Suspended Droplets Research Articles

Impact dynamics of a polystyrene suspension droplet on nonwetting surfaces measured using a quartz crystal microresonator and a high-speed camera

We investigated the impact dynamics of droplets containing various sizes and concentrations of polystyrene microparticles (PSM) on superhydrophobic surfaces using a high-speed camera and a quartz crystal microresonator (QCM). Gold nanoflake structures were synthesized on QCM surfaces and treated with perfluorooctane ethylthiol molecules to obtain superhydrophobic characteristics. Upon the collision onto the superhydrophobic surface, the droplet spread, retracted, and bounced off the surface due to its nonwetting characteristics. Within the experimental range of PSM size and PSM concentration, the high-speed camera detected only negligible changes in the impact dynamics; however, abnormally large changes in the resonance frequency were observed with the QCM when the concentration or sizes of the PSM exceeded a certain threshold. The abnormal change in frequency was attributed to the formation of a shear-induced PSM structure.

Numerical probing of suspended lactose droplet drying experiment

Single droplet drying (SDD) device has been extensively used to measure drying kinetics of droplets of 0.5–2 mm initial diameter. Despite the large size, it has deemed to be the most practical for such purposes in the past 40 years. There is a surplus of numerical investigations of SDD in the literature. However, this is the first time detailed computational fluid dynamics (CFD) simulations were carried out on the actual fluid behaviour inside and around a lactose solution droplet in SDD by glass suspension experiment. The full set of the governing equations were solved and the fluid flow was investigated. A lactose concentration gradient was developed within the droplet due to momentum transfer and water removal. The CFD predictions of lactose concentrations could be used to predict the formation of crust or crystallization in the droplet. Furthermore, accumulation of solute within the droplet indicated the tendency to form solid structure. The simulated overall drying rate curve compared well with the available laboratory data. The predicted temperature results have also been benchmarked against the experimental measurements generated from the thermal history of lactose droplet drying. More importantly, CFD simulations have been extended to similar small droplet sizes occurring during spray drying, which are experimentally inaccessible with SDD. These analyses are relevant to spray drying to validate simpler 1D models.

Frequency dependence of the vaporization threshold of sonosensitive perfluorocarbon droplets varying their liquid core and size

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound have received increasing attention for therapeutic and diagnostic applications. The ultrasound activation pressure required for the phase change of these droplets into echogenic microbubbles is termed acoustic droplet vaporization (ADV). This study systematically investigates the effect of frequency of excitation¾2.25 MHz, 10 MHz and 15 MHz¾on ADV and inertial cavitation (IC) thresholds of a suspension of PFC droplets by varying the physical properties of the liquid droplets (size and boiling point) in a tubeless setup. We prepared lipid-coated droplets with three different liquid cores, perfluoropentane (PFP), perfluorohexane (PFH) and perfluorooctyle bromide (PFOB), of two different size ranges ¾ one with diameter smaller than 3 I¼m and the other with diameter larger than 10 I¼m. We found that the ADV threshold increases with frequency for the most volatile PFC liquid, PFP, for both large and small size droplets. While for the s...

The effect of acetylene black on droplet combustion and flame regime of petrodiesel and soy biodiesel

The addition of nanomaterials to base liquid fuels for the enhancement of combustion properties has long generated interest, since it is linked to improvement in combustion properties in petroleum-derived fuels. This work investigates the effect of acetylene black nanoparticles on soy-based biodiesel and petrodiesel. These nanoparticles were added in various proportions to base fuels to make suspensions. Sub-millimeter-sized spherical droplets of such suspensions were ignited using hot wire loops, and the combustion process was recorded using a high-speed camera. An infrared imaging camera was also used to record the droplet combustion process. Properties such as ignition delay, combustion rate, total combustion time, flame stand-off ratio, and droplet temperature were measured with post-processing of the resulting images. The results showed an increase in ignition delay of up to 10% in biodiesel and up to 8% in petrodiesel. An up to 13% decrease in biodiesel combustion rate and 1.5% decrease in diesel combustion rate was also observed. Moreover, a maximum of 14% decrease in the total combustion time of biodiesel was observed, compared to a 13% decrease in that of petrodiesel. The results are expected to spark further interest in the exploration of acetylene black as an additive for other liquid fuels.

