Droplet Chain Research Articles

Ferrofluid Droplet Chains in Thermotropic Nematic Liquid Crystals.

Dispersing ferrofluids in liquid crystals (LCs) produces unique systems which possess magnetic functionality and novel phenomena such as droplet chaining. This work reports the formation of ferrofluid droplet chains facilitated by the topological defects within the LC director field, induced by the dispersed ferrofluid. The translational and rotational motion of these chains could be controlled via application of external magnetic fields. The process of the droplet chain formation in LCs can be stabilized by the addition of surfactants. The magnetic colloidal particles in the ferrofluid located at the interface between the ferrofluid and the LC are arranged so that a boundary layer was formed. The velocities and boundary layer thickness values of ferrofluid droplet chains in nematic 5CB (4-Cyano-4'-pentylbiphenyl) were investigated for varying average droplet sizes and number of droplets in a chain. The creation and behaviour of ferrofluid droplet chains in 5CB with the addition of the surfactant polysorbate 60 (Tween-60) and without, was comparatively investigated. The integration of liquid crystals and ferrofluids along with the incorporation of functional materials facilitates the innovative development of advanced materials for future applications.

Two-dye two-color laser-induced fluorescence spectroscopy on droplets of green solvent water/ethanol mixtures for thermometry and mixture composition

This work shows new insights on the application of two-color laser-induced-fluorescence (2c-LIF) thermometry in a droplet chain. A two-dye mixture is used in ethanol, water and ethanol/water mixtures in order to reach a high-temperature sensitivity and avoid the detection of lasing effects in the droplets. Various droplet sizes are recorded in regard to the limitation of the detection system for very small micrometric droplets. The breakup of a droplet chain is measured to assess the spectral detection system in applications with liquid structures of different sizes. Additionally, a proposal to expand the 2c-LIF application for studying ethanol/water droplets regarding mixture composition with a third color channel is presented. Forming two intensity ratios, the spectra can be used to obtain information on the mixture composition of the solvent. Measurements in different ethanol/water mixtures containing 0–100 vol% water are evaluated to show this possibility.

Application Of Two Dye Two-Color Laser-Induced Fluorescence Spectroscopy On Droplets Of Green Solvent Mixtures Containing Water And Ethanol

In order to reach a higher sustainability, the utilization of green solvents is decisive in many technical applications. Water and ethanol substitute more complex solvents, showing advantages like ecological manufacturing, reusability or biodegradation. Both liquids are found in various industrial areas, ranging from a use as additives in biofuels, coolants or solvents for extraction in pharmaceutical, medical or food industries. Regarding these industrial processes, important variables are the liquid phase temperature and the composition in binary fluids. The focus of this work is two dye two-color laser induced fluorescence (2c-LIF) measurements in ethanol, water and their mixtures. Liquid phase thermometry via 2c LIF requires a selection of suitable fluorescence dyes regarding solubility, temperature sensitivity and the interaction among the dyes themselves. A mixture of a temperature sensitive and a temperature insensitive dye is used in order to reach a large change in signal ratio for thermometry. In this work, two dyes – fluorescein disodium (FL) and sulforhodamine 101 (SRh), as proposed by Chaze et al. (Chaze et al. 2016) – are analyzed regarding their absorption and emission behavior in various ethanol/water mixtures, which are examples of "green solvents". The absorption spectra of the dissolved dye mixture are measured with a spectrophotometer at temperatures from 293 K to 343 K. Information on the temperature dependent absorption of the single dyes and reabsorption effects in ethanol/water mixtures is obtained. Additionally, the fluorescence of the mixtures is analyzed spectrally and by an imaging system. Therefore, a fiber coupled spectrometer and a system with two sCMOS cameras equipped with a long distance (LD) microscope are applied for simultaneous detection of the signal of tempered solutions a monodisperse droplet chain. A study of small droplets with sizes below 50 μm are evaluated and the limit of the spectral detection system is determined and discussed. The application of the measurement technique to several solvent mixtures is investigated. Additionally, an application for composition investigations of binary water/ethanol droplets via 3c-LIF is proposed.

