Wide Range Of Droplet Sizes Research Articles

Fuel effects on the electrostatic control of charged droplet trajectories

The use of electrostatic fields to control the trajectory of charged droplets is investigated as a new technology to enhance mixing in liquid-fuelled combustors. The canonical configuration of a spray in crossflow is numerically studied with a focus on the effects of the fuel type on the onset of charge-induced breakup and droplet trajectories for a range of bulk flow velocities and electrostatic field strengths at conditions relevant to gas turbine applications. The investigated fuels are ethanol, n-heptane, and n-decane. Operational maps for each fuel are provided to assist the selection of the external electrostatic field required to achieve a balance between the drag and electrostatic forces, and enable a system design that considers fuel flexibility. The results demonstrate that the fuel type has an important impact on the diameter at which the charge-induced breakup is achieved, which mainly depends on the droplet equilibrium temperature. It is also shown that, for cases where the droplet net charge is fixed to a given fraction of the maximum possible charge (based on the Rayleigh limit), the temperature of the droplet at injection could be used as a parameter to control the onset of secondary breakup. Analysis of the strength of the electrostatic field necessary to achieve droplet stabilisation in a bulk flow shows that a balance between the electrostatic and drag forces can only be achieved for relatively low values of the bulk flow velocity, if the strength of the electrostatic field is kept below the breakdown limit of the carrier phase. This balance mainly depends on the droplet net charge and flow conditions, whereas the effect of the type of fuel on the drag force is less important. When the charge is imposed as a fraction of the maximum possible value at injection, for low values of the bulk flow velocity, the strength of the electrostatic field that balances the drag tends to become independent of the droplet diameter for a wide range of droplet sizes. Further investigation of the trajectories of evaporating droplets demonstrates that, although affected by the type of fuel through the evaporation rate, with the same settings of the electrostatic field, it is possible to achieve evaporation in a confined region for all the fuels and ambient conditions studied in this work once the initial droplet charge and initial droplet diameter are fixed. The present findings offer new insights for the development of future technologies for fuel preparation with enhanced mixing and fuel flexibility.

Vibration‐Enabled Mobility of Liquid Metal

AbstractDirected liquid metal (gallium‐based) manipulation and actuation are paramount for copious applications, including soft robotics, soft electronics, and targeted drug delivery. Although there are several strategies available to achieve mobility of liquid metals in a “wet” environment. Strategies to achieve and improve mobility of liquid metal droplets and puddles in a “dry” environment are scarce and rely on metallophobic surface design or liquid metal marbles. Here, high mobility of Galinstan is elucidated by combining metallophobic surface design and vertical vibrations. Vibration frequencies between 20 and 30 Hz are conducive to droplet movement and threshold inclination angles of 0.5° to 1° are observed upon actuation by these vibrations. The method itself is applicable for a wide range of droplet sizes (30 and 2000 µL) and very robust. The droplet movement typically comprises of periodic receding and advancing of the droplet and commences via a rolling mechanism rather than a gliding mechanism. Finally, it is shown that small (0.5 mm height) obstacles can be traversed by this method, indicating that it can be used in concert with other strategies, such as surface structuring strategies, which open up pathways for mobility and controlled actuation of liquid metal droplets in air.

Advances in Microscale Droplet Generation and Manipulation

Microscale droplet generation and manipulation have widespread applications in numerous fields, from biochemical assays to printing and additive manufacturing. There are several techniques for droplet handling. Most techniques, however, can generate and work with only a limited range of droplet sizes. Furthermore, there are constraints regarding the workable variety of fluid properties (e.g., viscosity, surface tension, mass loading, etc.). Recent works have focused on developing techniques to overcome these limitations. This feature article discusses advances in this area that cover a wide range of droplet sizes from subpicoliter to microliter.

Particle Size Analysis in Aerosol-Generating Dental Procedures Using Laser Diffraction Technique.

