Composite Droplets Research Articles

Resisting effects of alkanes on the stability and deformation of W1-O-W2 droplets

W1-O-W2 composite droplets are prepared using droplet microfluidics and core-hollow poly(α-methylstyrene) (PAMS) shells are fabricated by microencapsulation. This work examines the effects of alkanes on stability and deformation of double droplets. Results manifest that alkanes benefit resisting the deformation of the double droplets and improving the sphericity of the resulting polymer shells. Moreover, with shortening the chain length of the alkane, the deformation of double droplets is resisted and the sphericity of the solidified shells is improved. However, the addition of alkanes promotes the coalescence between the droplets, decreasing the yield of individual shells. The results presented in this work provide a feasible method to fabricate individual polymer shells with high sphericity.

A simple model for puffing/micro-explosions in water-fuel emulsion droplets

A new simple model for the puffing/micro-explosion of water-fuel emulsion droplets is suggested. The model is based on the assumption that a spherical water sub-droplet is located in the centre of a larger fuel droplet. The fuel is approximated by n-dodecane. The fuel droplet surface temperature is assumed to be fixed, and fuel evaporation is ignored. The heat conduction equation is solved inside this composite droplet with the Dirichlet boundary condition at the surface of the fuel droplet. The time instant when the temperature at the interface between water and fuel reaches the boiling temperature of water is associated with the start of the puffing process leading to micro-explosion. This time is referred to as the time to puffing, or micro-explosion delay time. When the fuel surface temperature is equal to the boiling temperature of fuel then this time is expected to be the shortest of the possible times. The predictions of the model are shown to be in agreement with available experimental data. The model predicts an increase in this time with increase in the fuel droplet size for fixed water and fuel mass fractions, in agreement with observations.

Designer liquid-liquid interfaces made from transient double emulsions

Current methods for generating liquid-liquid interfaces with either controlled composition or coverage often rely on adsorption equilibria which limits the freedom to design such multiphase materials, in particular when different components are used. Moreover, when interfaces become densely populated, slowing down of adsorption may impose additional constraints. Up to now, it is not possible to control surface coverage and composition of droplet interfaces at will. Here, we report a generic and versatile method to create designer liquid-liquid interfaces, using transient double emulsions. We demonstrate how the surface coverage in Pickering emulsions can be controlled at will, even for dense particulate layers going up to multilayers. Moreover, composite droplet interfaces with compositional control can be generated, even with particles which would have intrinsically different or even opposite adsorption characteristics. Given its simplicity, this method offers a general approach for control of composition of liquid-liquid interfaces in a variety of multiphase systems.

Relaxation Behavior of Core-Shell Particles in PP/PMMA/PS Ternary Blends: The Effects of Composition, Viscosity Ratio and Temperature

AbstractThe relaxation behavior of PMMA-PS core-shell particles in PP matrix was studied. The effects of composition and viscosity ratio of core and shell forming components on relaxation processes of PP/(PMMA+PS) 80/20 ternary blends were evaluated at three different temperatures (200, 225 and 250°C) and discussed. For the ternary blends containing low-viscosity PMMA (PP/L-PMMA/PS), the shape relaxation time, λ, sharply reduced at first with PS content and then increased after passing through a minimum with further increase in the PS content. The corresponding relaxation strength, λ.H(λ), increased suddenly at first and then monotonically decreased upon further incorporation of PS. The changes of λ and λ.H(λ) with PS content were attributed to a change in the interfacial energy (shell formation) and average viscosity of the dispersed composite droplets upon progressive addition of PS into PP/L-PMMA blend so that the minimum λ corresponding to the maximum λ.H(λ) was observed at shell completion composition with finest core-shell- particles (12 wt% of PS phase). The core-shell particles of all the ternary blends containing L-PMMA exhibited a shape relaxation at 200°C, while at the same temperature for the blends containing high-viscosity PMMA (H-PMMA) no shape relaxation peak was detected when the PS shell content was lower than 62 wt%. This was attributed to restriction effect of non-deformable H-PMMA core droplets, which led to no droplet relaxation. At higher PS contents, appearance of the shape relaxation peak was related to the relaxation of PS shell, suggesting that there is a critical shell thickness above which the relaxation behavior of core-shell droplets is dominated by the low-viscous PS shell. Increase in temperature from 200 to 250°C reduced the PS shell content corresponding to onset of deformation and accelerated the relaxation of composite droplets.