Modular Droplet‐Based Fluidics for Large Volume Libraries of Individual Multiparametric Codes in Lab‐On‐Chip Systems

AbstractDroplet‐based lab‐on‐a‐chip systems offer vast possibilities in the manipulation, guidance, tracking, and labeling of individual droplet‐based bioreactors. One of the targeted application scenarios is in drug discovery where millions of unique codes are required, which is out of reach for current technologies. Here, a concept for the realization of multiparametric codes, where information is stored in distinct physical and chemical parameters, is proposed and validated. Exemplarily, the focus is on the use of impedance and magnetic sensing by monitoring ionic concentration as well as magnetic content per droplet and droplet volume. Codes based on aqueous ferrofluid droplets are prepared using a tubing‐based millifluidic setup and consist of up to six droplets of different combinations of volumes and magnetic concentration. It is demonstrated that a droplet chain of three single droplets of different volumes with nine different magnetic nanoparticle concentrations accompanied with four different ionic concentrations per droplet offers up to 3 million unique codes. The developed fluidic platform can be readily extended to other types of sensors including optical ones to boost the coding capacity even further.

A Tri‐Droplet Liquid Structure for Highly Efficient Intracellular Delivery in Primary Mammalian Cells Using Digital Microfluidics

Automated techniques for mammalian cell engineering are needed to examine a wide range of unique genetic perturbations especially when working with precious patient samples. An automated and miniaturized technique making use of digital microfluidics to electroporate a minimal number of mammalian cells (≈40 000) at a time on a scalable platform is introduced. This system functions by merging three droplets into a continuous droplet chain, which is called a triDrop. In the triDrop configuration, the outer droplets are comprised of high‐conductive liquid while an inner or middle droplet comprising of low‐conductivity liquid that contains the cells and biological payloads. In this work, it is shown that applying a voltage to the outer droplets generates an effective electric field throughout the tri‐droplet structure allowing for insertion of the biological payload into the cells without sacrificing long‐term cell health. This technique is shown for a range of biological payloads including plasmids, mRNA, and fully formed proteins being inserted into adherent and suspension cells which include primary T‐cells. The unique features of flexibility and versatility of triDrop show that the platform can be used for the automation of multiplexed gene edits with the benefits of low reagent consumption and minimal cell numbers.

Droplet thermometry based on an optimized two dye two-color laser-induced fluorescence concept

In this work two-color laser-induced fluorescence (2c-LIF) is applied to calibrate and measure the temperature in a micrometric monodisperse ethanol droplet chain. A dye mixture of fluorescein disodium (FL) and sulforhodamine 101 (SRh) is used, in order to form a sensitive signal ratio of one temperature dependent and one independent fluorophore. Spectral and planar fluorescence detection via a fiber-coupled spectrometer and a camera system, consisting of two sCMOS cameras, is set up around the droplet chain. Additionally, absorption measurements of the dye mixture in ethanol are conducted using a photo-spectrometer to analyze the temperature sensitivity of the dyes as well as potential re-absorption and fluorescence cross-talk effects. The spectral setup allowed an investigation of the wavelength range in which morphology dependent resonances (MDR) occur at the phase boundary of the droplet. Thus, the optical filters, determining the color channels in the camera system, are chosen to avoid detection of the lasing signal and providing maximal temperature sensitivity at the same time. The calibrated signal ratios are applied in temperature measurements of evaporating heated droplets in the droplet chain, showing the cooling with larger distance from the nozzle.