The global outbreak of coronavirus disease 2019 (COVID-19) has raised concerns about the risk of airborne infection during dental treatment. Aerosol-generating dental procedures (AGDP) produce droplets and aerosols, but the details of the risks of COVID-19 transmission in AGDP are not well-understood. By discriminating between droplets and aerosols, we devised a method to measure particle size using laser diffraction analysis and evaluated aerosols generated from dental devices for providing a basis for proper infection control procedures. The droplets and aerosols generated from dental devices were characterized by multimodal properties and a wide range of droplet sizes, with the majority of droplets larger than 50 μm. AGDP emitted few aerosols smaller than 5 μm, which are of concern for pulmonary infections due to airborne transmission. In addition, the use of extraoral suction was found to prevent the spread of aerosols from high-speed dental engines. This study suggests that the risk of aerosol infections is considerably limited in regular dental practice and that current standard precautions, such as mainly focusing on protection against droplet and contact infections, are sufficient. While several cases of airborne transmission of COVID-19 in general clinics and emergency hospitals have been reported, cluster outbreaks in dental clinics have not yet been reported, which may indicate that AGDP does not pose a significant threat in contributing to the spread of SARS-CoV-2.

Energy Component Analysis of Electric Field-Induced Shape Change in Water Nanodroplets

We present a comprehensive classical molecular dynamics study of water nanodroplets under the influence of an externally applied electric field. Our simulations cover a wide range of droplet sizes and electric field strengths, which allows for a thorough exploration of the structural and energetic behavior of nanodroplets in the presence of an external electric field. Our analysis reveals the molecular-level mechanism behind the shape extension of a nanodroplet from a spheroid to a highly prolate ellipsoid as the propensity of the water dipoles to align with the electric field while simultaneously restructuring to minimize the dipole–dipole interaction energy. We also develop a quantitative theory that describes the energetic landscape for the nanodroplet shape extension process and allows predictions of the nanodroplet behavior based on its initial size and the strength of the applied field.

A double inclusion model for liquid metal polymer composites

Liquid metal elastomer composites have gained significant attention in advanced technologies including wearable electronics, soft robotics, and human-computer interactions. This is due to the combination of metallic conductivity and fluidic properties of liquid metal (LM) inclusions in addition to their facile fabrication process. With the emergence of gallium-based liquid metal nanocomposites and advances in synthesis and integration of LM nanoparticles in a variety of polymer matrices, there is a pressing need for a materials design tool to accelerate the development of these multifunctional composites. Here, we introduce a double inclusion (DI) model capable of predicting the properties of polymer composites with core-shell liquid metal droplets. The size-dependent elasticity of LM inclusions is modeled by considering the solid gallium oxide interphase between the liquid metal core and the solid polymer matrix. As the size of inclusions reduces from tens of microns to tens of nanometers, the role of the oxide interface (shell) becomes more dominant. The results of the DI model show excellent agreement with finite element analysis and experimental results for a wide range of droplet sizes and volume fractions. This model provides a design framework for the synthesis of LM composites with tailored multifunctional properties.

Microfluidic generation of ATPS droplets by transient double emulsion technique.

An aqueous two-phase system (ATPS) is considered as a promising candidate for biological applications due to its excellent permeability, selective separation, rapid mass transfer and all-biocompatible nature. However, it remains difficult to generate ATPS emulsions with controllable and prolonged stability due to ultra-low interfacial tension. Here, we present a transient double emulsion (TDE) technique to address this challenge. The method involves two steps: (i) water-in-oil-in-water (W1/O/W2) transient double emulsion droplets are first produced with droplet microfluidics by introducing an additional middle oil phase, and (ii) the W1/O/W2 droplets dewet into oil-in-water and ATPS droplets in a controllable manner. Using the TDE method, both dextran-in-polyethylene glycol (PEG) and PEG-in-dextran ATPS droplets can be generated with high uniformity (coefficient of variation (CV) ranging from 0.66% to 2.55%), a wide range of droplet sizes (∼100 to 250 μm in radius), and tunability in the generation frequency (∼4 to 170 Hz). Tuning the oil viscosity controls the destabilization time for on-demand dewetting and releasing of aqueous core droplets. The stability of ATPS droplets is significantly improved by storing aqueous droplets in double emulsion vessels, which would benefit various applications, such as small molecule encapsulation.

