Droplet Chain Research Articles

Effect of crystallization kinetics on the properties of spray dried microparticles

ABSTRACTA droplet chain technique was used to study the influence of the crystallization process on the morphology of spray dried microparticles. A piezoceramic dispenser produced a chain of monodisperse solution droplets with an initial diameter in the range of 60–80 µm. Aqueous solutions of sodium nitrate were prepared in concentrations ranging from 5 mg/ml to 5⋅10−5 mg/ml. The solution droplets were injected into a laminar flow with gas temperatures varying from 25 to 150°C, affecting the droplet temperature and the evaporation rate, accordingly. Dried particles with diameters between 0.3 and 18 µm were collected. The properties of the collected microparticles were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization. The model accounted for the dependence of the diffusion coefficient of sodium nitrate in water on droplet viscosity. The viscosity trend for sodium nitrate solutions was determined by studying the relaxation time observed durin...

Analysis of the Particle Formation Process of Structured Microparticles.

The particle formation process for microparticles of cellulose acetate butyrate dried from an acetone solution was investigated experimentally and theoretically. A monodisperse droplet chain was used to produce solution microdroplets in a size range of 55-70 μm with solution concentrations of 0.37 and 10 mg/mL. As the droplets dried in a laminar air flow with a temperature of 30, 40, or 55 °C, the particle formation process was recorded by two independent optical methods. Dried particles in a size range of 10-30 μm were collected for morphology analysis, showing hollow, elongated particles whose structure was dependent on the drying gas temperature and initial solution concentration. The setup allowed comprehensive measurements of the particle formation process to be made, including the period after initial shell formation. The early particle formation process for this system was controlled by the diffusion of cellulose acetate butyrate in the liquid phase, whereas later stages of the process were dominated by shell buckling and folding.

Application of small-angle neutron scattering to the study of forces between magnetically chained monodisperse ferrofluid emulsion droplets

The optical magnetic chaining technique (MCT) developed by Leal-Calderon, Stora, Mondain-Monval, Poulin & Bibette [Phys. Rev. Lett.(1994),72, 2959–2962] allows precise measurements of force profiles between droplets in monodisperse ferrofluid emulsions. However, the method lacks anin situdetermination of droplet size and, therefore, requires a combination of separately acquired measurements of droplet chain periodicityversusan applied magnetic field from optical Bragg scattering and droplet diameter inferred from dynamic light scattering (DLS) to recover surface force–distance profiles between the colloidal particles. Compound refractive lens (CRL) focused small-angle neutron scattering (SANS) MCT should result in more consistent measurements of droplet size (form factor measurements in the absence of field) and droplet chaining period (from structure factor peaks when the magnetic field is applied), and, with access to shorter length scales, extend force measurements to closer approaches than possible by optical measurements. This article reports on CRL-SANS measurements of monodisperse ferrofluid emulsion droplets aligned in straight chains by an applied field perpendicular to the incident beam direction. Analysis of the scattering from the closely spaced droplets required algorithms that carefully treated resolution and its effect on mean scattering vector magnitudes in order to determine droplet size and chain periods to sufficient accuracy. At lower applied fields, scattering patterns indicate structural correlations transverse to the magnetic field direction owing to the formation of intermediate structures in early chain growth.

A role for autophagy during hepatic stellate cell activation

Autophagy is a metabolic process that degrades and recycles intracellular organelles and proteins with many connections to human disease and physiology. We studied the role of autophagy during hepatic stellate cell (HSC) activation, a key event in liver fibrogenesis. Analysis of the autophagic flux during in vitro activation of primary mouse HSCs was performed using a DsRed-GFP-LC3B encoding plasmid. The effect of autophagy inhibition by bafilomycin A1 on the in vitro activation process of human and mouse HSCs was examined by measuring proliferation, presence of activation markers by RT-qPCR, immunofluorescence, and Western blotting. Analysis of lipid droplet and microtubule-associated protein light chain 3 beta (LC3B) colocalization in the presence of PDGF-BB was investigated by immunocytochemistry. A significant increased autophagic flux was observed during culture induced mouse HSC activation. Treatment of mouse HSCs and human HSCs with autophagy inhibitor bafilomycin A1 results in a significant decreased proliferation and expression of activation markers. In addition, lipid droplets and LC3B colocalization was increased after PDGF-BB treatment in quiescent HSCs. During HSC activation, autophagic flux is increased. The demonstration of partly inhibition of in vitro HSC activation after treatment with an autophagy inhibitor unveils a potential new therapeutic strategy for liver fibrosis.