Impact of Surface Roughness on the Impingement of Urea–Water Solution Droplets

The understanding of impingement processes is crucial for optimizing automotive selective catalytic reduction (SCR) systems. An accurate description of this behavior helps design exhaust systems and increases the validity of modeling approaches. A component test bench was set up, featuring a droplet chain generator for producing droplet sizes typically found in the urea–water solution sprays of SCR systems. A heatable impingement plate with an interchangeable surface enabled investigation of the influence of surface roughness. Data were acquired using a high-speed camera and image postprocessing. The droplet–wall interaction could be described using different regimes. An approach to characterizing impingement behavior based on weighted-regime superposition enabled gradual transitions between regimes, instead of step-like changes. It was observed that the surface roughness increased the droplet–solid contact area and generated thermal-induced secondary droplets at lower temperatures. A region of enhanced mechanical disintegration of the droplet was found, caused by peaks of the surface shearing off parts of the droplet. The probability of a droplet rebounding from the wall was reduced on a rough surface, due to the interference of the surface spikes with the droplet’s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface’s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed.

Classical-linear-chain behavior from dipolar droplets to supersolids

We investigate the classicality of linear dipolar droplet arrays through a normal mode analysis of the dynamical properties in comparison to the supersolid regime. The vibrational patterns of isolated-droplet crystals that time-evolve after a small initial kick closely follow the properties of a linear droplet chain. For larger kick velocities, however, droplets may coalesce and separate again, showing distinct deviations from classicality. In the supersolid regime the normal modes are eliminated by a counterflow of mass between the droplets, signaled by a reduction of the center-of-mass motion. Our study effectively captures the vibrational patterns of isolated droplets in the presence of three-body loss and can be generalized to systems with a larger number of droplet arrays.

Branched droplet clusters and the Kramers theorem

Scaling laws inherent for polymer molecules are checked for the linear and branched chains constituting two-dimensional (2D) levitating microdroplet clusters condensed above the locally heated layer of water. We demonstrate that the dimensionless averaged end-to-end distance of the droplet chain r[over ¯] normalized by the averaged distance between centers of the adjacent droplets l[over ¯] scales as r[over ¯]/l[over ¯]∼n^{0.76}, where n is the number of links in the chain, which is close to the power exponent ¾, predicted for 2D polymer chains with excluded volume in the dilution limit. The values of the dimensionless Kuhn length b[over ̃]≅2.12±0.015 and of the averaged absolute value of the bond angle of the droplet chains |θ|[over ¯]=22.0±0.5^{0} are determined. Using these values we demonstrate that the predictions of the Kramers theorem for the gyration radius of branched polymers are valid also for the branched droplets' chains. We discuss physical interactions that explain both the high value of the power exponent and the applicability of the Kramers theorem including the effects of the excluded volume, surrounding droplet monomers, and the prohibition of extreme values of the bond angle.

On the feasibility of spray-dried eudragit-trehalose microparticles for enteric drug delivery

Enteric infections have long constituted a silent epidemic responsible for hundreds of thousands of deaths around the world every year. Because of the global rise in antibiotic-resistant bacteria and the slow development of new small-molecule antibiotics, alternatives such as bacteriophage therapy have become a much sought-after option in the treatment of enteric infections. However, the administration of therapeutics through the oral route to target gastrointestinal infections poses challenges to dosage formulation because these active ingredients, particularly relatively fragile biological entities, require protection from the stomach’s harsh acids. Encapsulation of the therapeutics within a pH-responsive coating capable of surviving low pH conditions has the potential to provide such protection. In this study, we developed a spray-dried powder vehicle capable of withstanding low pH comparable to stomach conditions, using Eudragit® S100 as a protective particle coating and trehalose as a stabilizing excipient for a possible active component. A particle formation model and a monodisperse droplet chain technique were initially used to study the formation process of Eudragit-trehalose composite microparticles at different ratios and in different ratios of water–ethanol solvent, which showed formation of particles with Eudragit shells varying in thickness from 0.13 μm to 0.75 μm. Promising Eudragit-trehalose formulations were subsequently spray-dried and their survival in acidic and alkaline environments studied using a new shadowgraphic imaging method. The results demonstrated that Eudragit was capable of creating a protective shell in the particles irrespective of the type of solvent used to prepare the formulations. The trehalose cores of particles with higher than 5% w/w of Eudragit remained protected after one hour of exposure at pH 2, indicating the potential of Eudragit-trehalose formulations for enteric delivery of drugs.