Photoelectron Spectroscopy on Magnesium Ensembles in Helium Nanodroplets.

The electronic properties of magnesium embedded in helium nanodroplets are investigated as a function of the doping level by combining resonant two-photon ionization with photoelectron spectroscopy. Photon absorption near the atomic 31P1 ← 31S0 transition triggers the dynamics, which is probed on a nanosecond timescale by the subsequent absorption of the ionizing photon. Striking similarities in the photoelectron spectra for different doping levels and the population of atomic states well beyond 31P1 evidence an energy release process being relevant for a wide range of droplet sizes and doping conditions. From the statistical analysis, one can infer that instead of a single compact cluster size, a loosely bound Mg atom ensemble is responsible for the resulting spectra.

Numerical prediction of air-assisted primary atomization using Smoothed Particle Hydrodynamics

The well defined atomization of liquid fuels plays a key role in the combustion process of civil jet engines, in particular for minimizing toxic emissions. Presently, the breakup mechanisms leading to a wide range of droplet sizes and trajectories are not understood in detail. Furthermore, reliable numerical predictions have not been feasible due to the enormous computational costs associated with the simulation of air-assisted atomization. In this paper we present a Direct Numerical Simulation (DNS) of an experimentally investigated planar prefilming air-blast atomizer at one operating point. The numerical investigation is carried out using the Smoothed Particle Hydrodynamics (SPH) method. The spatial resolution of 5 µm yields a domain size of 1.2 billion particles. Due to the superior serial and parallel performance of the method, 122 channel flow-through times of the air flow could be realized consuming a rather small amount of computational resources. Within the simulated physical period of time of 14.6 ms, two main breakup events could be detected. As a converged statistical analysis of the resulting fuel spray properties is not possible, the quantitative comparison to the experimental findings is based on single event statistics. In this work we analyze the characteristic volumetric droplet diameters DV0.1,DV0.5 and DV0.9 and the Sauter Mean Diameter D32. Both quantitatively as well as with regard to the phenomenology of the breakup processes, the simulation matches extremely well the experimental observations. This work demonstrates, that reliable simulations of air-assisted atomization have come into reach by combining the highly efficient numerical method SPH with massive computing power.

Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age.

Aerosols are key components of the atmosphere and play important roles in many industrial processes. Because aerosol particles have high surface-to-volume ratios, their surface properties are especially important. However, direct measurement of the surface properties of aerosol particles is challenging. In this work, we describe an approach to measure the surface tension of picoliter volume droplets with surface age <1 ms by resolving their dynamic oscillations in shape immediately after ejection from a microdroplet dispenser. Droplet shape oscillations are monitored by highly time-resolved (500 ns) stroboscopic imaging, and droplet surface tension is accurately retrieved across a wide range of droplet sizes (10-25 μm radius) and surface ages (down to ∼100 μs). The approach is validated for droplets containing sodium chloride, glutaric acid, and water, which all show no variation in surface tension with surface age. Experimental results from the microdroplet dispenser approach are compared to complementary surface tension measurements of 5-10 μm radius droplets with aged surfaces using a holographic optical tweezers approach and predictions of surface tension using a statistical thermodynamic model. These approaches combined will allow investigation of droplet surface tension across a wide range of droplet sizes, compositions, and surface ages.

