Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U29: Electrostatic Manipulation of Fluids and Soft Matter I: Electrohydrodynamics |
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Sponsoring Units: DSOFT DPOLY DBIO DFD Chair: Jonathan Singer, Rutgers University, New Brunswick Room: 501 |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U29.00001: Control and characterization of thin film formation by electrospray deposition techniques Kristof Toth, Gregory Doerk, Kevin Yager, Chinedum Osuji Electrospray deposition (ESD) offers a precise and continuous manner by which to deliver sub-micron sized droplets of soft matter in dilute solution to a substrate. Preparation of thin films by ESD have offered advantages of making sequentially deposited, continually equilibrated or even compositionally varying systems on a single substrate for high-throughput characterization. However, understanding and control of the thin film formation are important to limit the Marangoni effects of the droplets on the surface to ensure uniform deposition and coverage. We describe a simple feedback control for visual analysis of various electrospray modes and their corresponding film formation. We also model the surface coverage of the films as a function of deposition time of varying molecular weight homopolymer and block copolymer systems. Furthermore, by changing the geometry of the ESD setup, the area of deposition can be tuned to fit the needs of transmission or reflection X-ray scattering experiments. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U29.00002: Morphological Control of Multifunctional Melting Gel Coatings via Electrospray Deposition Arielle Marie Gamboa, Lin Lei, James Mercado, Jennifer Guzman, Lisa C Klein, Andrei Jitianu, Jonathan Singer Melting gels (MG) are hybrid organic-inorganic gels with glass transition temperatures ~28°C and consolidation temperatures ~150°C. They exhibit thermoplastic behavior below their consolidation temperature and undergo complete cross-linking to form organically modified silica networks upon consolidation. By tuning surface properties, these glass sprays can be used as protective coatings in electronics and anti-corrosion coatings in metals. Electrospray deposition was used to spray solutions of 1wt% MG in 2-butanone. The high voltage produces charged, monodisperse droplets that, due to the low solid content of solutions, deliver small amounts of MG at a continuous rate. The pH of MG synthesis, solution viscosity, and spray polarity were varied to study the effects of charge injection on the consolidation of MG into hybrid glasses. The low MG content of solutions allows for the addition of viscosity modifiers such as terpineol which alter the ability of the spray to dissipate charge. Optical images, film measurements, and FT-IR were used to evaluate the effects of these variables on both the physical morphology along with the chemical structure of the final coatings. |
Thursday, March 5, 2020 2:54PM - 3:30PM |
U29.00003: Using Multiplexed Electrosprays to Manipulate Fluids and (Soft) Materials Invited Speaker: Alessandro Gomez The cone-jet electrospray has revolutionized the field of mass spectrometry but, despite the unique ability of this device to produce quasi-monodispersed particles in a phenomenal size range down to the nanoscale, it has not been applied to other fields, except for small scale proof-of-concept demonstrations. The primary reason for the lack of widespread use is the low flow rate at which the spray is dispersed. Using fluid mechanics and electrostatics, we a) developed criteria for compact multiplexing to increase flow rate drastically, b) microfabricated systems with high packing densities (1.1 104 sources/cm2), reducing the cost per electrospray source, and c) demonstrated successful operation of these devices in the synthesis of (soft) materials (e.g., polymer nanoparticles of controlled shape for drug delivery and battery materials) and in microchip cooling. I will review design criteria and applications. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U29.00004: Self-Limiting Electrospray Deposition for the Surface Modification of Additively Manufactured Parts Dylan Kovacevich, Lin Lei, Daehoon Han, Christianna Kuznetsova, Steven E Kooi, Howon Lee, Jonathan Singer Electrospray deposition utilizes a high voltage to atomize a flowing solution into charged microdroplets loaded with solute. Our previous research investigated the conditions necessary to minimize charge dissipation and deposit a thickness-limited film through self-limiting electrospray deposition (SLED). Such sprays possess the ability to conformally-coat complex three-dimensional objects without changing the location of the spray needle or orientation of the object. This makes them ideally suited for the post-processing of materials fabricated through additive manufacturing (AM), opening a paradigm of independent bulk and surface functionality. Having demonstrated three-dimensional coating with film thickness in the range of 1-50 µm on a variety of conductive objects, in this study we employed model substrates and finite element method simulations to quantitatively study the technique’s limits with regard to geometry and scale. Specifically, we examined recessed features with gaps ranging from 50 µm to 1 cm, as well as the ability to coat surfaces hidden from the line-of-sight of the spray needle. This was then extended to the coating of hydrogel structures printed by AM, as a means to create hydrophobic surfaces without affecting the absorption-driven humidity response. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U29.00005: The Role of Surface Charging in Electrospray Printing of Thin Films Yaqun Zhu, Paul Chiarot We report on the use of electrospray printing to create thin continuous multiscale films from nanoparticle aggregates. Nanomaterials are delivered to the target substrate in a dry state with an excess electric charge. The charge accumulates on the target and its decay rate plays a significant role in the film formation. Films printed on conductive substrates are thick and exhibit a periodic island structure, while films on insulating substrates are thickness-limited and more uniform. We leveraged the surface charging from electrospray to direct the assembly of nanoparticles using electrostatic focusing. This was achieved by patterning a photoresist layer on the target substrate. Charge accumulates on this layer and electrostatic forces direct the emitted material to the exposed parts of the substrate, which created high aspect ratio porous structures when the target substrate was silicon. The deposit growth was inhibited on glass substrates. To quantify the accumulated electric charge and its decay rate, we measured the spray and image charging for nylon and glass substrates. A scaling analysis revealed the importance of the processing conditions in determining the maximum accumulated charge and its decay rate. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U29.00006: Effect of Processing Conditions on the Formation and Microstructure of Electrospray Printed Polymer Films Bryce Kingsley, Paul Chiarot An electrospray uses high electric potential to atomize a liquid suspension. The droplets evaporate in-flight, leaving behind dry solute particles that can be printed on a target substrate. We report on the use of electrospray to print thin films of polytetrafluoroethylene (PTFE) on silicon substrates. The coupled effects of ambient humidity (50%, 10%), suspension conductivity (9, 25, 85 μS/cm), suspension volatility (20%, 60%; alcohol in suspension), and flow rate (0.5, 1 μL/min) on the growth and microstructure of the films is reported. The suspension conductivity governed the net electric charge of the emitted droplets and particles, while humidity and volatility influenced the in-flight droplet dynamics. Films printed in dry air were denser with a well-defined boundary compared to films printed in humid air. Semi-spherical polymer aggregates were delivered to the substrate when the suspension conductivity was low. Suspensions with higher conductivity produced films with a woven structure of interconnected particle groups. Similarly, sprays utilizing a more volatile suspension produced films with a woven structure, with strands of solute linking groups of aggregates. The use of higher flow rates increased the particle packing density in the printed film. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U29.00007: Electrospray deposition of nanowire forests through spinodal gellation Lin Lei, Catherine Nachtigal, Shensheng Chen, Tyler Moy, Xin Yong, Jonathan Singer The morphology of coatings created by electrostatic deposition can be generally divided into three categories: wire mats (electrospinning), particles (electrostatic spray, electrospray deposition), and films (all low-viscosity applications). There should exist nanowire forests as a mixture of wire and particulate deposition. Such a morphology has yet to be observed experimentally, which we propose is the result of spatially-varying viscosity in sprayed droplets. We utilized electrospray deposition (ESD) to explore the spray of methylcellulose (MC) in water:ethanol mixtures. MC possesses a lower critical solution temperature (LCST) in water and water:ethanol blends. Above the LCST, MC and water phase separate concurrently with the rapid evaporation of ethanol, forming a shear-thickening, homogeneous gel phase. This gel can undergo the elongation of electrospinning on a drop-by-drop basis to create forests of individual nanowires. Our study indicates that the homogenous evolution of viscosity is necessary for nanowire forest formation and that the specific viscosity (along with droplet size) further controls the morphology of the forests. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U29.00008: Modelling Nanowire Formation in Electrospray Deposition of Polymeric Droplets Shensheng Chen, Xin Yong The formation of nanowire forests was recently reported for the first time in electrospray deposition (ESD) of methylcellulose solutions. This novel morphology provides new opportunities for nanomaterial processing. The atypical nanowire geometry is a result of complex interplay among electrostatic interaction, solvent evaporation and polymer rheology. We conducted computer simulations to uncover the physics underpinning the morphological evolution of a highly charged polymeric droplets in ESD. We utilize electrostatic dissipative particle dynamics (EDPD) to simulate a model system that captures electrostatic interaction, evaporation, and polymer dynamics. We explored droplet morphological development under different evaporation conditions, which results in distinct modes of viscosity evolution. Our results show that the nanowire formation is a result of spinodal phase separation between polymers and solvents that homogeneously increases viscosity within the droplet. In contrast, a steep viscosity gradient formed leads to a bead-strings morphology. The coupling between electrostatically-driven deformation and evaporation-driven viscosity change in polymeric droplets can result in new and beautiful morphological evolution. