Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session S29: Soft Matter Electrified IIFocus
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Sponsoring Units: DSOFT Chair: Ignaas Jimidar, Vrije universiteit Brussel Room: 101J |
Thursday, March 7, 2024 8:00AM - 8:36AM |
S29.00001: Plastic Rockets: Stability and optimal design of resilient ion electrospray thrusters Invited Speaker: Paulo C Lozano Electrospray thrusters operating in the pure ionic regime are an attractive technology for a variety of space missions given their potential of high efficiency and scalability from very low to medium power levels. These thrusters are composed of a large number of individual microscopic “emitters” that transport an ionic liquid (a room-temperature molten salt), which is the thruster propellant. Each one of these emitters produce and accelerate an ion beam when a strong electric field is applied at the liquid-surface interface. The right selection of materials and manufacturing techniques of these devices is essential to make significant improvements in their lifetime, resilience, performance and scalability beyond what is possible today. To achieve this goal, it is most important that contemporary designs take advantage of the recent progress made in the scientific understanding of the electrospray ion emission process, while at the same time move away from monolithic designs, in which if a single emitter fails, the whole thruster array might fail. The first step is to define the properties of single emitter elements that could be independently clustered in a thruster array. Multi-physics models are used to establish optimal geometries and operational conditions that maximize the stable behavior of ion-emitting menisci on these single elements. A key finding of this approach is that electrified menisci working in the pure ionic regime, are exceptionally small and have only a limited range of voltages in which emission is stable. The understanding of such behavior and characteristics is essential to move towards the fabrication and testing of near-optimal emitter elements. These novel configurations will be made with a variety of materials not commonly used in the aggressive environment of conventional space thrusters and will have better performance and be significantly more robust to failure. |
Thursday, March 7, 2024 8:36AM - 8:48AM |
S29.00002: Uncovering the role of adsorbed water in the triboelectric effect with acoustic trapping Galien Grosjean, Scott R Waitukaitis Collisions between identical grains can cause large buildups of electric charge. However, the underlying causes of this charge separation process are still an open question. Acoustic trapping allows isolating a single grain, performing individual collisions on a substrate, measuring charge with high precision, and discharging grains using ionizing radiation to reset the system. Using this setup, we examine the fundamental processes behind contact electrification. In particular, we study the role played by adsorbed water on the contacting surfaces. We find that the direction of charge exchange correlates with how much water is on the surface, which itself is a function of the history of the sample. We investigate this by performing humidity ramps and selectively baking surfaces. |
Thursday, March 7, 2024 8:48AM - 9:00AM |
S29.00003: Annihilating Foam using an Applied Voltage Saurabh Nath, Maxime Costalonga, Valentina Negri, Sreedath Panat, Kripa K Varanasi Here we experimentally study the breaking of a 2D aqueous foam upon application of voltage. We find that the origin of the breaking of foam is not electrokinetically driven, rather electrolysis driven. High speed visualization of foam breakage reveals rupture is always initiated at the cathode where hydrogen evolves. We discuss the underlying mechanism and the two distinct regimes of breakage. Finally, our results guide us in designing a bench-scale foam mitigator that can suppress the foam growth using active electrolytic mechanism. |
Thursday, March 7, 2024 9:00AM - 9:12AM |
S29.00004: Self-assembly and control of water drops at an oil-air interface Paul R Kaneelil, Pedro de Souza, Amir Pahlavan, Howard A Stone From the beautiful organization of swarming birds to the growth of molecular crystals, self-assembly occurs all around us at a wide range of length scales. In this work, we reveal the self-assembly and pattern formation of electrically charged water droplets that are floating at an oil-air interface. We show that the assembly occurs because of capillary attraction, more commonly known as the Cheerios effect, and the opposing electrostatic repulsion between the drops. Using experiments and theory, we show that the depth of the oil bath plays a significant role in the distance between the drops assembled at the interface. We highlight the relevance of the type of the boundary containing the entire system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Furthermore, we present ways to control the motion and the assembly of the drops at an interface. |