Analysis of meniscus beneath metastable droplets and wetting transition on micro/nano textured surfaces**Project supported by the National Natural Science Foundation of China (Grant No. 21676041).

The expressions of interface free energy (IFE) of composite droplets with meniscal liquid–air interface in metastable state on micro/nano textured surfaces were formulated. Then the parameters to describe the meniscus were determined based on the principle of minimum IFE. Furthermore, the IFE barriers and the necessary and sufficient conditions of drop wetting transition from Cassie to Wenzel were analyzed and the corresponding criteria were formulated. The results show that the liquid–air interface below a composite droplet is flat when the post pitches are relatively small, but in a shape of curved meniscus when the piteches are comparatively large and the curvature depends on structural parameters. The angle between meniscus and pillar wall is just equal to the supplementary angle of intrinsic contact angle of post material. The calculations also illustrate that Cassie droplets will transform to Wenzel state when post pitch is large enough or when drop volume is sufficiently small. The opposite transition from Wenzel to Cassie state, however, is unable to take place spontaneously because the energy barrier is always positive. Finally, the calculation results of this model are well consistent with the experimental observations in literatures for the wetting transition of droplets from Cassie to Wenzel state.

Composite zein - propylene glycol alginate particles prepared using solvent evaporation: Characterization and application as Pickering emulsion stabilizers

There is great interest in identifying biopolymer-based particles to stabilize Pickering emulsions. Pure zein particles are easy to fabricate but are unsuitable for this purpose because of their high hydrophobicity. In this study, composite zein - propylene glycol alginate particles (ZPGAPs) were prepared using a solvent evaporation method. The potential of using these particles to stabilize oil-in-water emulsions through a Pickering mechanism was also studied. ZPGAPs prepared at a zein-to-PGA mass ratio of 10:1 had considerably smaller particle diameters (346 nm) than those prepared using pure zein (1400 nm) as determined by light scattering and scanning electron microscopy (SEM). The interface wettability of ZPGAPs mainly depended on the zein-to-PGA mass ratio. The three-phase contact angle (θ) of pure zein particles was around 68.1° whereas that of ZPGAPs was around 90°, suggesting that the composite particles had suitable wettability characteristics for stabilizing Pickering emulsions. Confocal laser scanning microscope (CLSM) images showed that ZPGAPs formed a densely packed layer around the surfaces of the oil droplets, further highlighting the potential of the composite particles as emulsion stabilizers. The elastic modulus (G′) was higher than the loss modulus (G″) in all the Pickering emulsions, which was indicative of the formation of an elastic gel-like structure. The rheological properties of the Pickering emulsions could be modulated by altering the composite particle concentration, droplet concentration, ionic strength, pH, and temperature. These results suggest that the ZPGAPs developed in this work may be a promising stabilizer of Pickering emulsions and may extend the application of zein in food products.

Study of the partial wetting morphology in polylactide/poly[(butylene adipate)‐co‐terephthalate]/polyamide ternary blends: case of composite droplets

AbstractThe prediction of the morphology of ternary polymer blends requires a good knowledge of the values of the three interfacial tensions. We selected three polymers, either biobased or biodegradable, polyamide (PA), poly[(butylene adipate)‐co‐terephthalate] (PBAT) and polylactide (PLA), and we accurately measured their interfacial tensions using the retraction method, varying the molar mass or inverting the phases. The following values of interfacial tension were obtained: γPBAT/PLA = 3.3 ± 0.7 mN m−1, γPA/PLA = 5.6 ± 0.3 mN m−1 and γPBAT/PA = 3.0 ± 0.4 mN m−1. These values were used to calculate the spreading coefficients giving rise to two negative coefficients and one coefficient close to zero. Ternary blends with various compositions, two different levels of viscosity for PBAT and different processing conditions were prepared. There was a very good agreement between the predictions of the spreading theory, when using the values of interfacial tension of the right order of magnitude, and the observed morphologies, whatever the polymer serving as a matrix. When PLA or PBAT was chosen as the matrix, the ternary blend morphology was composed of composite droplets, presenting a partial wetting morphology, dispersed in the polymer matrix. This morphology was observed whatever the composition, the viscosity of the PBAT phase and the processing conditions. A further calculation of the free energy confirmed this morphology. The formation process of this semi‐encapsulated morphology was observed during blending. © 2018 Society of Chemical Industry