Generation of water–ionic liquid droplet pairs in soybean oil on microfluidic chip

Droplet-based microfluidic chips have shown great advantages in fields of chemical and biological researches. Based on the different wettabilities of water, IL (ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate) and soybean oil in hydrophobic channels, three kinds of droplet pairs (including an alternating water-IL droplet chain, connected water-IL droplet pairs and separated water-IL droplet pairs) were generated for the first time in soybean oil on microfluidic chips containing double flow-focusing regions. The influences of fluid flow rate and channel geometry on droplet pair formation were carefully studied.

Ion acceleration by short high intensity laser pulse in small target sets

AbstractIon acceleration by short, high intensity laser pulses in sets of small targets is treated by an analytical model developed here, and by two-dimensional particle-in-cell simulations. When an intense short laser pulse illuminates a thin foil target at normal incidence, electrons in the target are accelerated by the ponderomotive force. At the rear surface of the foil they generate a strong electric field that accelerates the ions, and generates an ion beam of small divergence. Using a mass-limited small target like a droplet enhances the ion energy, but increases divergence at the same time. In this paper, a combination of several-micron targets in a periodic structure (for example, a droplet chain) is proposed in order to increase the conversion efficiency from the incident laser beam to the emergent protons. Improvement of the energy flux conversion efficiency from the laser to the ion beam at optimal conditions is demonstrated.

Experimental and numerical investigation of droplet evaporation under diesel engine conditions

Evaporation of mono-disperse fuel droplets under high temperature and high pressure conditions is investigated. The time-dependent growth of the boundary layer of the droplets and the influence of neighboring droplets are examined analytically. A transient Nusselt number is calculated from numerical data and compared to the quasi-steady correlations available in literature. The analogy between heat and mass transfer is tested considering transient and quasi-steady calculations for the gas phase up to the critical point for a single droplet. The droplet evaporation in a droplet chain is examined numerically. Experimental investigations are performed to examine the influence of neighboring droplets on the drag coefficients. The results are compared with drag coefficient models for single droplets in a temperature range from T = 293–550 K and gas pressure p = 0.1–2 MPa. The experimental data provide basis for model validation in computational fluid dynamics.

Different mechanisms of nucleation and self-organization of droplets in ferroelectric smectic membranes

New mechanisms of droplet nucleation and self-organization in ferroelectric membranes are described. The droplets may be accompanied by different number of topological defects (zero, one, two) whose location may be on the droplet boundary or in the membrane. Nucleation and self-organization of droplets with total topological charge S = 0 , S = + 1 and S = - 1 were investigated. We found that an S = - 1 topological defect may be the center of both droplet nucleation and chain formation. This mechanism of chaining drastically differs from the droplet self-organization described earlier which is realized by attraction of droplet-defect pairs. Our observations demonstrate new possibilities for manipulating the inclusions and their self-organization in smectic membranes.

Laser sheet drop sizing using Raman and Mie scattering

The knowledge of droplet size distribution is very important in different medical, pharmaceutical and technical applications. Researchers in all these fields are interested in a temporally and spatially resolving two-dimensional measurement technique. A new approach for the two-dimensional determination of the droplet size is the use of two different imaging measurement techniques whose signal intensities have different dependences on the particle size. The combination of laser Raman scattering with Mie scattering is presented here. Whereas Raman scattering intensities are dependent on the measurement volume and Mie scattering on the surface, the ratio of the signal intensities of the inelastically and the elastically scattered light is therefore proportional to the droplet size. The challenge by the application of this combination of measurement techniques, the advantages and the evaluation strategy will be described exemplary shown by measurements of a single droplet chain.