Thursday, March 7, 2024 9:12AM - 9:24AM |
S29.00005: No time for surface charge: how bulk conductivity can hide charge patterns from KPFM Felix Pertl, Isaac Lenton, Tobias Cramer, Lubuna Shafeek, Scott R Waitukaitis Kelvin Probe Force Microscopy (KPFM) is a powerful tool for studying contact electrification (CE), using electrical signals from a nanoscale AFM tip to spatially map a voltage above a surface that is caused by the presence of charge. Among the most influential results obtained with KPFM are observations of surface charge heterogeneity, i.e. mosaic-like patterns of +/- polarity after CE. Such experiments are often done with PDMS, due to its ability to create conformal contacts. In trying to reproduce such results, we instead observe signatures of spatially uniform surface potential that displays prominent temporal decay over a few minutes. We propose that this is due to the material’s bulk conductivity, which lowers the KPFM potential via the movement of non-CE charge carriers in the electric field gradient between the surface and back electrode. With a simple capacitor model, where the only adjustable parameter is the bulk conductivity, we obtain a value that is consistent with electrical resistivity measurements of PDMS. As further support, we observe the same temporal decay on PMMA and SiO2 surfaces, but with time constants that scale in agreement with their significantly lower conductivities. Going further, we consider more sophisticated models beyond the simple capacitor to account for non-linearities in the conductive response. Our results call into question the presence/stability of surface charge patterns on certain materials, and highlight the role of bulk conductivity during and after CE. |
Thursday, March 7, 2024 9:24AM - 9:36AM |
S29.00006: Revealing large scale surface charge distributions with Scanning Kelvin Probe Microscopy Isaac Lenton, Felix Pertl, Scott R Waitukaitis Scanning Kelvin Probe Microscopy (SKPM) is a macroscale (100 μm - 10 cm) technique to map the electrostatic potential above a surface by scanning a vibrating conducting probe and measuring induced currents. Typically, SKPM is used to measure localized differences in a material's work function e.g. in metals or semi-conductors. For insulators, such as oxide layers or thin polymers, a measurable potential difference can arise from localized surface charge, hence it is an excellent tool for studying charge patterns caused by adhesion, liquid evaporation, etc. However, there is a problem—back conversion from the measured voltage signal to the surface charge density that causes it is extremely complicated. In this talk, I will show how quantitative charge density can be backed out from SKPM data by deconvolution with the experimentally determined Green's function of the system. This presents an advantage over similar techniques at smaller scales (e.g. Kelvin Probe Force Microscopy), where the Green's function is estimated with simulations. Using this approach, we have begun investigating the electrostatic charging patterns caused by several interesting large-scale surface phenomena including peeling of soft polymers from surfaces and evaporation of liquids/solvents. By backing out charge, we take a step towards a quantitative understanding of the charging mechanisms in these and other electrified soft materials. |
Thursday, March 7, 2024 9:36AM - 9:48AM |
S29.00007: Temporal Decay Characteristics of Nanopatterned Tribocharge on Elastomer Surfaces Myung Gi Ji, Jaeyoun Kim, Rana Biswas, In Ho Cho Replica molding turned out to be a promising and facile technique for generating nanopatterned triboelectric charge on elastomer surfaces. As the tribocharges tend to disappear over time, it is imperative to investigate their longevity and temporal decay characteristics for sustained utilization. Monitoring the temporal decay of the nanopatterned tribocharge is a challenging task which necessitates extended and repeated charge measurements at the nanoscale resolution. In this work, we conducted such an experimental study on the tribocharge nanopatterned on PDMS surfaces using Kelvin probe force microscopy. Through extensive curve-fitting of the measurement results, we not only reaffirmed the inherent temporal decay of the tribocharges but also separated them into fast and slowly decaying components. Based on computer modeling of the replica molding process, we further attributed the fast and slowly decaying components to two different tribocharging mechanisms, i.e., tangential sliding and surface-normal separation between the material interface, respectively. These findings will enhance our understanding of tribocharging mechanisms on elastomer surfaces and benefit future exploration of nanoscale triboelectrification. |
Thursday, March 7, 2024 9:48AM - 10:00AM |