Hydrodynamics of composite droplet with high-viscosity shell subjected to shear flow in the narrow channel

In this work, the deformation and breakup behavior of the single-core composite and homogeneous droplets subjected to a simple shear flow between two parallel walls have been investigated numerically. The attention was focused on the studying of the high-viscosity droplets, which can be noticeably deformed or broken only in narrow channels. The influence of a low-viscosity core on hydrodynamics of the composite droplet with a high-viscosity shell has been studied for the first time at different capillary numbers and confinement degrees (the ratio of a droplet diameter to a channel thickness). It was found that a composite droplet is more stable as compared to a similar high-viscosity homogeneous droplet and breaks up at a higher capillary number.

ID: 1054 Comparison between fresh and cryopreserved Human Amnion Mesenchymal Stem Cells (HAMCs) in terms of serial passaging, morphology and differentiation potential during long term culture

The therapeutic efficiency of Human Amnion Mesenchymal stem cells (HAMCs) is significantly promising. However, its sustainability in long term sub-cultivation has yet to be identified, especially post-cryopreservation. There are concerns whether stem cells which are cryopreserved will be able to retain its functions and differentiation potential efficiently. The aim of this study is to determine the effects of cryopreservation on the serial passaging, morphology and differentiation potential of HAMCs culture post-cryopreservation compared to fresh HAMCs culture. HAMCs was isolated through serial enzymatic digestion of the amnion membrane of human term placenta after delivery. Fresh HAMCs cultures were sub-cultivated until passage 15 while cryopreserved HAMCs samples was preserved at passage 2 of the cell culture and revived after 3 months of cryopreservation period. Through microscopic observation, the cryopreserved HAMCs started to flatten and become larger in size, losing it fibroblastic feature as early as passage 6. The enlarged and flatten morphological features of the HAMCs indicate that the cells began to lose its proliferative activity at this point. In comparison, the HAMCs normally began to enlarge when reaching passage 15 in non-cryopreserved HAMCs cultures. Fresh HAMCs cultures were able to be sustained up to 15 passages while cryopreserved HAMCs were not able to survive pass 10 passages of cell culture. Furthermore, it was observed that both fresh and cryopreserved HAMCs were able to retain its differentiation potential through osteogenesis and adipogenesis medium induction. This was experimentally visualized through positive staining of the calcium composite and lipid droplet in the induced HAMCs culture using Von Kossa stain solution and Oil Red-O stain solution, respectively. Despite retaining it differentiation potential, cryopreserved HAMCs were only able to survive a few passages after being revived. The result suggests that fresh HAMCs is a more suitable candidate to be used in cellular therapy and various clinical application as it were able to retain all its function in long term culture.

Ultrafast ultrasound localization microscopy

The resolution of ultrasound imaging is limited by the classical wave diffraction theory and corresponds roughly to the ultrasonic wavelength (from 0.2 to 1 mm for clinical applications). Current methods for in vivo microvascular imaging are limited by trade-offs between the depth of penetration and resolution. Inspired by the optical localization techniques (FPALM), we developed the technique ultrafast ultrasound superlocalization (uULM) where the resolution is not limited by the wavelength (Couture et al. 2011, Desailly et al. 2013). The use of ultrafast ultrasound acquisitions, based on plane wave transmissions at the rate of thousand frames per second, enabled the separation of million microbubbles achieving a resolution of about 8 μm at 12 mm depth for the vascular structure of the rat brain in-vivo (Errico et al., 2015). Moreover, we have lately demonstrated that by combining acoustic vaporization of composite droplets and rapid ultrasound monitoring, ultrasound drug-delivery can also be attained with subwavelength precision (Hingot et al., 2016).

