Extremely-fast electrostatically-assisted direct ink writing of 2D, 2.5D and 3D functional traces of conducting polymer Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate- polyethylene oxide (PEDOT:PSS-PEO).

HYPOTHESIS: Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is an attractive conducting polymer, albeit its rheological properties are inappropriate for direct ink writing (DIW). Here it is hypothesized that a suspension of PEDOT:PSS with a non-conducting highly spinnable viscoelastic polymer, e.g., polyethylene oxide (PEO), will significantly facilitate printability and enhance the electrical conductivity (EC) of PEDOT:PSS-PEO. It is also hypothesized that high-humidity post-treatment will enhance the EC even further, and the application of the electric field can facilitate the DIW speed beyond the capabilities of current commercial 3D printers. EXPERIMENTS: The rheological behavior of PEDOT:PSS suspensions with several non-conducting polymers was explored in the experiments. The EC of the suspensions was measured, including the effect of high-humidity post-treatment. High-speed DIW of the optimal suspension was experimentally demonstrated with the applied electric field. FINDINGS: The findings revealed that PEO serves as a secondary dopant, and the suspension of 4.33 wt% PEDOT:PSS-52 wt% PEO possesses the EC > 15 times higher than that of PEDOT:PSS. Many 2D, 2.5D and 3D functional traces were printed at high resolution at the DIW speed up to 8.64 m/s (>10 times faster than current commercial printers), facilitated by the applied electric field. Post-treatment at 80-90% relative humidity enhanced the EC more than twice.

Electrospun membranes filtering 100 nm particles from air flow by means of the van der Waals and Coulomb forces.

Nonwoven fibrous filter membranes are widely used in filtration because of their low cost. They are less effective in intercepting airborne particles of the order of 100 nm, which is of the SARS-CoV-2 (COVID-19) virus's size. Many diseases, including COVID-19, predominantly spread by droplets released by breathing, coughing, sneezing, or medical procedures. It was shown that the smallest droplets can evaporate in air before settling, thus, making viruses airborne and easily penetrating even the best masks and filters. As a result, air-filtering membranes, which are capable of effective interception of ∼100 nm nanoparticles are highly desirable. A traditional way to improve filtration efficiency by overlapping several layers of nonwoven fabrics increases the required pressure drop, and thus, should be avoided as much as possible. Here, we propose and demonstrate an innovative approach to enhance performance of filtration membranes based on (i) a dramatic reduction in the fiber size, and (ii) metal coating of the fibers. The first component of this approach allows one to incorporate a novel physical mechanism of filtration, the short-range van der Waals forces, whereas the second one adds the long-range electric Coulomb forces if the oncoming nanoparticles are pre-charged and the metal-plated membrane grounded. In the present work, the ∼100 nm aluminum nanoparticles are filtered as a model of commensurate airborne single COVID-19 viruses, and Platinum is used as the sputter-coated material for the fiber coating. The resulting filtration efficiency enhanced by the electric Coulomb forces alone is increased by the factor of 1.77, while the filtration efficiency additionally facilitated by the van der Waals forces increased by the factor of 2.44. In comparison to the filter membranes with ∼500 nm fibers without the electric forces involved, the van-der-Waals-electric filter membrane with fibers ∼90 nm is 2.24 × 1.77 = 3.96 times more effective. The quality factor of a membrane which combines the van der Waals and Coulomb forces is 10.6 psi-1, which is almost three times that of a comparable membrane without the electric Coulomb force (with only van der Waals forces being used).

Control of Direct Written Ink Droplets Using Electrowetting.