Recent Developments on Colloidal Deposits Obtained by Evaporation of Sessile Droplets on a Solid Surface

Understanding flow patterns and coupled transport phenomena during evaporation of droplets loaded with colloidal particles is central to design technical applications such as organizing proteins/DNA on a solid surface. We review recent reports on evaporating sessile droplets of colloidal suspensions on a solid surface. Starting from the classical mechanism of formation of a ring-like deposit, we discuss the influence of several problem parameters. Notably, thermal or solutal Marangoni effect, particle size, particle concentration, particle shape, substrate wettability, pH of the suspension etc have been found important in controlling the deposition pattern. The deposit pattern complexity and shape have been attributed to the underlying coupled transport phenomena during the evaporation. We discuss important regimes maps reported for different types of deposit, which allow us to classify the deposits and coupled physics. We also present studies that have demonstrated particles sorting in an evaporating bi-dispersed colloidal suspensions on a solid surface. Finally, some remarks for the future research opportunities in this arena are presented.

Droplet condensation and jumping on structured superhydrophobic surfaces

A complete cycle of droplet nucleation, growth, coalescence and jumping on different textured hydrophobic and superhydrophobic surfaces is studied for the first time, using a 2-D double distribution function thermal lattice Boltzmann method. First, droplet nucleation mechanism on smooth and rough surfaces is studied in detail. The results reveal the presence of cooled vapor layer instability in the condensation on completely smooth surfaces. However, on the rough surfaces and near the roughness a completely different mechanism is observed and the nucleation occurs on the roughness wedges. Also, the condensation on different textured surfaces with nominal contact angles θa=90°,120°,155° is studied. On all these surfaces it is observed that for small textures the nucleation occurs normally near the structure wedges. However, increasing the height of the textures displaces the nucleation sites to the top of the structure, near the structure edge. After growing the droplets different behaviors are observed for different contact angles, i.e., the Wenzel or suspended droplets are formed and the droplets even jump out. Moreover, based on the non-dimensional numbers a complete study of droplet jumping is carried out and a method for proper designing the textured surfaces is presented. Finally, the benefits of using hybrid textured surfaces and the mechanism of droplet jumping on these surfaces are explained.

On growth of suspension plasma-sprayed coatings deposited by high-enthalpy plasma torch

Introduction of suspension plasma spraying (SPS) brought novel coating microstructures unachievable by other thermal spraying techniques. Both columnar and dense layers are of particular interest for various applications. Deeper understanding of the coating buildup mechanisms is essential for tailoring of the coatings microstructure and properties. This paper contributes to elucidation of the buildup mechanisms with focus on high enthalpy plasma torches, in particular the hybrid water/argon-stabilized plasma torch WSP-H 500. The main differences between the formation of finely structured dense coatings and columnar coatings were studied for alumina and yttria-stabilized zirconia (YSZ) by comprehensive evaluation of individual splat formation, gradual coating growth, and geometrical characteristics of the deposits.It was clearly demonstrated that the coating microstructure is generally controlled by morphology of individual deposited splats which is influenced namely by formulation of the suspension and mean trajectory of the fragmented suspension droplets in the plasma jet, both influencing size, flattening ratio and thermal history of deposited splats.Larger droplets having proper thermal treatment provided miniature lamellae similar to conventional plasma spraying, eventually leading to the development of low porosity (dense) structures. With decreasing splat size and increasing distance from the plasma jet axis, coatings tended to develop more porous columnar structures. After characteristic number of deposition cycles, lateral growth of the columnar structures became saturated, i.e. the equivalent diameter of the columnar structures became stabilized at about 75 μm, when observed from the free-surface. Material deposited from the plasma fringes was only loosely bonded and could be easily blown away from the surface which explains absence of interpass porosity in the deposited coatings.

A Novel Fabricating Process of Catalytic Gas Sensor Based on Droplet Generating Technology.

