Session D09: Filaments, Jets, and Aerosols

A Flow and Aerosol Guide to the Orchestra

In the midst of the COVID 19 pandemic, live musical activities have come to a standstill to protect both musicians and the public. Among musical groups, orchestral ensembles both personnel heavy and instrumentally diversified face the challenge of contamination. In particular, assessing whether wind instruments are possible vectors of contamination through the dispersion of aerosol from human origin is a chief concern in mitigating the effect of the spread of the disease.

This study, made possible by the participation of members of the Philadelphia Orchestra, brings insight on the modes of production and early life of aerosol droplets emitted during the use of wind instruments. We show that aerosol size distribution shares similar characteristics as normal speech and vocalization, and is sensitive to the material of the instrument. Furthermore flow characterization shows that although small, the flow rate at instrument orifices is sufficient for the aerosol to be spread in the immediate environment of the musician and be picked up by the ambient for longer distance of travel.   

Tone generation in an open-end organ pipe: How a resonating sphere of air stops the pipe

According to the classical Helmholtz picture, an organ pipe while generating its eigentone has two anti-nodes at the two open ends of a cylinder, the anti-nodes being taken as boundary condition for the corresponding sound. Since 1860 it is also known that the pipe actually sounds lower, which is to say the pipe "sounds longer," than it is, a long-standing enigma. As for the pipe end, we have resolved this acoustic enigma by detailing the physics of the airflow at the pipe's open end and showing that the actual boundary condition is the pipe's acoustically resonating vortical sphere (PARVS). The PARVS geometry entails a sound-radiating hemisphere based on the pipe's open end and enclosing a vortex ring. In this way we obtain a physical understanding of the dependence of the end correction upon the pipe's radius and also of the sound of the flute, mankind's oldest musical instrument.   

Spark-induced drops and jets

When a high-voltage capacitor is discharged between two electrodes in a conducting liquid, the resulting spark can create a rapidly expanding cavitation bubble that sends out a shock wave. We exploit this phenomenon in fine capillary tubes to create high-speed liquid jets that could be used for intradermal drug delivery. Depending on the voltage discharge, we can observe either a drop or a focused jet with speed in the range O(10¹-10²) m/s. We conducted a systematic study to understand the role of various parameters such as fluid viscosity, capillary geometry, and voltage on the speed and shape of the jet or drop. The transition between the droplet and jet regimes is primarily governed by the location of the spark relative to the open end of the capillary, as well as liquid viscosity, and discharge voltage. In the jet regime, the speed is dictated by the discharge voltage. We propose a robust system to generate drop/jet-on-demand using low voltage discharge techniques. This system is useful in various applications such as inkjet printing, coating techniques, and needle-free jet injections.   

Electrospray Printing of Polymeric Films onto Substrates with Non-Homogenous Electrical Properties

Electrospray printing is an additive manufacturing technique for creating thin polymeric films. A unique aspect of electrospray is that the electrical properties of the target substrate govern the thickness of the film. For example, a thicker film will be created on conductive portions of a surface compared to sections that are insulating. In this work, polyimide is delivered to a glass substrate with a periodic array of patterned gold electrodes. The emitted particles track the electric field lines from the electrospray nozzle to the grounded electrode array. The polymer is preferentially deposited onto the gold surface and is defined by a preference ratio (i.e. gold versus glass deposition). A comparison is made with an ungrounded electrode array, which showed thinner films but still a very high preference ratio. By tuning the flow rate of the precursor solution, a range of film thicknesses and microstructures are obtained. For sufficiently high flow rates, the precursor solvent does not fully evaporate in-flight, resulting in the polymer depositing in a semi-dry state. This modified the microstructure of the films, creating a thinner layer with higher density. The function of the printed films is characterized by measuring their adhesion and dielectric breakdown strength.   

Passively Focused Electrospray Printing of Polymeric Films

In electrospray printing, strong electric fields are used to atomize a liquid solution into a plume of charged microdroplets containing the print material. Owing to their small size, the solvent rapidly evaporates to leave behind dry particles that are delivered to a target substrate to create a film. Here, we report on the use of a high-throughput electrospray printing system to deploy polyimide onto silicon. Passive electrostatic focusing was used to increase the rate of film growth, resulting in dense particulate films in relatively short print times. The film growth was asymptotic and dependent on the emitter-substrate separation distance. Films printed at a closer separation distance grew more rapidly and had a greater (maximum) thickness than those printed from further away. The functional performance of the films was characterized by wettability and dielectric breakdown testing. The wettability was independent of the print conditions due to the negligible variation in surface roughness, resulting in an average contact angle of 110°. Breakdown potentials exceeding 1 kV were achieved with the thickest films, however the breakdown field strength decreased with increasing film thickness. As such, the thinner films achieved the highest breakdown field strength of ~142 V/??m.   