Characterization and modeling of aerosol deposition in Vitrocell® exposure systems - exposure well chamber deposition efficiency

Multi-well aerosol exposure systems are used in modern toxicology assessment studies to deliver aerosol to a large number of tissue/cell culture samples simultaneously. These systems are designed to control the experimental conditions of a delivered aerosol. In these systems (e.g., those developed by Vitrocell GmbH), the aerosol mixture is delivered perpendicularly to the tissue culture through a trumpet-shaped (flared) pipe. In the well chamber where the tissue/cell culture is exposed, the flow is smooth and laminar, which limits shear forces and potential moisture loss that may damage the tissue/cells. These operating flow conditions also determine the aerosol dynamics and deposition mechanisms within the system. The utility of these systems to evaluate biological responses depends on the quantity of tissue culture. With limited experimental data, evaluating the aerosol deposition via computational means is necessary to predict the deposition efficiency. For our investigations, we employed a recently developed Eulerian Computational Fluid Dynamics solver (available at www.aerosolved.com) for simulations of polydisperse multi-species aerosol transport and deposition. We investigated deposition efficiency using various exposure distances to the tissue culture, aerosol properties, and operating conditions. Terms associated with drag, gravitation, and Brownian diffusion were included in the aerosol equations to predict the deposition of the polydisperse aerosol. Results were verified by comparisons with the available experimental data, and predictions were obtained from the Lagrangian simulations using commercially available software. Within the recommended operating conditions, inertial impaction was found not to affect aerosol deposition, which is driven mainly by the size-dependent sedimentation and diffusion mechanisms. An important implication is that for a wide range of droplet sizes, the delivered dose to the tissue is independent of sampled flow rate. Taking this into account, a simple and robust size-dependent theoretical model of the aerosol deposition efficiency was developed. This theoretical model is based on aerosol characteristics, flow, and geometry inputs without the use of any fitting parameter. It can be applied to various exposure system geometries under different operating conditions, as verified in comparisons with published deposition efficiency data obtained from experiments and computations.

Controlled and tunable polymer particles\u2019 production using a single microfluidic device

Microfluidics technology offers a new platform to control liquids under flow in small volumes. The advantage of using small-scale reactions for droplet generation along with the capacity to control the preparation parameters, making microfluidic chips an attractive technology for optimizing encapsulation formulations. However, one of the drawback in this methodology is the ability to obtain a wide range of droplet sizes, from sub-micron to microns using a single chip design. In fact, typically, droplet chips are used for micron-dimension particles, while nanoparticles’ synthesis requires complex chips design (i.e., microreactors and staggered herringbone micromixer). Here, we introduce the development of a highly tunable and controlled encapsulation technique, using two polymer compositions, for generating particles ranging from microns to nano-size using the same simple single microfluidic chip design. Poly(lactic-co-glycolic acid) (PLGA 50:50) or PLGA/polyethylene glycol polymeric particles were prepared with focused-flow chip, yielding monodisperse particle batches. We show that by varying flow rate, solvent, surfactant and polymer composition, we were able to optimize particles’ size and decrease polydispersity index, using simple chip designs with no further related adjustments or costs. Utilizing this platform, which offers tight tuning of particle properties, could offer an important tool for formulation development and can potentially pave the way towards a better precision nanomedicine.

Enabling Biocatalysis by High-Throughput Protein Engineering Using Droplet Microfluidics Coupled to Mass Spectrometry.

Directed Evolution is a key technology driving the utility of biocatalysis in pharmaceutical synthesis. Conventional approaches to Directed Evolution are conducted using bacterial cells expressing enzymes in microplates, with catalyzed reactions measured by HPLC, high-performance liquid chromatography-mass spectrometry (HPLC-MS), or optical detectors, which require either long cycle times or tailor-made substrates. To better fit modern, fast-paced process chemistry development where solutions are rapidly needed for new substrates, droplet microfluidics interfaced with electrospray ionization (ESI)-MS provides a label-free high-throughput screening platform. To apply this method to industrial enzyme screening and to explore potential approaches that may further improve the overall throughput, we optimized the existing droplet–MS methods. Carryover between droplets, traditionally a significant issue, was reduced to undetectable level by replacing the stainless steel ESI needle with a Teflon needle within a capillary electrophoresis (CE)–MS source. Throughput was improved to 3 Hz with a wide range of droplet sizes (10–50 nL) by tuning the sheath flow within the CE–MS source. The optimized method was demonstrated by screening reactions using two different transaminase libraries. Good correlations (r2 ∼ 0.95) were found between the droplet–MS and LC–MS methods, with 100% match on hit variants. We further explored the capability of the system by performing in vitro transcription–translation inside the droplets and directly analyzing the intact reaction mixture droplets by MS. The synthesized protein attained comparable activity to the protein standard, and the complex samples appeared well tolerated by the MS. The success of the above applications indicates that the MS analysis of the microfluidic droplets is an available option for considerably accelerating the screening of enzyme evolution libraries.