Catalytic gas sensors are widely used for measuring concentrations of combustible gases to prevent explosive accidents in industrial and domestic environments. The typical structure of the sensitive element of the sensor consists of carrier and catalyst materials, which are in and around a platinum coil. However, the size of the platinum coil is micron-grade and typically has a cylindrical shape. It is extremely difficult to control the amount of carrier and catalyst materials and to fulfill the inner cavity of the coil, which adds to the irreproducibility and uncertainty of the sensor performance. To solve this problem, this paper presents a new method which uses a drop-on-demand droplet generator to add the carrier and catalytic materials into the platinum coil and fabricate the micropellistor. The materials in this article include finely dispersed Al2O3 suspension and platinum palladium (Pd-Pt) catalyst. The size of the micropellistor with carrier material can be controlled by the number of the suspension droplets, while the amount of Pd-Pt catalyst can be controlled by the number of catalyst droplets. A bridge circuit is used to obtain the output signal of the gas sensors. The original signals of the micropellistor at 140 mV and 80 mV remain after aging treatment. The sensitivity and power consumption of the pellistor are 32 mV/% CH4 and 120 mW, respectively.

Shape and rheology of droplets with viscous surface moduli

We develop perturbation theories to describe the flow dynamics of a droplet with a thin layer of insoluble surfactant whose mechanics are described by interfacial viscosity, i.e. a Boussinesq–Scriven constitutive law. The theories quantify droplet deformation in the limit of small capillary number, large viscosity ratio, or large shear Boussinesq number, to a sufficient level of approximation where one can extract information about nonlinear rheology and droplet breakup. In the first part of this manuscript, we quantify the Taylor deformation parameter and inclination angle in shear and extensional flows, developing expressions that resolve discrepancies between current analytical theories and boundary element simulations. Interestingly, the theories we develop appear to accurately describe the inclination angle of a clean droplet over a wider range of viscosity ratios and capillary numbers than previous works. In the second part of the manuscript, we calculate how interfacial viscosity alters the extra stress of a dilute suspension of droplets, in particular the shear stress, normal stress differences, shear thinning and extensional thickening. The normal stresses are intimately related to the lateral migration of droplets in wall-bound shear flow, and we explore the influence of interfacial viscosity on this phenomenon. We conclude by discussing how one can use these theories to describe droplet breakup, and how one can incorporate additional effects into the perturbation theories such as viscoelastic membranes and/or Marangoni flows.

Viscosity of protein-stabilized emulsions: Contributions of components and development of a semipredictive model

Protein-stabilized emulsions can be seen as mixtures of unadsorbed proteins and protein-stabilized droplets. To identify the contributions of these two components to the overall viscosity of sodium caseinate o/w emulsions, the rheological behavior of pure suspensions of proteins and droplets was characterized, and their properties were used to model the behavior of their mixtures. These materials are conveniently studied in the framework developed for soft colloids. Here, the use of viscosity models for the two types of pure suspensions facilitates the development of a semiempirical model that relates the viscosity of protein-stabilized emulsions to their composition.

Experimental study of sulfur solubility in silicate–carbonate melts at 5–10.5 GPa

Sulfur solubility in Ca–Mg–Fe silicate–carbonate melt was experimentally determined at 5–10.5 GPa and 1400–1600 °C. Melt was produced by reaction between natural carbonates (calcite and magnesite) and San Carlos olivine (XFo ~ 0.9), which was used as a container, and equilibrated with either Fe-Ni sulfide liquid under reducing conditions (in the presence of graphite) or CaSO4 liquid under oxidizing conditions (Re–ReO2 buffer). To overcome the problem of the presence of sulfide droplets within the silicate–carbonate melt and to reverse the equilibrium, two sample configurations were employed: (1) a carbonate–sulfide mixture placed in the center of the olivine container and (2) sulfide placed in the olivine container and carbonate initially positioned outside the container near the wall of the external Pt capsule. In the former case, silicate–carbonate melt with suspended sulfide melt droplets infiltrated through the olivine capsule, and the two melts were separated owing to their different physical properties. In the latter case, silicate–carbonate melt migrated through thin fractures in the olivine container to come into contact with sulfide. The results of the two experimental series constrained the solubility of sulfide S in silicate–carbonate melt at 0.02–0.10 wt%. The S-solubility increases with increasing FeO content in the melt and is less sensitive to pressure, temperature and other compositional parameters. The solubility of S in the presence of the Re–ReO2 buffer is much higher (up to 2–3 wt%) and dominated by S6+. The obtained results indicate that S is relatively inert during carbonate mantle metasomatism under reduced conditions, but can be transported under oxidizing conditions.