Electrospray Printing of Conformal Polyimide Films onto Complex Geometries

Electrospray printing is an additive manufacturing technique that utilizes a high electric potential to atomize a liquid suspension into a plume of droplets. In this work, the droplets consist of polyimide nanoparticles encapsulated in a volatile solvent, dimethylformamide, that evaporates in-flight due to the high surface area to volume ratio. The dry polymer nanoparticles are deposited onto a target, aggregating over time to build continuous films. Electrospray creates conformal coatings by targeting areas that are not directly within the line of sight of the emitter, making it an ideal process to coat complex geometries. We use electrospray to conformally coat 200 micrometer diameter copper wires and 20 micrometer diameter bond wires and pads. We report on how film thickness and coverage are governed by the print conditions, including spray time, delivery rate, and sprayed mass. We show that electrospray provides a uniform coating around the perimeter of the wires along the majority of the exposed length. Scanning electron microscopy is used to measure the film thickness and microstructure. A salt immersion assay showed that a bonded copper wire with an electrospray-printed polyimide coating provides 25 times better corrosion resistance compared to an uncoated wire.   

Probing electrically driven nanojets by energy and mass analysis in vacuo

Time of flight (TOF) and energy analysis in vacuum are used in series to determine jet velocity Uj, diameter dj, electrical potential Vj and energy dissipated V at the breakup point of electrified nanojets of the ionic liquid 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMI-FAP). The full spray is periodically gated by a grid held at a high voltage Vg, and received at a collector where the measured flight times provide the distribution of drop speeds u. Varying Vg provides the bivariate distribution of drop energies ξ and velocities. The collector plate, centred with the beam axis, is divided into eight concentric rings, yielding the angular distribution of the spray current, and high resolution (u,ξ ) values in the whole spray. The energies of various particles of given u are all well defined, but depend uniquely on u, even though u and ξ are in principle independent experimental variables. Slow and fast particles have energies respectively well above and below the capillary voltage Ve (1.64 kV). As previously shown by Gamero-Castaño & Hruby (J. Fluid Mech., vol. 459, 2002, pp. 245–276), this behaviour is due to the 2-stage acceleration process, first jointly in the jet for all particles, and then separately for free flying drops or ions of different mass/charge. The measured two-dimensional distributions of u and ξ provide the jet velocity Uj (∼0.44 km s−1) and electrical potential Vj (1.2 kV) at the breakup point. All molecular ions originate near the breakup point rather than the meniscus neck. A measurable fraction of anomalously fast drops is observed that must come from Coulomb fissions of the main drops.   

Can conical nozzles improve print quality in embedded 3D bioprinting?

Embedded 3D printing is a fabrication technique wherein a nozzle is embedded into a support bath and extrudes either continuous filaments (Embedded Ink Writing) or droplets (Embedded Droplet Printing). The support bath, which is usually a viscoelastic hydrogel, holds the form of the printed part until the part is cured and removed from the bath. Embedded 3D printing enables more flexibility in the rheology of the ink than freestanding direct ink writing, allowing for low-viscosity inks and inks without yield stresses. As a result, this technique is particularly useful for bioprinting. In this work, we use computational fluid dynamics simulations in OpenFOAM to compare the effects of conical and cylindrical nozzles on print quality. Commonly, conical nozzles are used to improve cell survival during printing of bio-inks. Confirming this practice, these simulations indicate that with increasing nozzle angle, the shear stresses inside of the nozzle decrease. However, larger nozzle angles can lead to surface roughness and anisotropy in printed parts. As such, during nozzle selection for bioprinting applications, the trade-offs between shear stress and filament shape defects should be considered.   

Visualization of Tattooing: What happens beneath the surface?

Since the Stone Age, the tattooing process has been used for medical and aesthetical purposes. This process involves a needle (or an array of needles) being used to deposit ink into the dermis. Over the past decade, several studies have reported the prospects of using tattooing as an intradermal (ID) drug delivery technique, however an understanding of the fluid dynamics involved in the delivery of fluid into the skin is still lacking. In this study, we probe into this process through in vitro experiments. We use both arrays, e.g., a “flat-line” 5-needle array, that is typically used by tattoo artists, as well as single-needle devices for more detailed investigation of the dynamics, to inject fluids into transparent gels with moduli ~ 15 kPa. High-speed imaging was used to visualize the injection process, along with image analysis to estimate the amount of fluid delivered after each injection up to the 50^th injection. We investigate the role of reciprocating frequency (f=O(10-100)Hz) and physical properties of the fluids on the volume infused (Vo) after injection. Based upon our observations, we hypothesize that the principal delivery mechanism is capillary imbibition.   

