Vibrating Orifice Droplet Generator for Studying Fast Processes Associated with Microdroplets

ABSTRACT

A light scattering technique, based on resonances, has been applied to a linear stream of highly monodisperse droplets generated by a modified vibrating orifice aerosol generator to study evaporation of ethanol droplets. The residence time of droplets was varied by altering the distance between the orifice and a laser beam. Frequencies at which intensity peaks (i.e., resonances) appear in elastic and Raman scattered light were determined by varying the frequency of droplet generation. The absolute droplet size and refractive index were determined by matching the observed resonances with theoretical resonances that minimize the difference between the observed and calculated resonance frequencies. The size and refractive index changes between two successive residence time observations were determined from the frequency shifts of the resonances. These procedures permit determination of size and refractive index with a precision of 2 parts in 10⁴, and their changes within 1 part in 10⁴ of the absolute values. For the examined residence time of 650 μs the evaporation process was unsteady. The evaporation rate as well as droplet temperature estimated from the refractive index was found to decrease with time. The experimental technique can be applied to study any fast process associated with microdroplets where observable changes occur over microsecond level time scales.     

Intra-Species Bacterial Quorum Sensing Studied at Single Cell Level in a Double Droplet Trapping System

In this paper, we investigated the intra-species bacterial quorum sensing at the single cell level using a double droplet trapping system. Escherichia coli transformed to express the quorum sensing receptor protein, LasR, were encapsulated in microdroplets that were positioned adjacent to microdroplets containing the autoinducer, N-(3-oxododecanoyl)- l-homoserine lactone (OdDHL). Functional activation of the LasR protein by diffusion of the OdDHL across the droplet interface was measured by monitoring the expression of green fluorescent protein (GFP) from a LasR-dependent promoter. A threshold concentration of OdDHL was found to induce production of quorum-sensing associated GFP by E. coli. Additionally, we demonstrated that LasR-dependent activation of GFP expression was also initiated when the adjacent droplets contained single E. coli transformed with the OdDHL synthase gene, LasI, representing a simple quorum sensing circuit between two droplets.     

Measurement of Activity Coefficients from Unsteady State Evaporation and Growth of Microdroplets

Techniques are presented for determination of activity coefficients of binary systems from unsteady state evaporation and growth of single microdroplets in controlled environments. A high-precision light scattering method based on resonances observed in light scattering by microdroplets was used to determine the size and composition of a microdroplet as functions of time. The techniques were validated through data on growth of glycerol microdroplets in slowly developing water vapor concentration fields and evaporation of microdroplets, containing volatile dimethylphthalate (DMP) and nonvolatile dioctylphthalate (DOP), in vapor-free atmospheres. When the water vapor concentration in the surrounding gas changes slowly a glycerol droplet maintains a dynamic equilibrium with the water vapor; thus the activity coefficient of water was determined from knowledge of the droplet composition and the water vapor saturation ratio in the gas phase. The activity coefficients of DMP were determined on the basis that the instantaneous evaporation rate of a DMP-DOP microdroplet in a vapor-free atmosphere is equal to the product of the activity of DMP and the evaporation rate of a pure DMP droplet. The activity coefficient values obtained from microdroplet experiments are highly reproducible and agree with data available in the literature.     

Parameter study of microdroplet formation by impact of millimetre-size droplets onto a liquid film

The objective of this work is to carry out an experimental investigation on the influence of different parameters such as liquid properties (viscosity and liquid–gas surface tension), impacting droplet properties (velocity v_i and diameter d_i) and the thickness of liquid film on the emission of airborne particles produced by the impact of millimetre-size droplets onto a liquid film. Our results show that in the variation range studied, the increase of v_i and d_i or the decrease in liquid film thickness produces an increase in the mean number of microdroplets emitted by impact in the size range 2–50 μm. Furthermore, it was also observed that an increase in viscosity involves a steep decrease in the mean number of microdroplets emitted. These microdroplets may be produced by the fingers pinching of the crown formed during impact. In the case of low surface tension liquid, the formation of bubbles involves high production of droplets smaller than 15 μm. The results presented here constitute a database of the micro-droplets produced to validate models of droplet impact outcomes.     

Measurement of Activity Coefficients from Unsteady State Evaporation and Growth of Microdroplets

Techniques are presented for determination of activity coefficients of binary systems from unsteady state evaporation and growth of single microdroplets in controlled environments. A high-precision light scattering method based on resonances observed in light scattering by microdroplets was used to determine the size and composition of a microdroplet as functions of time. The techniques were validated through data on growth of glycerol microdroplets in slowly developing water vapor concentration fields and evaporation of microdroplets, containing volatile dimethylphthalate (DMP) and nonvolatile dioctylphthalate (DOP), in vapor-free atmospheres. When the water vapor concentration in the surrounding gas changes slowly a glycerol droplet maintains a dynamic equilibrium with the water vapor; thus the activity coefficient of water was determined from knowledge of the droplet composition and the water vapor saturation ratio in the gas phase. The activity coefficients of DMP were determined on the basis that the instantaneous evaporation rate of a DMP-DOP microdroplet in a vapor-free atmosphere is equal to the product of the activity of DMP and the evaporation rate of a pure DMP droplet. The activity coefficient values obtained from microdroplet experiments are highly reproducible and agree with data available in the literature.     