Experimental understanding on the dynamics of micro-explosion and puffing in ternary emulsion droplets

Dynamics of puffing and micro-explosion phenomena occurring in ternary fuel emulsion droplets under high temperature environment were explored using high speed backlight imaging technique. A single droplet composed of diesel-biodiesel-ethanol emulsion was placed at the tip of a 75 µm gauge thermocouple and introduced rapidly into a furnace maintained at 500 °C. Several interesting features such as oscillation of suspended droplets, physical transformations occurring within the droplet, vapour expulsion, puffing, micro-explosion, sheet formation, perforations, growth of perforations, sheet disintegration and rotation of secondary droplets were observed. High resolution image analysis revealed separation of emulsion components within the core of the suspended droplet, which appeared either as a single nucleus or multiple nuclei. Two distinct types of micro-explosion were identified. For droplets encountering a single nucleus at the core resulted in a stronger vapour expulsion followed by intense micro-explosion. For droplets having multiple nuclei at the core resulted in a weaker vapour expulsion and slower growth of droplet prior to micro-explosion. Both types of micro-explosion process resulted in a number of child droplets. For the case of strong vapour expulsion nearly 80% of its child droplets have their sizes distributed within 150 μm compared to 60% for weaker vapour expulsion. The child droplets that were generated from the primary events of both puffing and micro-explosion cascaded further into secondary and tertiary events of puffing and micro-explosion in freely suspended environment.

Antireflective Transparent Oleophobic Surfaces by Noninteracting Cavities.

Oleophobic surfaces have been so far realized using complex microscale and nanoscale re-entrant geometries, where primary and secondary structures or overhang geometries are typically required. Here, we propose a new design to create them with noninteracting cavities. The suspension of liquid droplets relies on the mechanism of compression of air under the meniscus leading to stable composite oil-air-solid interfaces. To demonstrate the concept, we make oleophobic surfaces, with contact angle for oleic acid of about 130° (and hexadecane about 110°), using both microholes in silicon and nanoholes in glass. Thanks to the subwavelength dimensions and antireflection effect of the nanoholes, the glass substrate also shows a high degree of optical transparency with optical transmission exceeding that of the initial bare substrate. Crockmeter tests without any significant change in morphology, optical and wetting properties after more than 500 passes also confirm the high mechanical durability of the nanohole surface. The results indicate the possibility of using the proposed oleophobic surfaces for a wide range of applications, including self-cleaning transparent windows and windshields for automobiles and aircrafts.

Influence of Asphaltene Concentration on the Combustion of a Heavy Fuel Oil Droplet

Heavy fuel oils consist of a blend of middle distillates, mainly diesel fuel, and heavy oil residuals. Varying the fraction of the mixture changes the weight percentage of the asphaltene in the heavy fuel oil (HFO) sample. Asphaltene is a very high molecular weight complex component in the fuel that increases the fuel viscosity, surface tension, and chemical reaction rate. Here, we investigate the influence of high asphaltene concentration on the combustion of a single HFO droplet. In this experimental work, we used the thermogravimetric analysis (TGA) and the suspended droplet techniques. We tested HFO samples containing asphaltene at 8, 16, 24 wt % (HFO8, HFO16, and HFO24). The TGA result shows a residual amount of approximately 2.4 wt % of HFO24 compared to no residuals for HFO8 at the end of the process. The suspended droplet technique results reveal the following seven consecutive burning stages for the entire burning process of the liquid and solid phases: (1) preheating, (2) flame startup, (3) inne...

