Effect of a Surrounding Liquid Environment on the Electrical Disruption of Pendant Droplets.

The effect of a surrounding, dielectric, liquid environment on the dynamics of a suddenly electrified liquid drop is investigated both numerically and experimentally. The onset of stability of the droplet is naturally dictated by a threshold value of the applied electric field. While below that threshold the droplet retains its integrity, reaching to a new equilibrium state through damped oscillations (subcritical regime), above it electrical disruption takes place (supercritical regime). In contrast to the oscillation regime, the dynamics of the electric droplet disruption in the supercritical regime reveals a variety of modes. Depending on the operating parameters and fluid properties, a drop in the supercritical regime may result in the well-known tip streaming mode (with and without whipping instability), in droplet splitting (splitting mode), or in the development of a steep shoulder at the elongating front of the droplet that expands radially in a sort of "splashing" (splashing mode). In both splitting and splashing modes, the sizes of the progeny droplets, generated after the breakup of the mother droplet, are comparable to that of the mother droplet. Furthermore, the development in the emission process of the shoulder leading to the splashing mode is described as a parametrical bifurcation, and the parameter governing that bifurcation has been identified. Physical analysis confirms the unexpected experimental finding that the viscosity of the dynamically active environment is absent in the governing parameter. However, the appearance of the splitting mode is determined by the viscosity of the outer environment, when that viscosity overcomes a certain large value. These facts point to the highly nonlinear character of the drop fission process as a function of the droplet volume, inner and outer liquid viscosities, and applied electric field. These observations may have direct implications in systems where precise control of the droplet size is critical, such as in analytical chemistry and "drop-on-demand" processes driven by electric fields.

The high-throughput atomization of polymer solutions for fiber synthesis in a single step aided with corona ionizers.

Polymer microfibers are ubiquitous structures across virtually all technological fields. Their applications include, for instance, filter media, tissue regeneration, wound healing and dressing, and reinforcement materials. The most effective methods for fabrication of fibrous micro and nanomaterials rely on electric fields to spin a liquid jet into an ultrafine thread that rapidly dries up forming a fiber. Continuous spinning and collection leads to formation of fiber mats. Here we report a robust yet simple approach for the massive production of liquid threads, which upon acquiring electrical charges in-flight are collected downstream in the form of fibers. The entire process takes place on-line in a single step. The liquid threads are produced through the fragmentation of a polymer solution bulk due to a turbulent interaction of a gas-liquid interface in the interior of an engineered device, a so-called Flow Blurring atomizer. The particularity of this approach consists precisely in such vigorous interaction, at the micrometer scale, which triggers a bubbly motion in the interior of the device, that is a "micro-mixing". Subsequently, the threads are passed through ionized air currents, at ambient conditions, and then stretched to sub-micrometer dimensions by electric fields. Polyvinylpyrrolidone (PVP) as well as carbon nanotubes (CNTs) or graphene oxide sheets (GOSs)-containing PVP fibers, with diameters in the range 100-900 nm, were synthesized via this approach. In the cases studied herein the method was operated at liquid flow rates (i.e. production rates) of 0.2 mL/min but it could be readily increased up to a few tens of mL/min. The method requires further improvement and optimization, nevertheless it is a promising alternative for mass production of polymer fibers.

Integrating a micro-mixing mechanism and on-line thermal processing for the large-scale ejection of polymeric liquid threads for producing ultrafine fibers.

Micro/nanofibers are structures that nowadays have a wide range of cutting-edge applications including energy generation and storage devices, smart textiles, cell growth, and tissue engineering. These fibrous materials are mostly produced from polymer solutions spun, under laminar flow conditions, into nanofibers by external forces. However, the turbulent interaction of gas-liquid interfaces offers an innovative approach for the high-throughput production of nanofibers. Here, we present Flow Blurring (FB), a solely pneumatic approach for the massive production of liquid threads of polymer solutions, which relies on a micro-mixing mechanism that triggers a turbulent motion capable of fragmenting a viscous flow. The as-ejected threads are subsequently processed thermally, on-line in a single-step, thus producing micro/nanofibers that form mats. The method operates with relatively large liquid flow rates, equivalent of a high production rate, and is thus suitable for industrial production of engineered nanomaterials. In this work, we used solutions of poly(vinyl alcohol) (PVA) to study its ejection and fragmentation dynamics through computational fluid dynamics (CFD) simulations. In addition, the physics underlying the regulation of the liquid flow rate in FB atomizers are proposed. Fibers with typical diameters in the range 400-800 nm were produced by online heating of the liquid threads. Liquid ejection experiments were performed under different operating conditions thus verifying the capability of the method for synthesizing submicrometer-sized fibers with high uniformity and production rates suitable for scaling up.

Micromixing with In-Flight Charging of Polymer Solutions in a Single Step Enables High-Throughput Production of Micro- and Nanofibers.