Numerical investigation of the evaporation of two-component droplets

A numerical model for the complete thermo-fluid-dynamic and phase-change transport processes of two-component hydrocarbon liquid droplets consisting of n-heptane, n-decane and mixture of the two in various compositions is presented and validated against experimental data. The Navier-Stokes equations are solved numerically together with the VOF methodology for tracking the droplet interface, using an adaptive local grid refinement technique. The energy and concentration equations inside the liquid and the gaseous phases for both liquid species and their vapor components are additionally solved, coupled together with a model predicting the local vaporization rate at the cells forming the interface between the liquid and the surrounding gas. The model is validated against experimental data available for droplets suspended on a small diameter pipe in a hot air environment under convective flow conditions; these refer to droplet’s surface temperature and size regression with time. An extended investigation of the flow field is presented along with the temperature and concentration fields. The equilibrium position of droplets is estimated together with the deformation process of the droplet. Finally, extensive parametric studies are presented revealing the nature of multi-component droplet evaporation on the details of the flow, the temperature and concentration fields.     

气溶胶微滴的蒸发及界面现象

用电动平衡仪和光散射技术研究气溶胶微滴蒸发及其伴随的界面现象.研究的系统有缓慢蒸发的有机物微滴、快速蒸发的水滴和含表面活性剂的水滴.实验结果表明:蒸发速率影响着微滴的温度;表面活性剂显著地降低了水滴的蒸发速率;表面电荷密度的增加会导致微滴的分裂.这些研究结论对控制工业装置中液滴的蒸发和燃烧过程有着重要意义.     

Lipid Estimation of Surfactant-Extracted Microalgae Oil Using Nile Red

Nile red staining has been used as a lipid quantification technique in many microalgae growth and oil accumulation studies. However, its application in lysed microalgae cells is limited. Therefore, this study focused on lysed microalgae cells and utilized the Nile red staining technique to evaluate oil content and extraction. This study aims to provide a rapid and high-throughput alternative method particularly in the microalgae extraction screening process. Potential interferences such as chlorophyll, β-carotene, soluble protein, and phospholipids were evaluated. The hydrophobic Nile red dye was found to quench in water, therefore the fluorometric measurement has to be completed immediately or within 5 min of dye addition. The fluorescence intensity was also found to be Nile red concentration dependent. The optimum Nile red concentration of 656 ppb was used throughout the study. In microalgae samples containing chlorophyll and carotenoids (such as Nannochloropsis sp.), Nile red fluorescence intensity was significantly reduced in comparison to non-chlorophyll microalgae (Schizochytrium limacinum). Soluble proteins from defatted microalgae did not fluoresce significantly relative to lipids, therefore did not interfere with the method to a high degree. Comparing the optimized Nile red staining method with the gravimetric lipid quantification method, a good linear correlation was found in all three materials tested (soybean oil, Nannochloropsis sp., and Schizochytrium limacinum).     

Surface temperature of acoustically levitated water microdroplets measured using infra-red thermography

A 58 kHz acoustic levitator was fitted with an infra-red thermography camera to examine the drying behaviour of water microdroplets at various drying-air temperatures. The evaporation rate was greater with larger initial droplet size at otherwise identical drying-air temperature (T_da). Measurement of droplet aspect ratio indicate that this is caused by differing acoustic field strengths. The measured droplet surface temperature in dry air showed no dependence on initial droplet size, but deviated from the wet bulb and also from the droplet temperature predicted by acoustic levitation theory. The degree of deviation of drying rate from that predicted by the d²-law using the wet bulb was dependent on T_da. Use of measured droplet surface temperature instead of the wet bulb gave, however, good agreement with the d²-law in dry air. No substantial effects of acoustic field streaming on drying rate could therefore be seen, even at the sound pressure levels of 106-165 dB used. Interpretation of evaporation rates of acoustically-levitated droplets requires therefore the measured droplet surface temperature.     