Aluminum nitride–filled elastic silicone rubber composites for drag reduction

This study presents aluminum nitride/silicone rubber composite as a drag reduction material, inspired by the boundary heating drag reduction mechanism of dolphin skin. Aluminum nitride was added as a thermal conductive filler at weight fractions of 16.67, 21.05, and 28.57 wt% to pristine silicone rubber. Tests of the thermal conductivity and tensile properties showed that the thermal conductivity of all three aluminum nitride/silicone rubber composites were increased 27.9%, 41.4%, and 43.7% than that of the pristine silicone rubber, and the elastic modulus of the composites was increased with the aluminum nitride content. Droplet velocity testing, which can reflect the drag reduction mechanism of the heating boundary controlled by the aluminum nitride/silicone rubber composites, was performed between all three aluminum nitride/silicone rubber composites and pristine silicone rubber. The results showed that the droplet velocity of all three aluminum nitride/silicone rubber composites were higher than pristine silicone rubber, implying that the composites had a drag-reducing function. In terms of the drag-reducing mechanism, the heat conductivity performance of the aluminum nitride/silicone rubber accelerates the heat transfer between the aluminum nitride/silicone rubber composite surface and droplet. The forces between the molecules and droplet dynamic viscosity are reduced, which result in drag reduction. The application of aluminum nitride/silicone rubber composite to control fluid medium will have important value for fluid machinery.

Formation process of core‐shell microparticles by solute migration during drying of homogenous composite droplets

Particle formation process during spray drying profoundly impacts particle morphology and microstructure. This study experimentally investigated the formation of core‐shell‐structured microparticles by one‐step drying of composite droplets made of Eudragit® RS (a polycationic acrylic polymer in nanoparticle form) and silica sol. The formation of an incipient surface shell was monitored using single droplet drying technique, and the freshly formed shell was recovered for subsequent analyses. Adding silica to RS precursor increased the shell formation time and altered the properties of the incipient shell from water‐dispersible to nondispersible. The incipient shell formed from RS/silica droplets with mass ratios of 1.5%:1.5% and 0.5%:2.5% showed ingredient segregation with a preferential accumulation of RS, similar to the shell region of dried microparticles. After shell formation, excess silica sol in the liquid phase could flow inward, producing a dense core. This mobility‐governed solute migration would be useful for constructing core‐shell microparticles using other precursor systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3297–3310, 2017

In situ targeted activation of an anticancer agent using ultrasound-triggered release of composite droplets

The efficiency of a drug is usually highly dependent on the way it is administered or delivered. As such, targeted-therapy, which requires conceiving drug-delivery vehicles that will change their state from a relatively stable structure with a very slow leak-rate to an unstable structure with a fast release, clearly improves the pharmacokinetics, the absorption, the distribution, the metabolism and the therapeutic index of a given drug. In this context, we have developed a particularly effective double stimuli-responsive drug-delivery method allowing an ultrasound-induced release of a monomethylauristatin E-glucuronide prodrug and its subsequent activation by a β-glucuronidase. This led to an increase of cytotoxicity of about 80% on cancer cells.

Droplet Merging on a Lab-on-a-Chip Platform by Uniform Magnetic Fields.

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

Subwavelength far-field ultrasound drug-delivery

The theoretical diffraction-limit of resolution for ultrasound imaging has recently been bypassed in-vitro and in-vivo. However, in the context of ultrasound therapy, the precision of therapeutic beams remains bound to the half-wavelength limit. By combining acoustic vaporization of composite droplets and rapid ultrasound monitoring, we demonstrate that the ultrasound drug-delivery can be restricted to a subwavelength zone. Moreover, two release zones closer than the wavelength/4 can be distinguished both optically and through ultrafast ultrasound localization microscopy. This proof-of-concept let us envision the possibility to treat specific tissues more precisely without compromising on the penetration depth of the ultrasound wave.