Trileucine as a dispersibility enhancer of spray-dried inhalable microparticles

The formation of trileucine-containing spray-dried microparticles intended for pulmonary delivery was studied in depth. A single-particle method was employed to study the shell formation characteristics of trileucine in the presence of trehalose as a glass former, and an empirical correlation was proposed to predict the instance of shell formation. A droplet chain instrument was used to produce and collect monodisperse particles to examine morphology and calculate particle density for different levels of trileucine. It was observed that the addition of only 0.5 mg/mL (10% w/w) trileucine to a trehalose system could lower dried particle densities by approximately 1 g/cm3. In addition, a laboratory-scale spray dryer was used to produce batches of trileucine/trehalose powders in the respirable range. Raman spectroscopy demonstrated that both components were completely amorphous. Scanning electron microscopy and time-of-flight secondary ion mass spectrometry were used to study the particle morphologies and surface compositions. For all cases with trileucine, highly rugose particles with trileucine coverages of more than 60% by mass were observed with trileucine feed fractions of as little as 2% w/w. Moreover, it was seen that at lower trileucine content, smaller and larger particles of a polydisperse powder had slightly different surface compositions. The surface activity of trileucine was also modeled via a modified form of the diffusion equation inside an evaporating droplet that took into account initial surface adsorption and eventual surface desorption due to droplet shrinkage. Finally, using the Flory-Huggins theory, it was estimated that at room temperature, liquid-liquid phase separation would start when the trileucine reached an aqueous concentration of about 18 mg/mL. Besides the surface activity of trileucine, this low concentration was assumed to explain the substantial effect of trileucine on the morphology of spray-dried particles due to early phase separation. The methodology proposed in this study can be used in the rational design of trileucine-containing microparticles.

Non‐coalescence and chain formation of droplets under an alternating current electric field

AbstractThe dynamic behaviors of two droplets and droplet cluster under an alternating current (AC) electric field are investigated. Two droplets generally undergo transformation from complete coalescence to partial coalescence and finally to non‐coalescence as the electric capillary number Cap increases. The critical electric capillary number Capc for complete coalescence in the AC electric field remains unchanged and is twice as large as that in the direct current (DC) electric field when the frequency f ≥ 250 Hz. Charge transfer and reversal of electric field result in the reversal of the direction of electric force, which is the fundamental mechanism of non‐coalescence of two droplets and chain formation in droplet cluster. The number of rebounds dramatically increases as f increases, promoting the stability of droplet chain. The droplet chains in the high‐frequency AC electric field are longer and more stable than those in the low‐frequency AC electric field.

On the particle formation of leucine in spray drying of inhalable microparticles

The particle formation of L-leucine, a dispersibility-enhancing amino acid used in the spray drying of inhalable pharmaceutical aerosols, was extensively studied using three experimental methods, and the results were interpreted with the aid of theory. A comparative-kinetics electrodynamic balance was used to study the shell formation behavior in single evaporating microdroplets containing leucine and trehalose. Different concentration thresholds of solidification and shell formation were determined for trehalose and leucine, which were then used in the particle formation model to predict the properties of spray-dried particles. Furthermore, a droplet chain instrument was used to study the particle morphologies and particle densities that were not accessible in the single particle experiments. Lab-scale spray drying was also used to produce powders typical for actual pharmaceutical applications. Raman spectroscopy confirmed that a glass former, such as trehalose, can inhibit the crystallization of leucine. The surface compositions of these spray-dried powders were analyzed via time-of-flight secondary ion mass spectrometry. The leucine surface coverage in a polydisperse powder was determined to be a function of the particle size or the initial droplet diameter of each respective particle. This observation confirms the important role of leucine crystallization kinetics in its shell-forming capabilities. A critical supersaturation ratio of 3.5 was also calculated for leucine, at which it is assumed to instantaneously nucleate out of solution. This ratio was used as the threshold for the initiation of crystallization. Crystallinity predictions for the leucine-trehalose particles based on this supersaturation ratio were in good agreement with the solid-state characterizations obtained by Raman spectroscopy. This study improves the fundamental understanding of the particle formation process of leucine-containing formulations, which can apply to other crystallizing systems and potentially facilitate the rational design of such formulations with reduced experimental effort.