Fundamental photopolymer additive manufacturing informed by a uniform, calibrated light engine

Photopolymer additive manufacturing (PAM) is fueling an explosion of opportunities and capabilities for US production of 3D printed products in a wide range of industries including automotive, healthcare, energy, and consumer goods. While digital light processing (DLP), a subset of PAM, relies heavily on patterned light sources with up to millions of individual pixels in the 1 µm to 100 µm length scale, recent research has revealed the dramatic dependence of the polymerization process on slight variations in photo-exposure conditions. Reliance by the PAM industry on non-uniform, poorly-characterized light engines can convolve exposure variability with fundamental photochemistry and photo-physics, hindering advancements. Specifically, a lack of single pixel characterization and multi-pixel uniformity in the light engine hinders fundamental characterization of the photopolymerization reactions that underpin PAM. By developing a uniform, well-characterized light engine, this work provides a foundation to inform the comprehensive models required to improve the fidelity and integrity of PAM products. This presentation will provide an overview of the development of a state-of-the-art light engine, comparing prints from commercially available DLP systems to those from the uniform light-engine, and highlight the need for rigorous characterization of sources to streamline PAM resin development for reliable, high performance PAM.    

Flow-induced stretch, alignment, and relaxation of semi-crystalline polymers in material extrusion additive manufacturing

Most soft matter additive manufacturing processes produce parts with asymmetric material properties due to the weak interface between layers. In thermoplastic material extrusion, these anisotropic properties are attributed to rapid cooling and subsequent limited diffusion time across the interface. However, the thermal history and flow-field have an additional effect on semi-crystalline polymers, resulting in changes to the crystal structure due to non-quiescent or flow-induced crystallization. These effects are most dramatic at the interface and can produce varying and asymmetric crystal morphologies in the extrudate. To quantify these effects, we use a combination of in-situ thermography and polarized light imaging to characterize cooling rate and residual stress during printing and ex-situ polarized imaging and micro-beam wide-angle x-ray scattering to characterize the non-equilibrium state of the polymer and final crystalline morphology. From these measurements, we see high extrusion speeds and low extrusion temperatures leave the polymer in a stretched and aligned state, which changes the nucleation density and crystal morphology at the interface between layers. In contrast, printing at high temperatures provides sufficient mobility for the chains to relax to an equilibrium state before crystallizing or cooling to the glass transition temperature.   

A study on the pulsation reduction effect of a valve composed of elastic bodies.

The peristaltic pump is the most widely used actuator in a situation where constant deliver is important. The purpose of this study is to completely control the flow pulsation generated in the process of suctioning and transmitting fluid by the peristaltic pump. The study was began based on the flow that comes out when a small hole is made in a water filled balloon. We have manufactured a valve composed of an elastic body such as rubber balloon, it could be possible to remove all pulsation of the fluid passing through the pump. In this paper, the manufacturing and experimental method of the elastic body is explained. We measured a pulsation using piezoelectric effects for comparing with commercial constant valve. We prove that pulsation can be effectively reduced when the elastic body is used. The obtained maximum reduction ratio of flow fluctuation is 98.03%. Maximum reduction ratio of Commercial constant valve is 85.46%. This means that the softvalve we manufactured is useful.   

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Swirling flow is one of the handful of problems in fluid dynamics which involves wide range of applications in combustion chambers, cyclone separators, turbomachines etc.Swirl enhances mixing and flame stability in combustion chambers.This study is an approach to understand the near field entrainment characteristics of a swirling liquid jet.Volume of fluid based three dimensional numerical simulations have been carried out for Swirl numbers ranging from 0.5 to 1.55. There are two main objectives of this paper (1)To understand the impact of recirculation zones on entrainment of air and (2)To identify the near field entrainment characteristics of the swirling jet.The results indicates that, multiple recirculation zones develops in the flow near the shear layers which enhances entrainment of the ambient air. For swirl numbers varying from 0.5 to 1.1, a consistent increment in the entrained mass of air has been observed .Moreover the entrainment coefficient ranges from 0 to 0.4 for the present range of swirl numbers and Reynolds number considered.