Here, we investigate the feasibility and effectiveness of electrowetting in the motion control of droplets of different liquids, which are widely used as inks in direct writing (DW)-based three-dimensional (3D) printing processes for various applications. To control the movement of DW ink droplets on dielectric substrates, the electrodes were embedded in the substrate. It is demonstrated that droplets of pure liquid inks, aqueous polymer solution inks, and carbon fiber suspension inks can be moved on multi-angled surfaces. Also, experimental results reveal that droplets of a commercial hydrogel, agar-agar, alginate, xanthan gum, and gum arabic can be moved by electrowetting. Droplets of sizes 200 µm-3 mm were manipulated and moved by the electric field on different dielectric substrates accurately and repeatedly. Effective electrowetting-based control and movement of droplets were observed on horizontal, vertical, and even inverted substrates. These findings imply the feasibility and potential application of electrowetting as a flexible, rapid, and new method for ink droplet control in 3D printing processes.

Implications of two backward blood spatter models based on fluid dynamics for bloodstain pattern analysis.

Bloodstain pattern analysis (BPA) is an integral part of crime scene investigation. For violent crimes involving gunshots, standard practice in police departments worldwide have some physical limitations. For instance, the effect of gravity and air drag on trajectories of blood droplets are neglected using current reconstruction methods, which results in a well-known overestimation of the height of the source of blood. As a consequence, more sophisticated models for blood spatter trajectory reconstruction are being developed, two of which are highlighted in the present work. They allow the prediction of bloodstain patterns produced from backward spattered blood droplets from blunt and sharp bullets. Our recent models attribute the splashing of blood to the Rayleigh-Taylor instability which arises when blood is accelerated towards lighter air. This physically-based description comes with the powerful predictive capability to correlate features of bloodstain patterns with the specific bullet and gun that produced them, as well as with the body position. The results of the numerical models were compared with four experiments simulating blood spatter deposition on a vertical wall through the number of stains produced, average stain area, and average impact angle at the surface, and the agreement found is fairly good. Moreover, further insight is obtained by probing and explaining the influence of observable parameters on the resulting spatter pattern, with the goal of aiding BPA experts evaluating a crime scene.

Exponential vaporization fronts and critical heat flux in pool boiling.

Here we study the inception and propagation of vaporization fronts and transition to the critical heat flux (CHF) and film-boiling regime triggered by a steep, almost instantaneous increase in the heat release from a strip heater submerged in Novec 7300 liquid. The propagation path of the resulting vaporization front along the strip heater is measured and shown to be exponentially increasing in time, in distinction from the previous reports in literature claiming that the increase is only linear. Since the previous experiments employed such liquids as water, refrigerants, acetone, ethanol, and alkali metals, which possess relatively high latent heat of evaporation and thus require a relatively high power to be supplied, the heater burnout at the inception of the CHF and film boiling was too fast to allow for longer-time observations. Accordingly, the previous works observed only an extremely short-time asymptotics of the propagation process, which means the short-time expansion of the exponential function, which is linear. On the other hand, in the experiments with Novec 7300 liquid, the heater burnout is delayed due to a much lower latent heat of evaporation, thus allowing for a much longer observation of the propagation path, which appears to be exponentially increasing in time. The experiments were preceded by our theoretical prediction of such a behavior, and this theory is also described in the present work. Due to the fact that this theory has been corroborated by the experimental data, the theory yields an adequate explanation and description of the CHF trigger and film boiling inception.

High-speed video analysis of forward and backward spattered blood droplets.

High-speed videos of blood spatter due to a gunshot taken by the Ames Laboratory Midwest Forensics Resource Center (MFRC) [1] are analyzed. The videos used in this analysis were focused on a variety of targets hit by a bullet which caused either forward, backward, or both types of blood spatter. The analysis process utilized particle image velocimetry (PIV) and particle analysis software to measure drop velocities as well as the distributions of the number of droplets and their respective side view area. The results of this analysis revealed that the maximal velocity in the forward spatter can be about 47±5m/s and for the backward spatter - about 24±8m/s. Moreover, our measurements indicate that the number of droplets produced is larger in forward spatter than it is in backward spatter. In the forward and backward spatter the droplet area in the side-view images is approximately the same. The upper angles of the close-to-cone domain in which droplets are issued in forward and backward spatter are, 27±9° and 57±7°, respectively, whereas the lower angles of the close-to-cone domain are 28±12° and 30±18°, respectively. The inclination angle of the bullet as it penetrates the target is seen to play a large role in the directional preference of the spattered blood. Also, muzzle gases, bullet impact angle, as well as the aerodynamic wake of the bullet are seen to greatly influence the flight of the droplets. The intent of this investigation is to provide a quantitative basis for current and future research on bloodstain pattern analysis (BPA) of either forward or backward blood spatter due to a gunshot.

