Formation Of Nanodroplets Research Articles

Ion-Beam-Directed Self-Ordering of Ga Nanodroplets on GaAs Surfaces.

Ordered nanodroplet arrays and aligned nanodroplet chains are fabricated using ion-beam-directed self-organization. The morphological evolution of nanodroplets formed on GaAs (100) substrates under ion beam bombardment is characterized by scanning electron microscopy and atomic force microscopy. Ordered Ga nanodroplets are self-assembled under ion beam bombardment at off-normal incidence angles. The uniformity, size, and density of Ga nanodroplets can be tuned by the incident angles of ion beam. The ion beam current also plays a critical role in the self-ordering of Ga nanodroplets, and it is found that the droplets exhibit a similar droplet size but higher density and better uniformity with increasing the ion beam current. In addition, more complex arrangements of nanodroplets are achieved via in situ patterning and ion-beam-directed migration of Ga atoms. Particularly, compared to the destructive formation of nanodroplets through direct ion beam bombardment, the controllable assembly of nanodroplets on intact surfaces can be used as templates for fabrication of ordered semiconductor nanostructures by droplet epitaxy.

Tailor-made thermoreversible functional polymer via RAFT polymerization in an ionic liquid: a remarkably fast polymerization process

Polymerization of furfuryl methacrylate (FMA) in an ionic liquid (IL) using CPDTC as RAFT reagent leads to a remarkably fast rate of polymerization due to formation of nano-droplets.

Self-nanoemulsifying drug delivery system of docosahexanoic acid: development, in vitro, in vivo characterization

Context: Docosahexanoic acid (DHA) is an essential omega-3 fatty acid for normal brain development and its use has increased considerably in recent years.Objective: The aim of this study is to develop and evaluate self-nanoemulsifying drug delivery systems (SNEDDS) of DHA for improved palatability, dispersibility and bioavailability.Methods: The SNEDDS were prepared and evaluated for miscibility, employing different combinations of olive oil and soyabean oil as oil phase, Span 80, Span 20, soya phosphatidylcholine, Labrafil M 1944 CS as surfactants while Tween 80, PEG 400, Cremophor RH40 and propylene glycol as cosurfactants. Thermodynamically stable SNEDDS were characterized for dispersibility, self-emulsification time, droplet size, zeta potential along with sensory analysis. The optimized formulation was subjected to ex vivo and in vivo evaluation such as intestinal permeability, memory performance test, brain concentration and histopathology studies.Results: The optimized SNEDDS formulation showed emulsification time of 27 ± 4.7 s with droplet size of 17.6 ± 3.5 nm and zeta potential of −37.6 ± 0.5 mV. Intestinal absorption study depicted 18.3%, 21.5%, 41.5%, 98.7% absorption of DHA with SNEDDS-based formulation in comparison to 8.2%, 15.1%, 28.8%, 46.1% absorption of DHA with oil-based marketed formulation after 0.5, 1, 2 and 4 h. DHA concentration in brain homogenate was found to be increased to 2.6-fold in comparison to DHA-marketed formulation. This could be ascribed to enhanced dispersibility and bioavailability of DHA from nanosized formulation.Conclusion: The developed formulation led to enhanced dispersibility and bioavailability of DHA due to the formation of nanodroplets.

Gravitational Effect on the Formation of Surface Nanodroplets.

Nanoscale droplets at a solid-liquid interface are of high relevance for many fundamental phenomena and applied processes. The solvent exchange process is a simple approach to produce, e.g., oil nanodroplets over a large surface area on a substrate, by exchange oil-saturated ethanol by oil-saturated water, which has a lower oil solubility than ethanol. In this process, the size of the nanodroplets is closely related to the flow conditions. To achieve control of the droplet size, it is essential to fully understand the nucleation and growth of nanodroplets under different flow conditions. In this work, we investigate the gravitational effect on the droplet formation by the solvent exchange. We compared the droplet size as the substrate was placed on the upper or lower wall in a horizontal fluid channel or on the sides of a vertical channel with an upward or downward flow. We found significant difference in the droplet size for the three substrate positions in a wide channel with height h = 0.21 mm. The difference of droplet size was eliminated in a narrow channel with height h = 0.07 mm. The relevant dimensional control parameter for the occurrence of the gravitational effects is the Archimedes number Ar and these two heights correspond to Ar = 10 and Ar = 0.35, respectively. The gravitational effects lead to a nonsymmetric parabolic profile of the mixing front, with the velocity maximum being off-center and thus with different distances α(Ar)h and (1 - α(Ar))h to the lower and upper wall, respectively. The ratio of the total droplet volume on the lower and upper wall is theoretically found to be (α(Ar)/(1 - α(Ar)))(3). This study thus improves our understanding of the mechanism of the solvent exchange process, providing guidelines for tailoring the volume of surface nanodroplets.

