pronounced with colloidal SNP on Au-sputtered glass surfaces than pure liquid on bare glass surface. Potential application of this study was also investigated in the surface control of uniform silica microwires from colloidal SNP on Au-sputtered glass surface under low temperature conditions.

Yielding to stress in Pickering emulsions at dilute and intermediate volume fractions

Emulsions yield and flow (or lose their shape) when sufficient stress is applied to force the drops to move past each other. The drops in emulsions stabilised by nanoparticles do not move all at once, unlike in emulsions stabilised by surfactants. This is because particles that are not attached to the oil-water interface can aggregate into networks in the continuous phase. The networks around the drops must be disrupted before the drops can move. We used a combination of steady and oscillatory shear, and creep measurements to probe Pickering emulsion yielding behaviour at different drop volume fractions. We characterised the heterogeneous structure of the emulsions on the micro- and nano-scale using confocal fluorescence and electron microscopy techniques. A key finding is that two processes occur during the transition from solid to fluid flow in Pickering emulsions. This is best revealed by oscillatory measurements. We also found evidence of structure recovery in emulsions while they were being sheared. These results have implications for designing Pickering emulsions which retain their shape, rather than slumping.

The ejection of large non-oscillating droplets from a hydrophobic wedge in microgravity

When confined within containers or conduits, drops and bubbles migrate to regions of minimum energy by the combined effects of surface tension, surface wetting, system geometry, and initial conditions. Such capillary phenomena are exploited for passive phase separation operations in micro-fluidic devices on earth and macro-fluidic devices aboard spacecraft. Our study focuses on the migration and ejection of large inertial-capillary drops confined between tilted planar hydrophobic substrates (a.k.a., wedges). In our experiments, the brief nearly weightless environment of a 2.1 s drop tower allows for the study of such capillary dominated behavior for up to 10 mL water drops with migration velocities up to 12 cm/s. We control ejection velocities as a function of drop volume, substrate tilt angle, initial confinement, and fluid properties. We then demonstrate how such geometries may be employed as passive no-moving-parts droplet generators for very large drop dynamics investigations. The method is ideal for hand-held non-oscillatory ‘droplet’ generation in low-gravity environments.

Experimental and theoretical analysis of water spreading on horizontal grooved surfaces

In this study, experiments were conducted to investigate the liquid drop spreading behavior on the grooved surface. The critical volume and the critical angle at which the water drop started to spread on the grooved surfaces were determined by the shadow method. It was found that the drop spreading on the grooved surface is a catastrophe process and the critical values vary with the groove structures. A thermodynamic method was proposed for predicting the critical drop volume spreading on the grooved surfaces with different structures. The consistency between the theoretical and experimental results is concluded to be satisfactory.

Morphological characteristics of the drop on the travelling substrate with a sharp stepped configuration

The shape of a drop on a partially wetting substrate strongly depends on not only its wetting velocity but also the surface characteristics of the substrate. The unexpected deformation or even breakup of the drop on the heterogeneous travelling substrate is a common phenomenon and needs to be carefully controlled in industrial processes. In this work, we designed an experiment to investigate the morphological characteristics of the drop on the substrate with a sharp stepped configuration. We experimentally studied the entire process of the drop motion on the substrate with a vertical-edged stepped structure and subdivided it into three stages: the collision stage, the inserting stage, and the stretching stage. For the collision stage, substrate velocity was determined as an important factor influencing the tail of the drop before and after the collision. For the inserting stage, it was found that the liquid is transferred to the pre-inserting zone with the reduced gap height due to the squeezing, resulting in an increase in the tail length; it was verified that the tail lengths vary with the position of the stepped frame by assuming that the drop volume is constant and the tail is simplified as a cone. For the stretching stage, three types of drop tails were observed by balancing the motion of the constraint tail in two directions: the one along the substrate movement and the one for anti-constraint of the stepped configuration; we experimentally analyzed the three types of drop tails and found that they are related to the substrate velocity, the gap height, and the height of stepped configuration.

Relaxation and contact angle dynamics during the coalescence of different sized vertically aligned water drops in different silicone oil viscosities

This paper reports an experimental investigation for the impact of the head drop size as well as the outer phase viscosity on the coalescence time of two water drops vertically aligned in silicone oil. Two types of silicone oil with different viscosities have been used in this work (47v350 and SilOil M40.165). A specific configuration of a sessile drop in direct contact with another drop placed above has been studied. The coalescence time is depending mainly, in this work, on the relaxation of the resulted drop together with its contact angle evolution during the merger process. For both liquid-liquid (LL) systems, the first stage of the coalescence process was dominated by the inertial forces induced by capillarity where the redefined Reynolds number Re is found to be much greater than the unity (Re>>1). It’s depicted that the smaller the head drop volume the faster the merger process. The viscosity of the outer phase has also found to have a main impact on the coalescence time and therefore on the relaxation of the resulted drop as well as its contact angle. It’s found that increasing much more the outer viscosity provokes a long coalescence time accompanied with an elimination of the harmonic movement characterizing the merger drop relaxation as well as a damping of the contact angle dynamics. Focusing on the first instants of the merger process where a liquid bridge linking the drops is formed, it’s shown that for the different head drop volumes used in the study that the variation of the dimensionless capillary pressure P* in function with the dimensionless time t* is the same for each LL system and it follows a power scaling law presented in the last section.