Charge-Transfer-Induced Noncoalescence and Chain Formation of Free Droplets under a Pulsed DC Electric Field.

Electrocoalescence technology is an important method for the demulsification of crude oil emulsion, but its development is restricted by the short circuit caused by droplet chain formation. To reveal the formation mechanism of droplet chains, the electrocoalescence behaviors of two droplets and droplet clusters under pulsed direct current (DC) electric fields are experimentally studied. The two droplets usually successively undergo complete coalescence, partial coalescence, and noncoalescence as the electric field strength increases. The critical electric field strengths for complete coalescence under pulsed DC electric fields with different frequencies are obtained. The effects of the electric field waveform and frequency on the noncoalescence characteristics of two droplets and the stability of droplet chains are explored. The droplet chains under a high-frequency electric field are more stable and longer than those under a low-frequency electric field due to the reduction of the movement distance and the generation of daughter droplets from tip streaming. The reversal of the composition of electric forces due to charge transfer is the fundamental mechanism of noncoalescence of two droplets and chain formation in the emulsion under a pulsed DC electric field.

Light Diffraction in Magnetic Emulsions with High Interfacial Tension

Specific features of small-angle light scattering by aqueous magnetic emulsions under the action of a magnetic field and a hydrodynamic field of a rotating fluid have been investigated. It has been shown that the experimentally observed diffraction lines that have no pronounced maxima and minima are due to diffraction on an irregular system of droplet chains arising in the magnetic field. Under the combined action of the magnetic and hydrodynamic fields, a rotation of a diffraction line is observed. This phenomenon has been interpreted using the known models of orientation of extended particles having complex shapes. It has been shown that the experimental nonlinear dependence of the rotation angle of a droplet chain on the angular rate of fluid rotation may be explained by S-shaped deformations of a chain.

Measurement of evaporation of hydrocarbon droplets by laser absorption spectroscopy

Concentration measurements of evaporated hydrocarbon species by infrared laser absorption spectroscopy at a monodisperse droplet chain are presented. A droplet generator was installed within a flow channel and operated with cyclohexane, iso-octane, n-heptane, n-pentane and 1-butanol. The flow channel was flushed with a laminar flow of air at different temperatures. The absorption of a HeNe laser beam at $$\lambda = 3.39\,\upmu \hbox {m}$$ traversing through the flow channel at varying distances from the droplet chain was exploited to determine the vapor concentrations of the hydrocarbons. Measurements of the absorption cross sections in a heated gas cell ($$T = 300$$–773 K) enabled the quantification of the absorption signals from the droplet chain. Vapor concentrations were determined in planes perpendicular to the droplet chain. From the increase of vapor concentration between the planes, the evaporation rate could be determined. The evaporation rates were measured in dependence of co-flow temperature, droplet velocity, droplet generation frequency and droplet spacing. In the investigated temperature range of the air (313–430 K) the evaporation rates increased linearly with temperature. The order of the fuels with respect to evaporation rates corresponded with the boiling points of the individual fuels. In addition to the presentation of the results, the paper discusses the performance of vapor concentration measurements by laser absorption spectroscopy at droplet chains which has not been tested before in such a configuration. Particular attention was paid to the spatial resolution of the measurement. The results are well suited to validate models and numerical simulations.