Spontaneous Pattern Formation of Surface Nanodroplets from Competitive Growth.

Nanoscale droplets on a substrate are of great interest because of their relevance for droplet-based technologies for light manipulation, lab-on-chip devices, miniaturized reactors, encapsulation, and many others. In this work, we establish a basic principle for symmetrical arrangements of surface nanodroplets during their growth out of oversaturated solution established through solvent exchange, which takes place under simple and controlled flow conditions. In our model system, nanodroplets nucleate at the rim of spherical cap microstructures on a substrate, due to a pulse of oversaturation supplied by a solvent exchange process. We find that, while growing at the rim of the microcap, the nanodroplets self-organize into highly symmetric arrangements, with respect to position, size, and mutual distance. The angle between the neighboring droplets is 4 times the ratio between the base radii of the droplets and the spherical caps. We show and explain how the nanodroplets acquire the symmetrical spatial arrangement during their competitive growth and why and how the competition enhances the overall growth rate of the nucleated nanodroplets. This mechanism behind the nanodroplet self-organization promises a simple approach to control the location of droplets with a volume down to attoliters.

Solvent Effects on the Formation of Surface Nanodroplets by Solvent Exchange.

Solvent exchange is a simple process to form oil nanodroplets at solid-liquid interfaces with well-defined location and morphology. In this process, a good solvent of the oil is displaced by a poor solvent, leading to the nucleation and growth of oil droplets from a transient oversaturation at the mixing front. Our recent work has shown that the final volume of the droplets is related to the flow conditions. In this work, we investigate the effects of the type and the composition of solvents on the droplet formation under the same flow conditions. Water nanodroplets were produced by ethanol/cyclohexane (solution A) and cyclohexane (solution B) on a hydrophilic substrate. We found that the droplet size increases first and then decreases with an increase of the initial ethanol concentration in solution A. This is attributed to the phase separation of ethanol-cyclohexane-water; in particular, the composition of solution A on the phase boundary above the Ouzo region. The same reason also contributes to the lower efficiency in droplet formation for a longer alkane. The important implication from this work is that the maximal droplet volume is limited by the phase separation of the solvents used in the solvent exchange.

Liquid nanodroplet formation through phase explosion mechanism in laser-irradiated metal targets.

Some quantitative aspects of laser-irradiated pure metals, while approaching phase explosion, are still not completely understood. Here, we develop a model that describes the main quantities regulating the liquid-vapor explosive phase transition and the expulsion of liquid nanodroplets that, by solidifying, give rise to nanoparticle formation. The model combines both a thermodynamics description of the explosive phase change and a Monte Carlo simulation of the randomly generated critical vapor bubbles. The calculation is performed on a set of seven metals (Al, Fe, Co, Ni, Cu, Ag, and Au) which are frequently used in pulsed laser ablation experiments. Our final predictions about the size distribution of the liquid nanodroplets and the number ratio of liquid/vapor ejected atoms are compared, whenever possible, with available molecular dynamics simulations and experimental data.

Formation and lateral migration of nanodroplets via solvent shifting in a microfluidic device

Solvent shifting is a process in which a non-solvent is added to a solvent/solute mixture and extracts the solvent. The solvent and the non-solvent are miscible. Because of solution supersaturation, a portion of the solute transforms to droplets. In this paper, based on this process, we present an investigation on droplet formation and their radial motion in a microfluidic device in which a jet is injected in a co-flowing liquid stream. Thanks to the laminar flow, the microfluidic setup enables studying diffusion mass transfer in radial direction and obtaining well-defined concentration distributions. Such profiles together with the ternary phase diagram give detailed information about the conditions for droplet formation as well as their radial migration in the channel. The ternary system is composed of ethanol (solvent), de-ionized water (non-solvent), and divinylbenzene (solute). We employ analytical/numerical solutions of the diffusion equation to obtain concentration profiles of the components. We show that in the system under study droplets are formed in a region of the phase diagram between the binodal and the spinodal, i.e., via a thermally activated process. The droplets are driven to the channel centerline by the solutal Marangoni effect but are not able to significantly penetrate into the single-phase region, where they get rapidly dissolved. Therefore, the radial motion of the binodal surface carries the droplets to the centerline where they get collected.