Effect of inter-particle interactions on evaporation of droplets in a linear array

Effects of interparticle interactions on evaporation have been examined on droplets in a linear array generated by a modified vibrating orifice aerosol generator (VOAG). The size as well as the spacing between the droplets was varied by modulating the frequency of the generator, and the liquid flow rate through the orifice. Ethanol and methanol droplets of differing initial sizes and spacings were studied, and the instantaneous evaporation rates as well as droplet temperatures were measured using a resonance-based light scattering technique. The results show that at a fixed dimensionless spacing between the droplets (i.e., the ratio of the spacing to the droplet radius, l/a), the size and temperature versus time data are highly reproducible. The measured evaporation rates were normalized by the evaporation rates of an isolated droplet under identical conditions to obtain the interaction parameter (η) as a function of the dimensionless spacing between the droplets. The value of η was observed to decrease with a decrease in l/a as predicted by the theoretical models, and as expected, found to be the same for ethanol and methanol droplets.     

FORMATION OF HYDROBROMIC AND HYDROCHLORIC ACID AEROSOLS IN WET FLUE GAS CLEANING PROCESSES

The formation and behavior of hydrobromic and hydrochloric acid aerosols in a wet flue gas cleaning pilot plant were investigated. The optical three-wavelength extinction (3-WE) method was used to determine mean aerosol droplet diameters and droplet number concentrations. The experimental data are compared with theoretical results of the simulation tool AerCoDe (aerosol formation in contact devices). Results are presented for a raw gas temperature of 200°C and raw gas concentrations up to 260 mg/m³(STP) for HBr, and 2500 mg/m³ (STP) for HCl. Under these conditions aerosol formation for both species is initiated by heterogeneous nucleation. It is shown that during absorption processes HBr is forming essentially higher supersaturated gas phases in comparison to HCl, resulting in higher droplet number concentrations and smaller droplet sizes. For both species the number concentration is a strong function of the maximum degree of saturation, which corresponds to the classical theory of heterogeneous nucleation. HBr aerosol droplets cannot be detected by the 3-WE method directly after the first stage of the flue gas cleaning plant (quench column) because they are smaller than the detection limit ( ≈ 500 nm) of the 3-WE method for nonabsorbing particles. In this case the droplets are enlarged in a second step, and the number concentration is determined after enlargement. Using the actual number concentration as an input parameter, the mean diameter after the first stage can be calculated with the simulation tool AerCoDe.     

Attenuation of concentration fluctuations after a quench to a temperature in the single-phase region

Attenuation of concentration fluctuations in a polystyrene/poly(2-chlorostyrene) blend after a quench from a temperature in the spinodal region to one in the single-phase region was studied by using the time-resolved light scattering technique and a scanning electron microscope. The characteristic wave number q_m, where the maximum of the scattered light intensity was located, move towards smaller values during the attenuation process. The corresponding growth of the wave length of the dominant concentration fluctuation was clearly observed by the electron microscope. The scattered light intensities at small wave numbers were found to increase before they attenuated. In the later period the power-law behavior q_m∼t^−0.3 was observed. The exponent was smaller than the value (0.5) obtained on the basis of the non-linear theory by Akcasu and collaborators.     

13C nuclear spin-lattice relaxation study of anisotropic solvent rotational diffusion in the polystyrene/trans-decalin system

The rotational diffusion tensor for the solvent trans-decalin has been determined as a function of temperature (21°–55°C) and polystyrene concentration (0–13% w/w) from ¹³C spin-lattice relaxation data. The sensitivity of rotational diffusion behaviour of the trans-decalin molecule to changes in solution conditions was found to be essentially proportional to the axial ellipticities of the respective principal rotational axes. The degree of anisotropic motion was found to increase strongly upon increasing the polystyrene concentration and decreasing the temperature. Apparent activation energies for rotation about each of the principal axes of inertia have been calculated for the series of polystyrene concentrations.     

Halogenation par voie electrochimique de composes ethyleniques—I. Bromation du cyclohexene dans differents solvants nucleophiles

The electrochemical bromination of cyclohexene has been studied in several solvents : acetonitrile, dimethylformamide, dimethylsulfoxide and methanol.In the former three solvents, the first oxidation step of the bromide ion is not modified by the cyclohexene presence, whereas the intensity of the second step depends on the amount of the added cyclohexene. In these three solvents, the oxidation current leads to same value when cyclohexene concentration increases. In methanol, the intensity of the single oxidation step of the Br^? ion increases with the cyclohexene concentration.The preparative electrolyses were realised at the different oxidation steps of Br^?. In acetonitrile only the trans-dibromo compound is formed. In the other solvents beside the 1,2-dibromo cyclohexane, bifunctional compounds are isolated; the solvent participation in the reaction is thus proved.     