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U29.00009: Influence of ionic conductivity on unconfined melt electrospinning of thermoplastics Neelam Sheoran, Brenton Boland, Elnaz Shabani, Russell Gorga, Jason R Bochinski, Laura Clarke Ionic additives incorporated into thermoplastics melts can influence both viscoelastic properties and ionic conductivity. Such effects can be used to manipulate the process of melt electrospinning. In particular, fiber diameter is impacted by the melt viscosity as well as ionic motion within the melt under the influence of an applied electric field. Decreased fiber diameter could open pathways for fabrication of high strength mesoscale thermoplastic nanofibers, which have significant importance for filtration and biological applications. My talk will focus on how altered melt ionic conductivity due to commercial additives affects an unconfined melt electrospinning process. I will report ionic conductivity measurements obtained using broad-band impedance spectroscopy for two different grades of commercial linear low-density polyethylene with varying viscosities as a function of melt temperature and additive concentration. Corresponding viscosity measurements were performed by means of rotational plate rheometry. The rate of jet formation, the number of jets in steady-state, and the resulting fiber diameters were determined to characterize the effects of additive-doping of melts on the unconfined melt electrospinning process. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U29.00010: Unconfined melt electrospinning from molten polymer experiencing an electric discharge to manipulate ion density and ionic conductivity Brenton Boland, Neelam Sheoran, Laura Clarke, Jason R Bochinski Melt electrospinning is a promising way of achieving solvent-free mesoscale fibers for applications in tissue scaffolding, water filtration, and wound dressing; however, large fiber diameters is a persistent problem that must be understood. To address this, we are interested in the role of ionic conductivity in determining fiber diameter in melt electrospinning. We utilize an unconfined geometry where jets spontaneously form and organize on the edge of a melt-covered plate under the influence of the applied electrospinning electric field. Inspired by recent results from our group that show enhancement of ionic conductivity by additives resulted in reduced fiber diameter and by historical work on corona-assisted electrospraying, we controllably create a discharge near the plate edge to manipulate local ion density and alter the electrospinning process. This manipulation of local ion density by pulsed Dielectric Barrier Discharge (DBD) altered the electrospinning process in a way similar to the addition of ionic additives, suggesting a temporary enhancement of ionic conductivity. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U29.00011: Liquid Selection for Electrohydrodynamic Capillary Thermal Switches Tianxing Ma, Darrel Dsouza, Kyrsten McKenzie Ryerson, Matthew Signorelli, Yang Zhao, Chinedum Osuji, Jonathan Singer The deformation of sessile droplets and capillary bridging in a parallel-plate capacitor under DC field has been the subject of several scientific studies. Coaxially located droplets on opposing electrodes experience an attraction in the presence of an electric field. Application of a suitably large field will lead to either the droplets forming a liquid bridge or oscillation between bridged and de-bridged (i.e. droplet) states. We explored the bridging behavior of a variety of liquids in air. Among the liquids that could form a stable field-induced bridge, only a limited set could reversibly make and break the capillary bridge by switching the electric field on and off. The ability to form a switchable liquid bridge is a function of both the liquid’s properties, including surface tension, electric conductivity, and dielectric constant, and external conditions such as electrode separation, droplet volume, and the substrate selection. Taking advantage of this phenomenon, through periodically forming and breaking the capillary bridge, we demonstrate the realization of a planar thermal switch that can control the flow of heat. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U29.00012: Dielectric modulation of two-dimensional dipolar materials Ziwei Wang, Erik Luijten Spontaneous pattern formation plays an important role in a wide variety of natural phenomena and materials systems. A key ingredient for the occurrence of modulated phases is the presence of competing interactions, generally of different physical origins. We demonstrate that in dipolar films, a prototypical system for pattern formation, patterns can be induced by dielectric effects alone [1]. A rich phase diagram arises, where striped and circular morphologies emerge with geometric properties that can be controlled through variation of particle shape and substrate permittivity or permeability. These effects are particularly enhanced by metamaterial substrates. |
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