The Energy of COPI for Budding Membranes.

As a major actor of cellular trafficking, COPI coat proteins assemble on membranes and locally bend them to bud 60 nm-size coated particles. Budding requires the energy of the coat assembly to overcome the one necessary to deform the membrane which primarily depends on the bending modulus and surface tension, γ. Using a COPI-induced oil nanodroplet formation approach, we modulated the budding of nanodroplets using various amounts and types of surfactant. We found a Heaviside-like dependence between the budding efficiency and γ: budding was only dependent on γ and occurred beneath 1.3 mN/m. With the sole contribution of γ to the membrane deformation energy, we assessed that COPI supplies ~1500 kBT for budding particles from membranes, which is consistent with common membrane deformation energies. Our results highlight how a simple remodeling of the composition of membranes could mechanically modulate budding in cells.

Formation of surface nanodroplets under controlled flow conditions

Nanodroplets on a solid surface (i.e., surface nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, laboratory-on-chip devices, and near-field imaging techniques. Oil nanodroplets can be produced on a solid-liquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. In this work, we experimentally and theoretically investigate the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We find significant effects from the flow rate and the flow geometry on the droplet size. We develop a theoretical framework to account for these effects. The main idea is that the droplet nuclei are exposed to an oil oversaturation pulse during the exchange process. The analysis shows that the volume of the nanodroplets increases with the Peclet number Pe of the flow as ∝ Pe(3/4), which is in good agreement with our experimental results. In addition, at fixed flow rate and thus fixed Peclet number, larger and less homogeneously distributed droplets formed at less-narrow channels, due to convection effects originating from the density difference between the two solutions of the solvent exchange. The understanding from this work provides valuable guidelines for producing surface nanodroplets with desired sizes by controlling the flow conditions.

Structure and internal dynamics of nanodroplets in dilute solutions of chiral gelators

A gel lattice formed in solutions of N-trifluoroacetylated aminoalcohols (TFAAA) consists of solitary quasi-one-dimensional strings. It has been previously shown that, at gelator concentrations below the gelation threshold, the solution contains a dispersed phase consisting of two fractions (nanoscale and submicroscopic). The molecular dynamics method is used to simulate the formation of particles of the nanosized fraction. It is shown that its elements are liquid, solvent-containing nanodroplets with a diameter of 2–3 nm, exhibiting high internal molecular mobility and an extremely heterogeneous surface with a fractal dimension of ~1.6. Hydrogen bonding is demonstrated to play a marginal role in the formation of nanodroplets. Submicroscopic particles fractions were identified with the mesophase. The equilibrium between the nanodroplets and the mesophase observed over 105 s indicates the formation of mesophase particles due to the condensation of nanodroplets. An extreme heterogeneity is a factor that ensures the coexistence of dispersed-phase particles of different sizes. With increasing particle size, the high internal mobility of nanodroplets gives way to an orientational order characteristic of the mesophase, which apparently becomes long-range during further condensation of TFAAA, an order characteristic of the quasi-supramolecular strings.

Microfluidic Selective Concentration of Microdroplet Contents by Spontaneous Emulsification

The selective concentration of the contents in a microdroplet using spontaneous emulsification was proposed and demonstrated in a microfluidic channel. Aqueous microdroplets having a 40-μm diameter, in octane containing 100 mM of Span 80, shrank to 10 μm within 10 min with nanodroplet formation at the interface of the microdroplets. The microdroplets' contents either stayed in the microdroplet or partitioned into the nanodroplets, depending on their properties. The size and the hydrophobicity of the contents are two parameters that determine concentration/separation. In addition, this method was applied to a bound complex and free ligand (B/F) separation to demonstrate its applicability to biochemical analyses. Here we report the separation of water-soluble molecules in microdroplets for the first time. This method is expected to enhance the flexibility of the design of droplet analytical processes and widen their applicability.

Formation of nanojets and nanodroplets by an ultrashort laser pulse at focusing in the diffraction limit

A new model has been developed and calculations have been performed for the formation of nanodroplets after action of an ultrashort laser pulse on a thin (10–100 nm) gold film deposited on a glass substrate. The action of a laser results in the melting of the film in the region of a laser spot and in its thermomechanical separation from the substrate. The separated film acquires a dome shape because of a decrease in the temperature in the direction from the center of the laser spot. This theoretical model provides the explanation of the formation of nanodroplets. It has been established that, first, the separation speed of a gold film from glass decreases sharply because the acoustic impedance of gold is much larger than that of glass. Second, nanodroplets are formed owing to the capillary focusing of the substance, which is manifested in the appearance of the drag component directed toward the axis of symmetry of the dome. The surface tension becomes dynamically significant because of the indicated sharp decrease in the separation speed from glass and of the smallness of the diameter of the focal spot (D ∼ 1 μm), which is determined by the diffraction limit of optical radiation.