Evidence of Faradaic Reactions in Electrostatic Atomizers.

Any rational theory of electrostatic atomizers (EAs) would require a detailed understanding of the nature of the polarized layer near the electrode, since this is the source of the electric charge carried by the jets issued from the EAs. The polarized layer either is driven out as the electrically-driven Smoluchowski flow and/or entrained by the viscous shear imposed by the bulk flow. The standard Gouy-Chapman theory of polarized diffuse layers implies zero electric current passing across the layer, which is impossible to reconcile with the fact that there are leak currents in the EAs. Here, we show that the electric current through the EA is controlled by faradaic reactions at the electrodes. The experiments were conducted with stainless steel or brass pin-like cathodes and three different anode (the conical nozzle) materials, which were copper, stainless steel, and brass. The different electrode materials resulted in different spray, leakage, and total currents in all the cases. Accordingly, it is shown that the total electric current generated by EAs can be controlled by the cathode and anode materials, i.e., by faradaic reactions on them. This lays the foundation for a more detailed understanding and description of the operation of EAs.

Biodegradable and biocompatible soy protein/polymer/adhesive sticky nano-textured interfacial membranes for prevention of esca fungi invasion into pruning cuts and wounds of vines.

Adhesive biodegradable membranes (patches) for the protection of pruning locations of plants from esca fungi attacks were developed using electrospun soy protein/polyvinyl alcohol and soy protein/polycaprolactone nanofibers. Several different water-soluble adhesives were either added directly to the electrospinning solutions or electrosprayed onto the as-spun nanofiber mats. The nanofibers were deposited onto a biodegradable rayon membrane, and are to be pressed onto the pruned location on a plant. The pore size in the nanofiber mats is sufficient for physically blocking fungi penetration, while the outside rayon membrane provides sufficient mechanical support in handling prior to deposition on a plant. Diseases like Vine Decline are one of the most important cases where such a remedy would be needed. It should be emphasized that these novel biodegradable and sticky patches are radically different from the ordinary electrospun ultra-filtration membranes. The normal and shear specific adhesive energy of the patches were measured, and the results show that they can withstand strong wind without being blown off. On the other hand, the patches possess sufficient porosity for plant breathing.

Enhanced foamability of sodium dodecyl sulfate surfactant mixed with superspreader trisiloxane-(poly)ethoxylate.

Gravitational drainage from thin vertical surfactant solution films and gravitational drainage in a settler column are used to study the behavior of foams based on two-surfactant mixtures. Namely, solutions of the anionic sodium dodecyl sulfate (SDS) and nonionic superspreader SILWET L-77, and their mixtures at different mixing ratios, are studied. It is shown, for the first time, that solutions having a longer lifetime in the vertical film drainage process also possess a higher foamability. An additional and unexpected unique result is that when using a mixed surfactant system, the foamability can be much greater than the foamabilities of the individual components.

Superspreaders versus "cousin" non-superspreaders: disjoining pressure in gravitational film drainage.