Controlled ejection of liquids at capillary scales is a ubiquitous phenomenon associated with significant advances in, for instance, molecular biology or material synthesis. In this work, we introduce a high-throughput approach, which relies on a micromixing mechanism to eject and fragment viscous liquids, for production of microfibers from poly(vinyl alcohol) solutions. First, filaments were generated pneumatically with a so-called flow-blurring atomizer and using liquid flow rates of up to ∼1 L/min. Subsequently, the filaments were ionized online by corona discharge and consecutively manipulated with an electric field created by disc electrodes. Such charging of the filaments and the effect of the electric field allowed for their ultrafast elongation and diameter reduction from 150 µm down to fibers of 500 nm, which after collection exhibited fabric-like texture. The approach presented herein is a general procedure with potential for scalability that, upon proper adaptation, may be extended to various polymeric materials.

Massive production of fibroin nano-fibrous biomaterial by turbulent co-flow.

Among the different polymers (proteins, polysaccharides, etc.) that make up natural fibers, fibroin is a protein produced by silk spinning animals, which have developed an optimized system for the conversion of a highly concentrated solution of this protein into high-performance solid fibers. This protein undergoes a self-assembly process in the silk glands that result from chemical gradients and by the application of mechanical stresses during the last step of the process. In the quest for a process that could mimic natural spinning at massive scales, we have discovered that turbulence offers a novel and promising solution: a turbulent liquid jet can be formed by a chemically green and simple coagulating liquid (a diluted solution of acetic acid in etanol) co-flowing with a concentrated solution of fibroin in water by the use of a Flow Blurring nebulizer. In this system, (a) the co-flowing coagulant liquid extracts water from the original protein solution and, simultaneously, (b) the self-assembled proteins are subjected to mechanical actions, including splitting and stretching. Given the non-negligible produced content with the size and appearance of natural silk, the stochastic distribution of those effects in our process should contain the range of natural ones found in animals. The resulting easily functionalizable and tunable one-step material is 100% biocompatible, and our method a perfect candidate to large-scale, low-cost, green and sustainable processing of fibroin for fibres and textiles.

Sensitivity of micro-Raman spectrum to crystallite size of electrospray-deposited and post-annealed films of iron-oxide nanoparticle suspensions.

This paper demonstrates the sensitivity of the Raman spectra of electrospray-deposited iron-oxide nanoparticle films to their grain size. The grain size in the films was controlled using the temperature of annealing. The position and width of the bands at approximately 225, 612, 660, and 1317 Deltacm(-1) were investigated for their sensitivity to both grain size and laser-induced heating, using 532 nm excitation. The parameter most sensitive to grain size was found to be the difference between the band position of the 660 Deltacm(-1) LO and 225 Deltacm(-1) TO bands (X660-X225). The distance between the two bands narrowed with increasing annealing temperature of the films, suggesting that, as grain size increases, these two bands become closer. The 660 Deltacm(-1) band position was very stable regardless of variation in laser power and, thus, laser-induced heating effects. The films were prepared by electrospray deposition of a commercial maghemite (gamma-Fe2O3) nanoparticle suspension onto a silicon wafer followed by post-annealing in a temperature range of 600-1000 degrees C for 15 min in an air atmosphere.

Flow Blurring-Enabled Production of Polymer Filaments from Poly(ethylene oxide) Solutions.

Flow blurring (FB) atomizers are relatively simple yet robust devices used for the generation of sprays from solutions of a wide range of viscosities. In this work, we have demonstrated that FB devices may also be applied for massive production of liquid filaments from polymeric solutions. They can later be transformed into solid filaments and fibers, leading to the production of so-called fiber mats. The liquid precursors consisted of poly(ethylene oxide) (PEO) solutions of varying molecular weights (105 [100k] to 4 × 106 g/mol [4M]) and concentrations. The FB device was operated in the gas pressure range of 3-6 bar. Except for solutions of PEO 100k, all solutions exhibited a shear thinning behavior. For massive filament production, a threshold polymer concentration (c t) was identified for each molecular weight. Below such concentration, the atomization resulted in droplets (the classical FB functioning mode). Such a threshold value decreased as the PEO molecular weight increased, and it coincides with the polymer coil overlap concentration, c*. The viscoelastic nature of the solutions was also observed to increase with the molecular weight. A 3.2 dependency of the zero-shear rate viscosity on a so-called Bueche parameter was found for filament production, whereas a nearly linear dependency was found for droplet production. In general, the mean diameter of the filaments decreased as they traveled downstream from the atomization point. Furthermore, at a given distance from the atomizer outlet and gas pressure, the mean filament diameter slightly shifted toward larger sizes with increasing PEO molecular weight. The tendency agrees well with the calculated filaments' Deborah number, which increases with PEO molecular weight. The approach presented herein describes a high-throughput and efficient method for the massive production of viscous filaments. These may be transformed into fibers by an on-line drying step.