Formation and nitridation of InGa composite droplets on Si(1 1 1): In-situ study by high resolution X-ray photoelectron spectroscopy

We demonstrate applying synchrotron radiation based high resolution X-ray photoelectron spectroscopy as a versatile in-situ tool to study the step-wise formation and nitridation of indium gallium (InGa) composite droplets on Si(111)7 × 7. This includes analysis of initial interactions of Ga and In with Si(111)7 × 7, and characterization of formation and nitridation of InGa droplets. The results show that after stabilization of Si(111)7 × 7 structure with Ga and In at 750°C and formation of Ga nanodroplets as the bases, the InGa composite droplets (In:Ga ≈ 3:1) were formed at RT and 200°C in a coverage range 3–14 monolayers. The nitridation efficiency and structure of the droplets/Si(111) with NH3 were temperature dependent. At or below 350°C, the droplets were hardly nitridated. At 670°C, the nitridation of InGa was more completed, which however caused an unavoidable nitridation of Si surface. The optimum nitridation occurred around 480°C and proceed in a 2D mode. Further simultaneous growth and nitridation of the InGa droplets on the 2D nitridated surface at 400°C resulted in a complex surface composition and structure. An ex-situ atomic force microscopy reveals both the aligned metallic droplet-island assemblies and the fractured nitridated island bases with small droplets on top for the final surface.

Ion induced compositional changes and nanodroplet formation on GaN surface

Abstract Low energy argon ion bombardment of different fluence onto a GaN film grown on Al 2 O 3 (0 0 0 1) surface induces interesting compositional and morphological modifications. While atomic force microscopy (AFM) studies reveal the formation of a circular flat islands, Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) core-level and valence-band spectra confirm that these nanodroplets consist of pure Ga. Detailed XPS studies suggest breaking of Ga N bond, that releases Ga to diffuse on the surface and nucleate at the threading dislocation sites, defined by the interfacial surface free energy. The formation of droplet pattern formation is discussed in the light of the existing theories on ion induced surface modification.

A real-time AFM study on crystal nucleation and growth in nanodroplets of isotactic polypropylene

Isotactic polypropylene (iPP) nanodroplets were prepared by using the classical droplet method in this study. The formation of nanodroplets allowed the controlled observation of polymer nucleation as well as access to crystal growth at exceptionally high supercooling in iPP. Three cases including the heterogeneous nucleation and fast crystallization in iPP droplets, the formation of multiple independent homogeneous nuclei within a single droplet and a single nucleus within a single droplet were detected by using atomic force microscopy (AFM) during gradually cooling after remelting the nanodroplets. Moreover, it is found that when the volume of droplet is larger than the value of ca. 130000 nm3, the first case was observed. Otherwise, the latter two cases appeared. The temperature at which the onset of nucleation was observed in individual droplets was found to be mainly dependent on height of the droplets when the size scale of the droplet is comparable to the size of the critical nucleus in at least one dimension, which indicates the nucleation behavior under confinement.

Enhancement of surface finish using water-miscible nano-cutting fluid in ultra-precision turning

This paper presents an experimental study on the enhancement effect of the low viscosity cutting fluid enriched with nano-droplets (NDCF) in ultra-precision turning. Unlike the conventional method in which the enhancement is done by adding nano-metric particles, no suspension particles are mixed to produce the NDCF in this paper. Instead, the NDCFs are produced by atomisation of the cutting fluid that circulates in a closed-loop treatment device. The atomisation process is found to reduce the viscosity of the NDCF significantly by the formation of nano-droplets (NDs). The reduced viscosity of the NDCFs could be increased by diluting them with water. The bench tests on wetting properties reveal that the NDCF demonstrates two distinct properties: a very linear spreading rate and a relatively low contact angle. These two properties assist the NDCF in penetrating deeper into the tool–chip and tool–work interfaces and hence provides a better lubrication effect at these interfaces. In this paper, the ultra-precision cylindrical turning experiments reveal that the droplet size of the NDCF is a more important factor than viscosity on affecting the surface finish in ultra-precision machining.