Gravitational drainage of vertical films supported on a wire frame of two superspreaders SILWET L-77 and BREAK-THRU S 278 and their respective "cousin" non-superspreaders SILWET L-7607 and BREAK-THRU S 233 revealed drastic differences. The superspreader films showed complicated dynamic "turbulent"-like interferometric patterns in distinction from the ordered color bands of the "cousin" non-superspreaders, which resembled those of the ordinary surfactants. Nevertheless, the superspreader films stabilized themselves at the thickness about 35 nm and revealed an order of magnitude longer lifetime before bursting compared to that of the "cousin" non-superspreaders. Notably, the superspreaders revealed drastic differences from the non-superspreaders in aqueous solutions with no contact with any solid hydrophobic surface. The self-stabilization of the superspreader films is attributed to significant disjoining pressure probably related to long superspreader bilayers hanging from the free surfaces. The scaling law for the disjoining pressure was found as p(disj)(h) ~ h(-m) (with m ≈ 9-11) for the sufficiently concentrated superspreader solutions, and as p(disj)(h) ~ h(-s) (with s ≈ 6) for more dilute solutions (in both cases, concentrations were above the critical micelle concentration). The non-superspreaders do not possess any significant disjoining pressure even in the films with thicknesses in the 35-100 nm range. The results show that gravitational drainage of vertical films is a useful simple tool for measuring disjoining pressure.

Two-stage desorption-controlled release of fluorescent dye and vitamin from solution-blown and electrospun nanofiber mats containing porogens.

In the present work, a systematic study of the release kinetics of two embedded model drugs (one completely water soluble and one partially water soluble) from hydrophilic and hydrophobic nanofiber mats was conducted. Fluorescent dye Rhodamine B was used as a model hydrophilic drug in controlled release experiments after it was encapsulated in solution-blown soy-protein-containing hydrophilic nanofibers as well as in electrospun hydrophobic poly(ethylene terephthalate) (PET)-containing nanofibers. Vitamin B2 (riboflavin), a partially water-soluble model drug, was also encapsulated in hydrophobic PET-containing nanofiber mats, and its release kinetics was studied. The nanofiber mats were submerged in water, and the amount of drug released was tracked by fluorescence intensity. It was found that the release process saturates well below 100% release of the embedded compound. This is attributed to the fact that desorption is the limiting process in the release from biopolymer-containing nanofibers similar to the previously reported release from petroleum-derived polymer nanofibers. Release from monolithic as well as core-shell nanofibers was studied in the present work. Moreover, to facilitate the release and ultimately to approach 100% release, we also incorporated porogens, for example, poly(ethylene glycol), PEG. It was also found that the release rate can be controlled by the porogen choice in nanofibers. The effect of nanocracks created by leaching porogens on drug release was studied experimentally and evaluated theoretically, and the physical parameters characterizing the release process were established. The objective of the present work is a detailed experimental and theoretical investigation of controlled drug release from nanofibers facilitated by the presence of porogens. The novelty of this work is in forming nanofibers containing biodegradable and biocompatible soy proteins to facilitate controlled drug release as well as in measuring detailed quantitative characteristics of the desorption processes responsible for release of the model substance (fluorescent dye) and the vitamin (riboflavin) in the presence of porogens.

Gravitational drainage of foam films.

Gravitational drainage from thick plane vertical soap films and hemispherical bubbles is studied experimentally and theoretically. The experiments involve microinterferometry kindred to the one used in the experiments in the Scheludko cell. The following surfactants were used in the experiments: cationic dodecyltrimethylammonium bromide (DTAB), anionic sodium dodecyl sulfate (SDS), anionic Pantene shampoo which primarily contains sodium lauryl sulfate, nonionic tetraethylene glycol monooctyl ether (C8E4), and nonionic Pluronic (P-123) surfactants at different concentrations. The theoretical results explain the drainage mechanism and are used to develop a new method of measurement of the surface elasticity and to test it on the above-mentioned surfactants.

Foam consolidation and drainage.