Field Study of LWC and MVD Using the Droplet Imaging Instrument

The droplet imaging instrument (DII) is a new instrument for cost-effective in situ measurements of the size and concentration of water droplets. The droplet size distribution and the concentration of atmospheric liquid water are important for the prediction of icing on structures, such as wind turbines. To improve the predictions of icing, there is a need to explore cost-effective working solutions. Through imaging, a wide range of droplet sizes can be measured. This paper describes a study of the atmospheric liquid water content and the median volume diameter using the DII and a commercial reference instrument—the cloud droplet probe 2 from Droplet Measurement Technologies Inc. The measurement is done at a weather measurement station in mid-Sweden. For a second validation, the result is compared with predictions using a numerical weather prediction model. The size measurement of the DII is verified using polymer microspheres of four known size distributions. The study shows that the DII measurement is precise, but there is a systematic difference between the two compared instruments. It also shows that droplets larger than $50~\mu \text{m}$ in diameter are occasionally measured, which we believe is important for the prediction of icing.

Synthesis and Stability of Water-in-Oil Emulsion Using Partially Reduced Graphene Oxide as a Tailored Surfactant.

Graphene oxide (GO) is widely known as an amphiphile having hydrophilic oxygen functionality and unoxidized graphitic patches as the hydrophobic domains. Exploiting this amphiphilicity, GO serves as a surfactant to stabilize oil-water interfaces. While there are numerous reports on GO as a surfactant, most of these reports concern oil-in-water (O/W) emulsions, and there are very few on the formation of water-in-oil (W/O) emulsions. We prepared W/O emulsions using partially reduced graphene oxide (prGO) as a surfactant. The partial reduction introduces a subtle hydrophilic-lipophilic balance (HLB), which favors the formation of the W/O emulsion. The morphological features and rheological characteristics of the W/O emulsion with 75:25 water-to-oil ratio were investigated and analyzed in detail. The W/O emulsion was found to have polydispersity with wide range of droplet sizes varying between 2 to 500 μm. Using confocal microscopy, the role of parameters such as extent of reduction, continuous phase volume fraction and the concentration of GO on the stability, microstructure and variation of droplet size distribution of the W/O emulsion were carefully monitored. With prGO concentration as large as 0.05% (w/w), highly concentrated emulsion will form, and are stable up to 20 days from formation; destabilization occurred from sedimentation and subsequent coalescence as the partially reduced GO was limited by its dispersion ability in the oil-phase studied here. Understanding the mechanisms behind the transient stability will enable the development of novel emulsion compositions containing GO as a multifunctional additive.

The effect of Marangoni convection on heat transfer during dropwise condensation on hydrophobic and omniphobic surfaces