A Low-power Liquid-fuelled Burner Using a Novel Atomization Concept

ABSTRACTThe design of and experimental results for a low-power heating oil burner, operating between 1.25 and 5.1 kW with exhaust gas emissions within the normative regulations for domestic usage, are presented. In contrast to common domestic heating oil burners, the atomization and combustion sections are spatially separated to allow the implementation of a novel atomization concept for low-power liquid-fuelled burners proposed in a previous study. The main working principle for spray generation is the secondary atomization of a droplet chain impinging on a pin downstream. Through pulse-width modulation (PWM) of the droplet generator, modulation of the spray flowrate over a wide range with constant spray quality was achieved. Phase-Doppler anemometry measurements under non-reacting conditions were performed to obtain the droplet size distribution of the spray. Three model liquids were used to determine the influence of liquid properties on the droplet size distribution. The Sauter mean diameter (SMD) values obtained for all liquids and operation parameters studied are in the range of 10% of the primary droplet diameter, which is 30 µm for the setup in this study.

Multi-Solvent Microdroplet Evaporation: Modeling and Measurement of Spray-Drying Kinetics with Inhalable Pharmaceutics.

Evaporation and particle formation from multi-solvent microdroplets containing solid excipients pertaining to spray-drying of therapeutic agents intended for lung delivery were studied. Various water and ethanol co-solvent systems containing a variety of actives and excipients (beclomethasone, budesonide, leucine, and trehalose) were considered. Numerical methods were used to predict the droplet evaporation rates and internal solute transfers, and their results verified and compared with results from two separate experimental setups. In particular, an electrodynamic balance was used to measure the evaporation rates of multicomponent droplets and a monodisperse droplet chain setup collected dried microparticles for further analytical investigations and ultramicroscopy. The numerical results are used to explain the different particle morphologies dried from solutions at different co-solvent compositions. The obtained numerical data clearly show that the two parameters controlling the general morphology of a dried particle, namely the Péclet number and the degree of saturation, can change with time in a multi-solvent droplet. This fact complicates product development for such systems. However, this additional complexity vanishes at what we define as the iso-compositional point, which occurs when the solvent ratios and other composition-dependent properties of the droplet remain constant during evaporation, similar to the azeotrope of such systems during distillation. Numerical and experimental analysis of multi-solvent systems indicate that spray-drying near the iso-compositional ratio simplifies the design and process development of such systems.

Wetting Behavior of Polymer Droplets: Effects of Droplet Size and Chain Length

Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adsorbed at solid surfaces as a function of the number of chains making up the droplets and varying droplet sizes. The wetting behavior is quantified via the ratio of the perpendicular to the parallel component of the effective radii of gyration of the droplets and is analyzed further in terms of the adsorption behavior of the polymer chains and the monomers that constitute the droplets. This analysis is complemented by an account of the shape of the droplets in terms of the principal moments of the radius of gyration tensor. Single-chain droplets are found to lie flatter and wet the substrate more than chemically identical multichain droplets, which attain a more globular shape and wet the substrate less. The simulation findings are in good agreement with atomic force microscopy experiments. The present investigation illustrates a marked dependence of wetting and adsorption on certain structural arrangements and ...

Weakly nonlinear instabilities of a liquid ring

The weakly nonlinear instability of a liquid ring is investigated by perturbation method and illustrates several important features at the early stage of evolution after destabilization. When the centerline maintains circular, the ring would transform into a droplet chain during its shrinkage, but tends to possess some local pinching during the expansion. If there is an additional distortion of the centerline, an evident tendency to protrude spikes, on each of which a droplet may be formed, is predicted. The analysis concentrates on inviscid cases, because the viscosity of ring liquid is proven to merely affect the base state. Besides, the interaction between instability modes is studied to understand the irregularity of perturbed surface and centerline, and the four locations where the irregularity appears to the maximum extent are discovered. All these results are confirmed by numerical simulations of the same model and experimental observations on ring-shaped the rim of drop-splash ejecta, and hence are related intrinsically with the spike formation process.