Analysis of droplet dynamic behavior and condensation heat transfer characteristics on rectangular microgrooved surface with CuO nanostructures

Condensation is a ubiquitous phase-change phenomenon in nature and has been widely adopted in various energy-intensive industrial application. Many efforts have been focused on regulating droplet dynamics to enhance the condensation heat transfer by developing micro/nanostructured surface. In this work, a microgrooved surface with CuO nanostructures was fabricated by combination of simple machining and scalable self-limiting chemical oxidation method for regulating droplet dynamic behavior. The wettability, droplet dynamics and heat transfer characteristics on such surface were compared with that on plain and microgrooved hydrophobic surfaces. The anisotropic wettability was observed on microgrooved surface and enhanced by creating nanostructures. 15–43% higher heat flux was reached on microgrooved hydrophobic surface compared to plain hydrophobic surface due to an increase of effective heat transfer area, the sweeping effect of liquid columns for droplets on adjacent plateaus and cooperation of liquid columns flowing and liquid bridges sliding. Several novel droplet dynamic behaviors that the suspension of large droplets, suction of spindle-shaped droplets and strong self-oscillation of falling droplets were observed on the microgrooved surface with nanostructures and enhanced the condensation heat transfer. Compared with plain hydrophobic surface, the heat flux was enhanced up to 55–102%.

Three-Dimensional Modeling of Suspension Plasma Spraying with Arc Voltage Fluctuations

In this study, a three-dimensional DC plasma torch is modeled using Joule effect method to simulate the plasma jet and its voltage fluctuations. The plasma gas is a mixture of argon/hydrogen, and the arc voltage fluctuation is used as an input data in the model. Reynolds stress model is used for time-dependent simulation of the oscillating flow of the plasma gas interacting with the ambient air. The results are used to investigate the plasma oscillation effects on the trajectory, temperature, and velocity of suspension droplets. Suspensions are formed of ethanol and yttria-stabilized zirconia submicron particles and modeled as multicomponent droplets. To track the droplets/particles, a two-way coupled Eulerian–Lagrangian method is employed. In addition, in order to simulate the droplet breakup, Kelvin–Helmholtz/Rayleigh–Taylor (KH–RT) breakup model is used. After the completion of suspension breakup and evaporation, the sprayed particles are tracked to obtain the in-flight particle conditions including trajectory, size, velocity, and temperature. The arc voltage fluctuations were found to cause more than two times wider particle trajectories resulting in wider particle temperature, velocity, and size distributions compared with the case of constant voltage.

Evaporation of a suspended nanofluid droplet

The experimental results on evaporation of the suspended droplets of distilled water with the addition of 0.1% silicon dioxide with an average particle diameter of 10 nm in a stationary ambient and in an air flow of 0.1 m/s at a temperature of 25 °C to 26°C and 50°C are presented. A comparison is made with the evaporation droplet parameters of the base liquid. Experimental data show that during evaporation the change in the surface temperature of the nanofluid droplets is qualitatively similar to the dynamics of the surface temperature of the mixture droplets, consisting of two pure components with different degrees of volatility.

Experimental study of the evaporation of suspended droplets of a water-ethanol solution

The dynamics of the geometric parameters of droplets of a water-ethanol solution suspended on polypropylene filaments was investigated using high-speed microphotography. It was found that all suspended droplets had a spherical shape during the main time of evaporation. The higher the concentration of ethanol in the drop, the faster the droplet evaporates. The dynamics of the temperature of droplets of a water-ethanol solution suspended on polypropylene filaments was investigated using the method of infrared thermography. Three stages of temperature change of evaporating droplets of a water-ethanol solution were found. At the initial stage of evaporation, the droplet temperature changed like the temperature of an ethanol droplet, and then its behavior was similar to the temperature change of water droplets. The higher was the ethanol concentration in the droplet, the more similar was the temperature change with a change in the ethanol droplet temperature.