Chain expansion and chain localisation in the homogenous regime of blends of liquid low molar mass polysiloxanes as revealed by neutron scattering investigations

Abstract Using small angle neutron scattering and neutron spin echo spectroscopy, two isotopic blends of low molar mass polydimethylsiloxane (PDMS) and polyethylmethylsiloxane (PEMS), and the corresponding binary blends d-PDMS/ p-PEMS and p-PDMS/d-PEMS (d: deuterated, p: protonated) were studied at the critical composition in the homogeneous regime. From the scattering data it becomes evident that coil dimensions and collective dynamics of these both blend systems behave significantly different. Compared to the isotopic mixtures, which exhibit the expected unperturbed chain dimensions and the typical Rouse relaxation, in the binary blends, which differ by a large shift with respect to the critical temperature, considerable coil expansion and spatially restricted Rouse relaxation occur. Both these structural and dynamic effects are in agreement with the model of droplet formation and chain localization, resulting from the existence of microscopic heterogeneities within the spinodal boundaries of the phase diagram. In addition, the observation of Rouse relaxation, spatially restricted to microscopic length scales, provides a new access to the molecular understanding of the critical slowing down of the mutual diffusion process, observed by photon correlation spectroscopy.

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.

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.

Observation of Fluorescence Background Suppression in Raman Scattering on Single Microparticles

AbstractRaman scattering experiments were performed on single optically levitated microparticles. In comparison with bulk experiments an dramatic enhancement of the Raman scattering relative to the fluorescence scattering was observed. Two real‐world samples, a monomer for use in laser stereolithography and an engine lubricant, were investigated. Furthermore Dioctylphthalat (DOP), doped with fluorescent dye Coumarin 6, was studied by Raman spectroscopy in a droplet chain, on an optically levitated particle and in a cuvette. The combination of Raman spectroscopy and optical levitation could be a new offspring for analytical applications of Raman spectroscopy. Liquid and solid microparticles with a few nanograms in mass can be investigated by this technique with excellent signal‐to‐noise ratio.

The boundary layer on a regular droplet chain in controlled motion

The retardation of a droplet chain by air resistance has been examined. Lee's method gives nomograms for the boundary-layer thickness, velocity distribution near the chain, and chain retardation. Experiments show that the model is applicable to important cases.

Raman scattering on liquid aerosol particles:concentration measurements on droplet chains

A new in situ technique for aqueous two-phase systems is presented which allows concentration measurements with a micrometer space resolution. The method combines the advantages of imaging and spectroscopy. The Raman scattered light from a chain of freely moving, evaporating microdroplets has been collected with a CCD camera. The Raman signals from water vapour and from liquid water could be separated. After calibration, the absolute concentrations of water in both phases could be measured simultaneously. The optics which collected the scattered light were set up in such a way that the direction of the dispersion of the monochromator and the direction of the droplet chain propagation were parallel to one axis of the CCD sensor. Therefore, from the intensity distribution along the second axis the water concentrations can be derived as a function of the distance from the axis of the droplet chain. A spatial resolution of 2,5 μm can be achieved.

Drag coefficients of single droplets moving in an infinite droplet chain on the axis of a tube

The drag of monodisperse droplets moving in an infinite droplet chain is studied numerically and experimentally. The droplet chain produced by a vibrating orifice generator is composed of an infinite number of droplets with the same diameter σ P and a constant spacing d P. The droplets move with a velocity u 0 along the axis of a cylindrical tube of radius R. In the experiments the velocity decrease of the droplets is measured by a laser Doppler velocimeter. It is shown that near the wall the droplet chain produces the same flow field as a cylindrical rod with a diameter of σ P exp(−ƒ) , where ƒ is a function of the velocity u 0, the droplet diameter σ P, the spacing of the droplets d P and the radius of the tube R. With these assumptions the Navier-Stokes equations have been solved numerically. Drag coefficients c D have been computed for Reynolds numbers from 20 to 100. The Reynolds number is based in this case on the droplet velocity u 0 and the droplet diameter σ P. The dimensionless spacing d P σ P was varied between 2 and 12 and the dimensionless tube diameter 2 R/ σ P ranged from 20 to 1000. In addition, numerical results are presented for the radial coordinate δ of the boundary, at which the flow becomes essentially parallel to the tube, for the axial velocity at this boundary u z ( z, δ) and for the maximum value of the velocity along the axis of symmetry u z ( z, 0) max. The results of the numerical calculations are compared with the experiments.