On the Stability of Electrohydrodynamic Jet Printing Using Poly(ethylene oxide) Solvent-Based Inks.

Electrohydrodynamic (EHD) jet printing of solvent-based inks or melts allows for the producing of polymeric fiber-based two- and three-dimensional structures with sub-micrometer features, with or without conductive nanoparticles or functional materials. While solvent-based inks possess great material versatility, the stability of the EHD jetting process using such inks remains a major challenge that must be overcome before this technology can be deployed beyond research laboratories. Herein, we study the parameters that affect the stability of the EHD jet printing of polyethylene oxide (PEO) patterns using solvent-based inks. To gain insights into the evolution of the printing process, we simultaneously monitor the drop size, the jet ejection point, and the jet speed, determined by superimposing a periodic electrostatic deflection. We observe printing instabilities to be associated with changes in drop size and composition and in the jet's ejection point and speed, which are related to the evaporation of the solvent and the resulting drying of the drop surface. Thus, stabilizing the printing process and, particularly, the drop size and its surface composition require minimizing or controlling the solvent evaporation rate from the drop surface by using appropriate solvents and by controlling the printing ambient. For stable printing and improved jet stability, it is essential to use polymers with a high molecular weight and select solvents that slow down the surface drying of the droplets. Additionally, adjusting the needle voltages is crucial to prevent instabilities in the jet ejection mode. Although this study primarily utilized PEO, the general trends observed are applicable to other polymers that exhibit similar interactions between solvent and polymer.

The Role of Electrical Polarity in Electrospinning and on the Mechanical and Structural Properties of As-Spun Fibers.

Electric field strength and polarity in electrospinning processes and their effect on process dynamics and the physical properties of as-spun fibers is studied. Using a solution of the neutral polymer such as poly(methyl methacrylate) (PMMA) we explored the electrospun jet motion issued from a Taylor cone. We focused on the straight jet section up to the incipient stage of the bending instability and on the radius of the disk of the fibers deposited on the collecting electrode. A new correlation formula using dimensionless parameters was found, characterizing the effect of the electric field on the length of the straight jet, L˜E~E˜0.55. This correlation was found to be valid when the spinneret was either negatively or positively charged and the electrode grounded. The fiber deposition radius was found to be independent of the electric field strength and polarity. When the spinneret was negatively charged, L˜E was longer, the as-spun fibers were wider. The positively charged setup resulted in fibers with enhanced mechanical properties and higher crystallinity. This work demonstrates that often-overlooked electrical polarity and field strength parameters influence the dynamics of fiber electrospinning, which is crucial for designing polymer fiber properties and optimizing their collection.

Back to Normal: An Old Physics Route to Reduce SARS-CoV-2 Transmission in Indoor Spaces.

We advocate the widespread use of UV-C light as a short-term, easily deployable, and affordable way to limit virus spread in the current SARS-CoV-2 pandemic. Radical social distancing with the associated shutdown of schools, restaurants, sport clubs, workplaces, and traveling has been shown to be effective in reducing virus spread, but its economic and social costs are unsustainable in the medium term. Simple measures like frequent handwashing, facial masks, and other physical barriers are being commonly adopted to prevent virus transmission. However, their efficacy may be limited, particularly in shared indoor spaces, where, in addition to airborne transmission, elements with small surface areas such as elevator buttons, door handles, and handrails are frequently used and can also mediate transmission. We argue that additional measures are necessary to reduce virus transmission when people resume attending schools and jobs that require proximity or some degree of physical contact. Among the available alternatives, UV-C light satisfies the requirements of rapid, widespread, and economically viable deployment. Its implementation is only limited by current production capacities, an increase of which requires swift intervention by industry and authorities.

Ultrafast 3D printing with submicrometer features using electrostatic jet deflection.

Additive manufacturing technologies based on layer-by-layer deposition of material ejected from a nozzle provide unmatched versatility but are limited in terms of printing speed and resolution. Electrohydrodynamic jetting uniquely allows generating submicrometer jets that can reach speeds above 1 m s-1, but such jets cannot be precisely collected by too slow mechanical stages. Here, we demonstrate that controlling the voltage applied to electrodes located around the jet, its trajectory can be continuously adjusted with lateral accelerations up to 106 m s-2. Through electrostatically deflecting the jet, 3D objects with submicrometer features can be printed by stacking nanofibers on top of each other at layer-by-layer frequencies as high as 2000 Hz. The fast jet speed and large layer-by-layer frequencies achieved translate into printing speeds up to 0.5 m s-1 in-plane and 0.4 mm s-1 in the vertical direction, three to four orders of magnitude faster than techniques providing equivalent feature sizes.

Growth dynamics of granular films produced by electrospray.

Particulate coatings produced by electrospray deposition (ESD) of ethyl cellulose particles are found to widen over time. We hypothesize that during the ESD process, the electrospray expands due to repulsion caused by accumulated electrostatic charge in the film. The radial profiles of film thickness, mass density (per unit area), and porosity (gas volume fraction) have been determined as a function of several factors which influence electrostatic charging, namely, collection time, relative humidity, and deposition flux. The mass density has been determined from the local film thickness attained upon thermal annealing, which is largest near the center of the spray. The local porosity is lowest at the film center (at between 0.5 and 0.6), whereas across the film, it is roughly uniform (between 0.6 and 0.7) for deposition in a dry ambient, and slightly higher and more variable for humid ambient deposition. The granular films are compact (with low fractality) as expected for ballistic deposition (large Peclet number).

Turbulence in pneumatic flow focusing and flow blurring regimes.

An important paradigm in pneumatic atomization is the production of droplet sizes in the micron and submicron range, while achieving high energy efficiency by means of simple atomizer designs. Flow focusing (FF) and flow blurring (FB) [A. M. Gañán-Calvo, Appl. Phys. Lett.86, 214101 (2005).] are advancements toward this goal. Both FF and FB feature a fundamental macroscopic soft length scale, e.g., the diameter of the liquid stream formed at a discharge orifice by conversion of pressure into liquid kinetic energy. Droplet diameter distribution data compiled from many experiments reveal that turbulent flow regimes occur in both FF and FB. In FF, like in other jet-based droplet generation techniques, the jet breakup becomes asymmetric for Weber numbers over a transitional one (approximately 20 in FF), becoming turbulent through nonlinear interactions with the gas, downstream of the discharge orifice, for large enough Weber numbers. In FB, the liquid and gas phases interact inherently in a turbulent manner: air accelerates radially and implosively toward the liquid exiting a feeding tube, and mixes with it in a region immediately preceding discharge into ambient air. In our model, droplets form by the action of turbulent pressure fluctuations present in both phases, and a resulting droplet diameter distribution is obtained when coagulation and breakup events of the liquid blobs equilibrate. When the large scale of the turbulent inertial range is taken to be the fundamental soft scale, the model predicts a lower bound to the experimentally determined droplet volume median diameters. On the other hand, the histograms reflect the existence of additional hard length scales imposed by the atomizer outlet geometry.