Assessment of the Fire Dynamics Simulator Modeling for the Heating and Evaporation of a Single Water Droplet at Moderate and High Temperatures

The work described in this paper is undertaken with the purpose of providing a detailed assessment of the current modelling capabilities of the effects of fire suppression systems (e.g., sprinklers) in fire-driven flows. Such assessment will allow identifying key modelling issues and, ultimately, improving the reliability of the numerical tools in fire safety design studies. More specifically, we studied herein the heating and evaporation of a single water droplet. This rather ‘simple’ configuration represents the first step in a tedious and rigorous verification and validation process, as advocated in the MaCFP (Measurement and Computation of Fire Phenomena) working group (see https://iafss.org/macfp/). Such process starts ideally with single-physics ‘unit tests’ and then more elaborate benchmark cases and sub-systems, before addressing ‘real-life’ application tests. In this paper, we are considering the recently published comprehensive and well-documented experimental data of Volkov and Strizhak (Applied Thermal Engineering, 2017) where a single suspended water droplet of a diameter between 2.6 and 3.4 mm is heated up by a convective hot air flow with a velocity between 3 and 4.5 m/s and a temperature between 100 and 800°C. The high temperatures considered therein represent a strong element of novelty, since previous experimental studies on single droplets were limited to rather relatively ‘moderate’ temperatures, up to around 350°C. Furthermore, the monitoring of the time history of the droplet temperature field, in addition to the droplet lifetime, provides very useful information for model development and validation purposes. In this numerical study, 36 experimental tests have been simulated with the Fire Dynamics Simulator (FDS 6.6.0). The results show that the droplet lifetime is overpredicted with an overall accuracy of 31%. The accuracy in the range 300 to 800°C is even better, i.e., 7 %, whilst the cases of 200 and, more so 100°C, showed much stronger deviations. The measured droplet saturation temperatures did not exceed 70°C, even for high air temperatures of around 800°C, whereas the predicted values approached 100°C. Based on the current findings, further analysis is required on the modelling of the heat and mass transfer coefficients, and more specifically the sub-models for the Nusselt and Sherwood numbers.

A Detailed Investigation on the Effect of the Sherwood and Nusselt Number Modelling for the Heating and Evaporation of a Single Suspended Water Droplet

The work described in this paper presents a comprehensive analysis of the convective heat and mass transfer coefficient modelling around a single water droplet using an in-house code. The most widely used approach is to rely on sub-models for the non-dimensional heat and mass transfer numbers (called hereafter the Nusselt, Nu, and the Sherwood, Sh, numbers) and which are shown to take the theoretical and functional form of and where Red is the droplet Reynolds number, Pr and Sc are the Prandtl and Schmidt numbers of the surrounding gas and K1 and K2 are constants. This formulation, which is generally referred to in the literature as the Ranz-Marshall model (with K1 = K2 = 0.6), is the most used approach in Computational Fluid Dynamics (CFD) codes for fire safety engineering. In this paper, we first assessed this formulation based on 36 experimental tests carried out in [Volkov and Strizhak, Applied Thermal Engineering (2017)] and where a single suspended water droplet of a diameter between 2.6 and 3.4 mm is heated up by a convective hot air flow with a velocity between 3 and 4.5 m/s and a temperature between 100 and 800°C. The results showed that the overall model uncertainty in the droplet lifetime prediction is about 34% with particularly poor results when the air temperature is 100 or 200°C. The droplet saturation temperatures were overestimated by around 20 to 30°C. After this initial assessment, we performed a sensitivity analysis and selected a combination of values for K1 and K2 that provided an overall simultaneous good agreement for both the droplet lifetimes (model uncertainty of 5%) and the droplet saturation temperatures (around 10°C). This analysis showed that, for high air temperatures (i.e., Ta ≥ 300°C), the value of K2 = 0.6 remains suitable. However, for these cases, the value of K1 needed to be increased (to 1.8 and up to 4) in order to promote the evaporation-induced cooling and improve the predictions in terms of droplet saturation temperatures. For Ta = 100°C, the ‘best’ combination was found to be K1 = K2 = 1.8. Such combination allowed to reduce the initially overestimated droplet lifetimes by promoting mass transfer (through an increased value of K1) without slowing down the heating process that generates water vapor at the droplet surface (explaining the equally increased value of K2). The case of Ta = 200°C appeared to be an intermediate case. The present results indicate that the ‘classical’ Ranz-Marshall approach with K1 = K2 = 0.6 is not optimal. A more thorough analysis, with eventually additional experimental data, is required.