Adhesive water-in-oil nano-emulsions generated by the phase inversion temperature method

This study aims at exploring and understanding the formulation of reverse (water-in-oil) nano-emulsions by the phase inversion temperature method (PIT method). Over the past decade, low-energy nano-emulsification methods have been largely approached for the formulation of direct (oil-in-water) nano-emulsions. These studies have allowed better understanding in identifying the basic concepts and principles driving the nano-droplet formation processes. In fact, the water-in-oil low-energy nano-emulsification has been much less studied and understood. Recent advances in this field have demonstrated methods and processes based on spontaneous emulsification, however there have been no studies to date describing the driving principles in the production and formulation of reverse nano-emulsions and their link with direct nano-emulsification, especially by the PIT method which still remains challenging. Herein we report for the first time a comprehensive experimental approach that explores and understands the formulation of water-in-oil nano-emulsions by the phase inversion temperature method. The experimental system for emulsion phase inversion was composed of water–paraffin oil–C18E6 (lipophilic polyethoxylated surfactant), which was maintained in the phase inversion zone before its breaking-up by the dilution light oil (2-methylbutane), immediately generating reverse nano-emulsions. The results allowed disclosure of emulsification principles and mechanisms, in the same line as the ones acknowledged for oil-in-water nano-emulsification. The study emphasized that, similarly to direct low-energy nano-emulsification with the oil–surfactant ratio, here the water–surfactant ratio has a strong impact on the resulting nano-emulsion droplet size. Finally, these reverse nano-emulsions showed strong adhesive properties, characterized commonly with macro-emulsions but never observed with nano-emulsions, and thus leading to the formation of nano-emulsion-gels.

Development of Curcumin Based Ophthalmic Formulation

Problem statement: Ophthalmic drug delivery system has been a major challenge due to the distinctive and unique anatomy of the eye. The use of phytochemicals for the application in ocular disease has not been investigated much, due to poor pharmacokinetics of phytochemical and hence poor therapeutic efficacy. Approach: Curcumin is one such phytochemical known for its medicinal properties, but its usage has been limited due to its poor bioavailability. Therapeutic ocular application of curcumin for indications such as allergic conjunctivitis and use of nanotechnology based delivery system for increasing its bioavailability haven’t been explored much. Thus the objective of current study was to develop a nanoemulsification based formulation for curcumin with enhanced bioavailability and efficacy for ophthalmic therapeutic application. Different surfactants and oils were screened based on its ability to solubilize hydrophobic curcumin for developing formulation. Ultrasonication was used for dispersion of curcumin in surfactants and nanoemulsification. Charecterization of the selected formulation was done based on particle size, Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) analysis and tested for different physiochemcial properties. Results: A novel formulation was developed through nanoemulsification technology by ultrasonication using carefully selected nonionic surafactants. Acconon and Tween 80 were chosen after thorough screening studies. The ratio of curcumin: Acconon: Tween80: water (0.12:1:7:1) was found to be optimum for the formulation. Particle size analysis showed that the nano droplets are in the range of 8-22 nm. Further the TEM and SEM studies also confirmed that the nanodroplets are in spherical shape. Formulation was found to be stable as ascertained by phase separation and observational studies. Conclusion: The studies ensured the stability and formation of characteristic nanodroplets with curcumin entrapment as a result of the developed formulation. The so developed formulation is expected to have higher ophthalmic bioavailability, which further needs to be confirmed through in vitro and in vivo studies.

Nanodroplet formation and exclusive homogenously nucleated crystallization in confined electrospun immiscible polymer blend fibers of polystyrene and poly(ethylene oxide)

By utilizing electrospun blend fibers of polystyrene (PS) and poly(ethylene oxide) (PEO) with diameters in sub-microns, nanodroplets of the minor component (PEO) were obtained by annealing the blend fibers above the glass transition temperature ( T g ) of the matrix polymer (PS), as a result of the Rayleigh-Plateau instability in the melt. However, direct thermal annealing of the PS/PEO blend fibers led to poor Rayleigh breakup of the PEO fibers in the PS matrix, and fractionated crystallization with both homogeneous and heterogeneous nucleation was observed, probably due to a broad size distribution of PEO particles. On the contrary, after confining the PS/PEO blend fibers with a high T g polymer, poly(4- tert-butyl styrene) (P4tBS, T g ∼ 143 °C), well-defined Rayleigh breakup of the PEO fiber was achieved by annealing the P4tBS-coated PS/PEO blend fibers at 150 °C. Consequently, exclusive homogeneously nucleated PEO crystallization was observed at −20 °C. This report could provide a universal method to achieve nano-sized droplets for the study of nanoconfinement effect by utilizing electrospun immiscible polymer blend fibers without addition of any compabitilizers.