A theoretical model of foam as a consolidating continuum is proposed. The general model is applied to foam in a gravity settler. It is predicted that liquid drainage from foam in a gravity settler begins with a slow drainage stage. Next, a stage with faster drainage occurs where the drainage rate doubles compared to the initial stage. The experiments conducted within the framework of this work confirmed the theoretical predictions and allowed measurements of foam characteristics. Foams of three different concentrations of Pantene Pro-V Classic Care Solutions shampoo were studied, as well as the addition of polyethylene oxide (PEO) in one case. The shampoo's main foaming components are sodium lauryl sulfate and sodium laureth sulfate. It is shown to what extent foam drainage is slowed down by using higher shampoo concentrations and how it is further decreased by adding polymer (PEO).

Carbon nanofibers decorated with poly(furfuryl alcohol)-derived carbon nanoparticles and tetraethylorthosilicate-derived silica nanoparticles.

The present paper introduces a novel method to functionalize nanofiber surfaces with carbon or silica nanoparticles by dip coating. This novel approach holds promise of significant benefits because dip coating of electrospun and carbonized nanofiber mats in poly(furfuryl alcohol) (abbreviated as PFA) is used to increase surface roughness by means of PFA-derived carbon nanoparticles produced at the fiber surface. Also, dip coating in tetraethylorthosilicate (abbreviated as TEOS) is shown to be an effective method for decorating carbon nanofibers with TEOS-derived silica nanoparticles at their surface. Furthermore, dip coating is an inexpensive technique which is easier to implement than the existing methods of nanofiber decoration with silica nanoparticles and results in a higher loading capacity. Carbon nanofiber mats with PFA- or TEOS-decorated surfaces hold promise of becoming the effective electrodes in fuel cells, Li-ion batteries and storage devices.

Solution blowing of soy protein fibers.

Solution blowing of soy protein (sp)/polymer blends was used to form monolithic nanofibers. The monolithic fibers were blown from blends of soy protein and nylon-6 in formic acid. The sp/nylon-6 ratio achieved in dry monolithic nanofibers formed using solution blowing of the blend was equal to 40/60. In addition, solution blowing of core-shell nanofibers was realized with soy protein being in the core and the supporting polymer in the shell. The shells were formed from nylon-6. The sp/nylon-6 ratio achieved in dry core-shell fibers was 32/68. The nanofibers developed in the present work contain significant amounts of soy protein and hold great potential in various applications of nonwovens.

Thorny devil nanotextured fibers: the way to cooling rates on the order of 1 kW/cm2.

In the present work high-heat-flux surfaces, which should serve at temperatures of up to 200 °C, were covered by electrospun polymer nanofiber mats with thicknesses of about 30 µm. Then, four different metals were electroplated on separate polymer mats, namely, copper, silver, nickel, and gold. As a result, copper-plated nanofiber mats took on an appearance resembling that of a small Australian thorny devil lizard (i.e., they became very rough on the nanoscale) and acquired a high thermal diffusivity. Silver-plated nanofiber mats also became very rough because of the dendritelike and cactuslike nanostructures on their surfaces. However, nickel-plated nanofibers were only partially rough and their mats incorporated large domains of smooth nickel-plated fibers, and gold-plated nanofibers were practically smooth. Drop impacts on the hot surfaces coated with copper-plated and silver-plated nanofibers revealed tremendously high values of heat removal rates of up to 0.6 kW/cm(2). Such high values of heat flux are more than an order of magnitude higher that the currently available ones and probably can be increased even more using the same technique. They open some intriguing perspectives for the cooling of high-heat-flux microelectronics and optoelectronics and for further miniaturization of such devices, especially for such applications as UAVs and UGVs.

Thermo-responsive copolymer coatings for flow regulation on demand in glass microcapillaries.