S29.00008: Electrostatic discharges are the cause of bipolar charge mosaics at scales between 0.1 mm and 50 mm Yaroslav I Sobolev, Witold Adamkiewicz, Marta Siek, Bartosz A Grzybowski The phenomena of dielectrics becoming charged upon contact and separation have perplexed scientists and engineers over centuries. In traditional understanding, the charges that appear on the two surfaces are due to the qualities of the materials in contact, they are of opposing polarities and are generally evenly distributed. In recent decade or so, it was found that contact electrification can lead to uneven charge distributions -- occasionally even resulting in positively-charged and negatively-charged regions coexisting as neighbours on each of the two surfaces. It's commonly believed these patterns signify some omnipresent spatial irregularities within the contact materials. |
Thursday, March 7, 2024 10:00AM - 10:12AM |
S29.00009: Effects of Solvent Blends on the Microstructure of Polymer Coatings Produced by Self-Limiting Electrospray Deposition (SLED) Andrew Huth, Jonathan P Singer, Robert A Green-Warren, Sarah H Park, Isha Shah, Ayman Rouf We investigate the controlled engineering of microstructures in polymer films using self-limiting electrospray deposition (SLED), with implications for advanced coating applications. Functional coatings play a pivotal role in a wide range of sectors, including technical textiles, microfluidic devices, and microelectronics, where precise control of surface properties is essential. In this study, we utilize polystyrene (PS) as a representative glassy material to elucidate the ability of SLED to generate tunable porous polymer composite films. These coatings are formed of regular microstructures, characterized by intercalated hollow spheres typically measuring between a few hundred nm and up 10 μm in diameter. Common electrospray parameters such as flow rate, nozzle distance, and voltage have been previously investigated, but their ability to control the microstructure of the coatings is limited. By varying the ratios of 1% wt. PS solutions in solvents with different boiling points, namely 2-Butanone (MEK) with a boiling point of ~80°C and propylene glycol methyl ether acetate (PGMEA) with a higher boiling point (~146°C), we systematically explore the influence of solvent properties on film morphology. Specifically, we are able to modify the porosity, and thereby, degree of fusion of the powder structure in the same way as if we were to increase the temperature of the spray. Consequently, the charge retention properties that lead to the self-limiting effect are also reduced, requiring the use of additional processing approaches, such as substrate biasing, to recover the same conformal effects. |
Thursday, March 7, 2024 10:12AM - 10:24AM |
S29.00010: Electrospray Deposition of Photosensitive Polyimide with In-Situ UV Exposure Bryce J Kingsley, Paul R Chiarot Electrospray deposition uses strong electric fields to atomize a liquid suspension into a spray of charged micro-droplets, and solute material dispensed within the suspension will be contained in the droplets. While the droplets are in-flight, the volatile solvent evaporates, leaving behind solute material that is delivered to a target to grow a film. In this work, we use electrospray deposition to grow thin films of UV-photosensitive polyimide (PSPI). In-situ UV exposure was used to activate the UV-photosensitive components in the precursor, while simultaneously depositing the material on the surface to create a film. The characteristics of the in-situ UV films were compared to films with post-deposition UV exposure (post-UV). The microstructure of the in-situ UV films was different to that of post-UV exposure. In-situ UV exposure caused a wavy/dimpled surface (compared to a very flat surface for post-UV), resulting from the change in viscosity of the droplets under in-situ UV exposure. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) both showed that the UV activation of the in-situ UV was nearly identical to that of post-UV. Lastly, breakdown testing was conducted to evaluate the dielectric strength of the deposited films. Both the in-situ UV and post UV films exhibited similar performance, with high breakdown strengths of approx. 430 – 450 V/µm. |
Thursday, March 7, 2024 10:24AM - 10:36AM |
S29.00011: Electric Field and Potential Measurement Techniques for Surfaces Maciej Noras Electric field and potential measurements on surfaces are of interest to many scientists. This paper gives an overview of the electronic circuits used in sensing of fields and potentials. The intent is to assist the researchers with selection of the approach that is the most appropriate and best fitting the conditions of the measurements. Discussion and recommendations pertaining to sensor shielding, signal denoising and conditioning are also provided. Examples of the surface potential measurements are also given and explained. |
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