A multi-fold enhancement in the rate of heat transfer can be achieved by promoting the dropwise condensation mode (DWC) over the filmwise condensation mode. Recent material developments are increasing the chances that DWC will transition into industrial applications. Consequently, the ability to quantitatively model heat transfer rate during DWC of water and low surface tension liquids will become increasingly important in design of condensers. DWC heat transfer models developed so far consider only conduction inside the condensate droplets. However, scaling analysis shows that, in contrast to buoyancy driven flow, thermocapillary flow could be present in a wide range of droplet sizes in industrially relevant conditions. In the present work, we theoretically quantify the effect of Marangoni convection on heat transfer across individual condensing droplets as well as its impact on the overall DWC heat transfer. Specifically, we use Finite Element simulations to estimate the change in heat transfer that thermocapillary flow induces in condensing drops with spherical cap geometry. Besides water, we also study heat transfer across drops of organic liquids including toluene, ethanol, and pentane. Our results indicate that heat transfer rates across droplets are higher in the conjugate heat transfer case than the conduction only case (up to 6-fold increase for large water droplets on hydrophobic and superhydrophobic surfaces under extreme subcooling of 50K). However, irrelevant of fluid and contact angle, for smaller droplets with radius below 100µm at most a twofold thermocapillary heat transfer enhancement was obtained. When integrated with the dropsize distribution, these multi-fold increases in heat transfer across individual drops translate in most cases to a minor 10% or lower increase in the overall dropwise condensation heat transfer coefficient. Thus, with exception of a few special cases, the Marangoni flow contribution to DWC heat transfer coefficient is on the order of typical experimental uncertainties and can be neglected.

The effect of Weber number, droplet sizes and wall roughness on crisis of droplet boiling

Various boiling modes in a wide range of droplet sizes and wall temperature under droplet Tw were studied experimentally. Dynamics of droplets boiling is determined not only by Tw, Weber number We, wall roughness but also by the droplet shape. Application of two different methods of forming a suspended spheroid (ellipsoid) for We=0 and We>1 allowed separate investigation of the influence of the key factors on evaporation. To form the spheroids with We=0 (no droplet fall, V0=0m/s), the limiting rings were used. For We=0, an increase in pressure inside a droplet and its disintegration were excluded because there was no fall. An increase in roughness at We=0 promotes an increase in Leidenfrost temperature TL, and conversely, at We>1, high roughness leads to a decrease in TL due to the pressure drop in liquid and decay of a spheroid. With the growth in the initial diameter of a suspended drop at We=0, temperature TL increases significantly and approaches the value of TL at pool boiling in water. The lower value of this temperature is also consistent with theoretical predictions. Experimental data demonstrate the importance of taking into account the size maldistribution of droplets for the correct prediction of the uneven temperature field on the heated wall surface.

Spray Tip Effect on Glufosinate Canopy Deposits in Palmer Amaranth (Amaranthus palmeri) for Pulse-Width Modulation versus Air-Induction Technologies

<abstract> Five nozzle tip treatments included nozzle tip designs and selected use of blended pulse-width modulation (bPWM), with tip selections based on a maxim of reduced spray drift potential, either through droplet size classification or spray entrainment of dual sprays. Spray nozzle technologies prioritized for this study included bPWM of pre-orifice tips in various configurations of single and dual tips, and air-induction tips operated in constant (non-bPWM) mode. A self-propelled sprayer equipped with a 30.5 m boom and bPWM applied glufosinate-ammonium herbicide simultaneously through the five different spray nozzle tip treatments. Ambient wind veleocities ranged from 3.1 to 4.1 m s^-1during application. Herbicide deposits on leaves and water-sensitive paper (WSP) coverage, spot density, and droplet size characteristics at top and middle canopy locations in Palmer amaranth were used to compare nozzle tip treatments. Treatment 1 (pre-orifice tips, bPWM and non-bPWM) produced significantly greater herbicide deposits than all tips except Treatment 2 (pre-orifice tip, bPWM). The latter was not significantly different from all other tips. The numerically lowest deposit level, based on the overall means, was produced by Treatment 5 (air-induction deflector tip, non-bPWM), although the leaf deposit was not significantly different from most tips. Deposits of glufosinate-ammonium for top canopy locations were greater than middle canopy location deposits for a given nozzle tip treatment. Summation of deposits across locations sometimes exceeded the application rate of 4.5 μg cm^-2, attributed to spray cloud momentum due to the application speed of 24 km h^-1. The addition of more weed plants or crop plants could possibly alter spray cloud momentum or serve as interceptors of droplets at the expense of deposit levels on Palmer amaranth. Glufosinate-ammonium leaf deposits were inversely proportional to and significantly correlated with canopy coverage of the ground, which supported this hypothesis. The highest mean WSP coverage occurring at top location for Treatment 1 was significantly greater than the mean WSP coverage at top locations for Treatment 4 (air-induction extended-range tip, non-bPWM) and Treatment 5, and greater than mean WSP coverages for all tip treatments at middle locations based on p-level observations. The highest mean WSP spot deposit for the top location of Treatment 1 (57.9 cm^-2) was significantly greater than the mean WSP spot deposits for all other tip treatments and canopy locations. A wide range of droplet sizes resulting from the entrainment of a Fine spray into an Extremely Coarse spray may have contributed to high values of coverage and spot deposit for Treatment 1. In conclusion, use of a pre-orifice nozzle tip with bPWM plus a pre-orifice nozzle tip in constant (non-bPWM) mode that produced contrasting droplet sizes for spray entrainment between the two tips provided a means to increase glufosinate-ammonium deposits on leaves and increase coverage and spot density on WSP. On the other hand, Y-adapter mounted pre-orifice tips with bPWM and separate air-induction nozzle tips (extended-range and deflector designs) operated as non-bPWM applications did not result in high levels of glufosinate-ammonium deposits on leaves and did not result in high levels of coverage and spot density on WSP.