This study presents thermo-responsive on-demand regulation of water flow rate in glass microcapillaries with a recently developed water-stable, stimuli-responsive poly(methyl methacrylate/N-isopropyl acrylamide) [P(MMA/NIPAM)] copolymer grafted at the inner walls. It is shown that the grafted coatings are stable and can withstand significant tractions under temperature variation. Such microcapillaries allow flow regulation on demand by changing temperature across the lower critical solution temperature (LCST) of the copolymer layer, which makes it swell or shrink, thus changing the bore available for pressure-driven flow. The grafted copolymer layers were subjected to different pressure drops applied to the capillary open ends, as well as to periodic temperature variation across the copolymer LCST to determine the best grafting conditions for microfluidic operation. Then, by varying the temperature, the flow rate in the capillaries was changed periodically on demand due to the swelling/shrinkage of the grafted copolymer layer. It was also shown that the entrapped air bubbles are present in the coating which can result in an apparent slip.

Resins with "nano-raisins".

Thermosensitive hydrogels are materials which globally shrink/swell in water when the surrounding temperature crosses the lower critical solution temperature (LCST). We demonstrate here a novel class of cross-linked polymeric materials, which do not shrink/swell in water globally, but nevertheless reveal a hydrogel-like, stimuli-responsive behavior. In particular, they demonstate a positive thermosensitive release of the embedded fluorescent dye significantly modulated when temperature crosses the LCST. Using staining with copper, transmission electron microscopy and energy dispersive X-ray analysis, we show that this effect is associated with nanogel "raisins" dispersed in such materials (e.g., polymer nanofibers). Shrinkage of individual nanogel "raisins" at elevated temperatures increases nanoporosity via increased exposure of the existing nanopores to water, or formation of new nanopores/nanocracks in the overstretched polymer matrix in the vicinity of shrinking nanogel "raisins". As a result, the release rate of the embedded dye from the nanofibers increases at elevated temperatures. We suggest that similar functional materials with embedded nanogel "raisins" will find applications in nanofluidics and as drug carriers for controlled drug release.

Mechanistic examination of protein release from polymer nanofibers.

Therapeutic proteins have emerged as a significant class of pharmaceutical agents over the past several decades. The potency, rapid elimination, and systemic side effects have prompted the need of spatiotemporally controlled release for proteins maybe more than any other active therapeutic molecules. This work examines the release of two model protein compounds, bovine serum albumin (BSA) and an anti-integrin antibody (AI), from electrospun polycaprolactone (PCL) nanofiber mats. The anti-integrin antibody was chosen as a model of antibody therapy; in particular, anti-integrin antibodies are a promising class of therapeutic molecules for cancer and angiogenic diseases. The release kinetics were studied experimentally and interpreted in the framework of a recently published theory of desorption-limited drug release from nondegrading--or very slowly degrading--fibers. The results are consistent with a protein release mechanism dominated by desorption from the polymer surface, while the polycaprolactone nanofibers are not degrading at an appreciable rate.

Fluidic delivery of homogeneous solutions through carbon tube bundles.

A wide array of technological applications requires localized high-rate delivery of dissolved compounds (in particular, biological ones), which can be achieved by forcing the solutions or suspensions of such compounds through nano or microtubes and their bundled assemblies. Using a water-soluble compound, the fluorescent dye Rhodamine 610 chloride, frequently used as a model drug release compound, it is shown that deposit buildup on the inner walls of the delivery channels and its adverse consequences pose a severe challenge to implementing pressure-driven long-term fluidic delivery through nano and microcapillaries, even in the case of such homogeneous solutions. Pressure-driven delivery (3-6 bar) of homogeneous dye solutions through macroscopically-long (approximately 1 cm) carbon nano and microtubes with inner diameters in the range 100 nm-1 microm and their bundled parallel assemblies is studied experimentally and theoretically. It is shown that the flow delivery gradually shifts from fast convection-dominated (unobstructed) to slow jammed convection, and ultimately to diffusion-limited transport through a porous deposit. The jamming/clogging phenomena appear to be rather generic: they were observed in a wide concentration range for two fluorescent dyes in carbon nano and microtubes, as well as in comparable transparent glass microcapillaries. The aim of the present work is to study the physics of jamming, rather than the chemical reasons for the affinity of dye molecules to the tube walls.