Experimental determination and mathematical modeling of the drying kinetics of a single droplet of colloidal silica

ABSTRACT

Colloidal silica has been used frequently as a model material of drying in the past two decades. Several models of single droplet drying have been validated against the sole experimental evidence by Ne?i? and Vodnik (Kinetics of droplet evaporation. Chemical Engineering Science 1991, 46(2), 527–537), in which relatively scattered experimental data on drying of single droplet of colloidal silica were provided. Due to the importance of this sort of data, the drying of single droplet of colloidal silica was determined more accurately under more extensive conditions in this work. The effect of air temperature on the drying of single droplet of colloidal silica was probed as well as the evolution of particle morphology. The droplet of colloidal silica was found to shrink irregularly during drying due to uneven exposure of droplet surface to air stream. The moisture within the droplet appears to transfer freely to the surface, keeping the surface highly moist. For a large part of drying process, drying of single droplet of colloidal silica is similar to the evaporation of water droplet, which can be predicted well using a simple mathematical model.     

Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part I: Infra-red spectroscopic investigation and modelling of chain conformation and orientation changes on drawing

Quantitative infra-red data on oriented poly(ethylene terephthalate) (PET) films have been used to determine the changes in the proportions of the trans and gauche conformers of the glycol residues and the development of molecular orientation of the terephthaloyl groups as functions of draw ratio. The results are compared with predictions for the stretching of a rubber-like network based on rotational-isomeric-state (RIS) Monte Carlo (MC) modelling. It is shown that both sets of data are consistent with stretching an entangled molecular network of about 10 PET monomer units per network chain. The onset of crystallisation at a draw ratio of 2.5 affects the glycol trans-gauche conformer contents but has no detectable effect on terephthaloyl orientation.     

Diameter control of poly(p-oxycinnamoyl) microspheres prepared by self-organizing polycondensation

The diameter control of poly(p-oxycinnamoyl) (POC) microspheres was examined. POC microspheres were prepared by the polycondensation of (E)-4-acetoxycinnamic acid in liquid paraffin at 320 °C. The microspheres were formed through the formation of microdroplets by the reaction-induced liquid-liquid phase separation and the subsequent polymerization of oligomers in the microdroplets leading to the solidification. The size of the microspheres was actually governed by the coalescence of the microdroplets in the growth process. In order to control the coalescence of the microdroplets, 4-octadecyloxybiphenyl (ODB) was added to the polymerization solution as a coalescence inhibitor, which exhibited amphiphilicity to liquid paraffin and POC. ODB inhibited the coalescence of the microdroplets. The average diameter decreased with the increase of the ODB concentration, and it was tunable from 0.97 to 4.61 μm.     

Pulsed 2‐Dimensional Raman Spectroscopy on Microdroplets

In this study, we used a technique based on pulsed Raman spectroscopy to determine the average concentration in the liquid phase and concentration distribution in the gas phase surrounding desorbing linear arrays of binary microdroplets. For this study, monodisperse microdroplets consisting of acetylene dissolved in acetone were generated by a modified vibrating orifice aerosol generator (VOAG). The 2‐dimensional concentration fields of acetylene desorbed from the droplets were determined with a spatial resolution of about 40 μm both along the direction of the droplet chain and perpendicular to it. The detection limits for the gas and liquid phase concentrations were about 100 ppm and 1 mmol/l, respectively.     

Suppression of inhomogeneities in hydrogels formed by free-radical crosslinking copolymerization

Network microstructures of poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) hydrogels were compared by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebis(acrylamide) as a crosslinker. During the formation of PAAm gels, the reaction time dependence of the scattered light intensity exhibits a maximum at a critical reaction time, while in case of PDMA gels, both a maximum and a minimum were observed, corresponding to the chain overlap threshold and the gel point, respectively. This difference in the time-course between the two gelling systems is due to the late onset of gelation in the DMA system with respect to the critical overlap concentration. Compared to the AAm system, no significant scattered light intensity rise was observed during the crosslinking polymerization of DMA. It was shown that, regardless of the crosslinker ratio and of the initial monomer concentration, PDMA gel is much more homogeneous than the corresponding PAAm gel due to the shift of the gelation threshold to the semidilute regime of the reaction system. The results suggest that the spatial gel inhomogeneity can be controlled by varying the gel point with respect to the critical overlap concentration during the preparation of gels by free-radical mechanism.     

Light scattering Rayleigh linewidth measurements on some globular protein solutions

An apparatus previously described for investigating the Rayleigh linewidth of light scattered from macromolecular solutions by means of the optical homodyne technique has been modified so as to obtain the autocorrelation function of the fluctuations in scattered intensity. Its operation is described and some results presented for three commercially obtainable globular proteins: chymotrypsinogen, bovine plasma albumin and urease. Measurements were made in unbuffered 0.2M NaCl aqueous solutions and the diffusion constants, D_20w, found to be 7.4±0.3×10^?7cm²s^?1, 5.8±0.1×10^?7cm²s^?1 and 3.4±0.1×10^?7cm²s^?1 respectively. There was no detectable concentration dependence. In each case the results agreed well with theoretical predictions both as regards angular variation and shape of the autocorrelation function. The effects of polydispersity are discussed.