Contact angle and droplet evaporation on the smooth and structured wall surface in a wide range of droplet diameters

Droplet evaporation in a wide range of initial volumes of 1–1000μl on the structured and smooth surfaces was studied experimentally. It is found that the static contact angle on the structured surface for the steady equilibrium depends on the droplet shape and initial diameter; it has an extreme. With the increasing wetting diameter of water samples from 2 to 30mm, the contact angle increases first, reaches a maximum, then decreases and tends to a constant value. To determine the contact angle, the authors have performed the comparisons by different methods in a wide range of droplet sizes. A wide range of droplet sizes is usual for spray cooling. Approximation dependences for determining droplet volume were obtained in a wide range of droplet sizes. For the droplets with the initial diameter less than 1mm, middle and large droplets, as well as for the small initial contact angles and angles of about 85–90°, different kinetics of evaporation will be observed. Kinetics of droplet evaporation on the structured surface differs from evaporation on a smooth wall. Dimensionless Fourier number (Fo), derived by the initial droplet diameter and total time of evaporation, decreases with an increase in the wall temperature and initial droplet diameter. For large droplet diameter and a high wall superheating the derivatives d(Fo)/d(Tw) and d(Fo)/d(d0) tend to zero.

Linear and non-linear rheological behavior of polypropylene/polyamide blends modified with a compatibilizer agent and nanosilica and its relationship with the morphology

The morphology of immiscible 80/20 polypropylene/polyamide 6 (PP/PA) blends that contain different types of nanosilica and a compatibilizer agent was correlated with their linear and non-linear rheological behavior. Polypropylene grafted with maleic anhydride (PPgMA) was used as compatibilizer agent. Two types of modified silica nanoparticles were added to the blends, one hydrophilic (NSE) and the other hydrophobic (NSH). SEM and TEM microscopy were employed to observe the sea-island morphology of the 80/20 blends. The size of the PA droplets was reduced 12 times when the compatibilizer agent was added; and 25 times when hydrophobic nanosilica (NSH) was additionally included in the formulation. TEM results revealed that NSH particles are preferentially located at the PP-PA interface, whereas NSE particles remain inside the PA droplets. A good agreement between morphology and Small Amplitude Oscillatory Shear tests (SAOS) was observed. At low frequencies a suspension-like rheological behavior was identified for the blends containing nanosilica. The results at intermediate frequencies allowed the evaluation of the shape relaxation time of the droplets. Large Amplitude Oscillatory Shear tests (LAOS) results correlate well with droplets size. The coalescence promoted by flow was investigated, and in the case of the neat binary blend, coalesced blends were obtained. Employing LAOS results determined by measuring blends with a wide range of droplet sizes, an inversely proportional function of a characteristic LAOS parameter with the diameter of the droplets was obtained.