Evolution of shell formation process in PP/PMMA/PS ternary blends: correlation between the melt rheology and phase morphology

Correlation between the melt rheology and phase morphology of PP/PMMA/PS ternary blends during the shell formation process were studied in detail. In this PP-matrix ternary system, theoretical predictions in agreement with the direct SEM observations demonstrated the core-shell morphology with PMMA and PS phases as core and shell phases, respectively. Morphological observations revealed that the complete shell formation takes place at about 12 and 9 wt% of PS minor phase in ternary blends composed of low and high viscosity PMMAs, respectively. In terms of rheological properties, this was corresponding to the maximum value on the storage modulus versus PS content (shell thickness) curves. Encapsulation of high viscosity PMMA core particles at lower PS contents was related to the bigger particles and low interfacial area in this system compared to the system with low viscosity PMMA core particles. At high PS contents, single and multi-core structures were observed for composite droplets of ternary blends containing low and high viscosity PMMAs, respectively. The single core morphology of low viscosity PMMA particles was related to the coalescence of core particles after the coalescence of the corresponding shells, while high viscosity PMMA cores are less likely to coalescence, leading to creation of multi-core morphology in latter system.

Preparation and characteristic of gelatine/oxidized corn starch and gelatin/corn starch blend microspheres

Combinations of gelatin (G) and oxidized corn starch (OCS) were explored as a new microcapsule composite for single droplet spray drying. The blending solutions property, gel time, transparency and viscosity of G/CS (corn starch) and G/OCS blend solutions were compared at different ratios (10:0;9:1;8:2;7:3;6:4;5:5) and concentrations(1%wt; 3%wt; 5%wt). The drying and dissolution behaviors of composite droplet have been studied using the single droplet drying technique. Possible reaction mechanisms in the composite blend were elucidated by SEM and FTIR techniques. Blends solutions of G/OCS showed longer Gel time, higher transparency and lower viscosity; further displayed faster dissolution rate than that of G/CS under similar conditions. This was attributed to the formed Schiff base between the aldehyde group of OCS and amino group of G which improved the compatibility between G and OCS. All results indicated that the composites could be prepared with excellent properties by G/OCS (6:4) which would overcome some disadvantage such as thermodynamic incompatibility and phase separation by G/CS.

Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers.

We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependent monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed.

Structure and linear viscoelasticity of polymer nanocomposites with agglomerated particles

Abstract The connection between the structures and the linear viscoelasticity was revealed in this work for polymer nanocomposites containing agglomerated particles. A new image analysis method using the greyscale histogram of TEM images was suggested to determine the local volume fraction of nanoparticles inside an agglomerate, which represents the degree of agglomeration. It is found that the local volume fraction of nanoparticles is strongly related to the particle-polymer interaction. Such result was justified by a new rheological analysis based on the recently suggested modulus decomposition in the linear viscoelastic properties of polymer nanocomposites, from which the combined shift factor due to the hydrodynamic effect was used to quantify the local volume fraction of nanoparticles. Moreover, a two phase model was suggested to describe the linear viscoelasticity of such polymer nanocomposites. The success of this model in different polymer nanocomposites implies the rationality to regard the isolated agglomerate as a composite droplet in the polymer matrix.

Preparation of Millimeter-scale Alumina Hollow Spheres by Oil$lt;inf$gt;1$lt;/inf$gt;-in-water-in-oil$lt;inf$gt;2$lt;/inf$gt; Emulsions

To prepare millimeter-scale alumina hollow spheres by oil1-in-water-in-oil2 emulsions, the influence factors of formation of precursor and curing process were studied. Composite droplets with paraffin liquid as inner core and alumina sol as outer shell were used as precursor. Results showed that the com-droplets had homogeneous and com- plete core-shell structure when emulsion generator was designed that inner oil could be injected into water phase with the size of millimeters scale. Thickness and diameter of droplets were adjusted by both controlling the injection speed of oil and water phase in the range of 30-80 μm and 800-2200 μm. In the curing process, the semisolid spheres kept hollow structure and spherical when the speed of rotation between 20 r/min to 60 r/min and the flask being put in horizontal. Finally, the prepared hollow ceramic spheres with wall thickness in micrometer-scale and diameter in mil- limeter-scale formed complete hollow structure and high sphericity after calcining at 1200 ℃ for 4 h. The surface of sphere was relatively smooth with roughness of about 22 nm. The main crystal of the prepared ceramic hollow spheres was alpha alumina.