Kripa K. Varanasi

Oil-water separation is critical for energy and manufacturing industries, including crude refining and oil recycling, yet remains challenging for nanoscale emulsions where gravity-based methods are ineffective. Conventional demulsification techniques using immersed electrodes are limited by low electric field strengths to avoid shorting, requiring toxic demulsifiers and water-intensive desalting. Here, we propose a non-Laplacian electrocoalescence strategy using space-charge emitter electrodes with an air gap, enabling corona-based charge injection to achieve electric fields up to 8 kilovolts per centimeter and an eightfold enhancement over conventional methods. The enhanced electrostatic field enables a 64-fold increase in dipole-dipole attraction forces, leading to accelerated droplet coalescence and separation across water fractions ranging from 2 to 20%, without chemical demulsifiers. Furthermore, we demonstrate a scalable, flow-through space-charge emitter system capable of continuous demulsification of nanoscale emulsions. This approach provides a sustainable, chemical-free solution to address critical environmental and economic challenges in oil-water separation processes.

The culture of anchorage-dependent cells is crucial in the biomedical industry, yet traditional dissociation methods, including enzymatic and mechanical techniques, often reduce cell viability and induce cellular stress, particularly in sensitive primary cell populations. These approaches are also resource-intensive, generate considerable biological waste, and lack compatibility with scalable or automated platforms. Here, we propose an enzyme-free and on-demand cell detachment strategy utilizing alternating electrochemical redox-cycling. This approach induces reversible morphological changes that promote cell detachment while maintaining high viability and stable proliferation. When applied to MG63 human osteosarcoma cells on a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate nanocomposite biointerface, the application of voltage initiates redox-cycling, generating ion flux that disrupts cell adhesion and facilitates rounding within 5 min. Detachment efficiency increases from 1 to 95% at an optimal frequency of 0.05 Hz, with cell viability exceeding 90%, demonstrating the feasibility and effectiveness of this method. We introduce an efficient and enzyme-free solution for cell harvesting, which is compatible with automated cell culture and biomanufacturing workflows.

On-demand cell detachment is of great importance in various applications in biosensitive environments. Existing methods such as enzymatic treatments and mechanical scraping are often time-consuming, labor-intensive, and harmful to cells. In this work, we demonstrate a method of detaching cells from substrates using electrochemical bubble generation without biocide generation. We demonstrate that shear stress generated by fluid flow beneath a rising bubble is the primary mechanism for cell detachment. This strategy, relying solely on physical forces and independent of cell or surface chemistry, can therefore work with a large range of the media, surfaces, and cells. We successfully implement this discovery at the lab-scale by designing a prototype for on-demand cell detachment that maintains high cell viability. The developed principle could find applications in high-throughput culture settings, such as algae photobioreactors or cell culture environments.

In this letter, we characterize quantitatively the complex phenomenon of debubbling via aerophilic membranes by examining local interactions at the scale of single bubbles. We identify three asymptotic limits of evacuation dictated by Rayleigh, Ohnesorge and Darcy dynamics, the physics of which we capture using simple scaling laws. We show that beyond a threshold permeability, bubble evacuations become constant in time - a feature we understand as an inertio-capillary limit. Our experiments reveal that the fastest bubble evacuations require an interface that is nearly a liquid, but not quite.

Electrochemical CO 2 capture systems using hydroxide solutions face stiff performance trade-offs, as the hydroxide ions necessary for rapid CO 2 absorption reduce the current efficiency of subsequent electrochemical CO 2 release. In this work, we propose a carbonate/hydroxide separation step between CO 2 absorption and release to provide a concentrated carbonate stream for efficient electrochemical release and a separate hydroxide stream for rapid absorption. We combine experiments on CO 2 absorption, nanofiltration separation, and electrochemical release to build a comprehensive model that illustrates system performance trade-offs. We find that employing commercial nanofiltration membranes for separation increases the electrochemical current efficiency by as much as six-fold without sacrificing absorption rate. In the case of Direct Air Capture, the nanofiltration approach reduces costs by 20-30% and significantly increases the operational flexibility of the system. Such carbonate/hydroxide separations may also find use in other systems such as point source capture and integrated CO 2 capture and conversion to valuable products.

As hydrogen energy systems advance globally, management of hydrogen ingress remains a significant challenge. When hydrogen gas contacts a metal, it dissociates into hydrogen atoms, which subsequently enter and embrittle the metal. This detrimental process not only affects hydrogen pipelines but also metal components in aqueous corrosive environments, such as heat exchanger tubes in geothermal systems. Disrupting hydrogen ingress at the metal interface is therefore crucial to prevent subsequent embrittlement within the metal. Here, we show that cloaking liquid films as thin as 1 nm minimize hydrogen ingress into steel. These films, made from Krytox lubricant, are immiscible with water and preferentially wet steel in aqueous environments. Using a Devanathan–Stachurski electrochemical permeation cell, we show that in mildly acidic electrolytes, cloaking liquid films reduce hydrogen diffusivity in steel by 80% and the subsurface hydrogen concentration, a measure of hydrogen uptake in the steel, is decreased by 86%. X-ray photoelectron spectroscopy confirms the stability of the cloaking barrier film after 25 h of accelerated electrochemical permeation testing. In alkaline environments, similar reductions in hydrogen diffusivity (82%) and subsurface hydrogen concentration (90%) were observed, demonstrating the versatility of these films as barriers to hydrogen ingress. Additionally, we apply these thin cloaking films to enhance the barrier properties of defective zirconia coatings on steel. Although zirconia impedes hydrogen ingress, pinhole defects in the zirconia coating can provide pathways for hydrogen entry. We show that thin cloaking liquid films combined with a 75 nm zirconia coating reduce hydrogen diffusivity in steel by 84%, even when defects are present. Thicker composite coatings of zirconia and lubricant-impregnated microtextured surfaces could further enhance long-term protection against hydrogen ingress in practical applications.

Biofouling, commonly referred to as the unwanted deposition of cells on wetted solids, is a serious operational and environmental issue in many underwater and biomedical applications. Over the past decade, lubricant-impregnated surfaces (LIS) arose as a potential solution to prevent fouling, owing to their unique layer of lubricant masking the solid from the outer environment, thereby preventing biofouling. However, living microorganisms alter their environment by reproducing and secreting biomolecules, which can threaten the stability of such coatings over time. In this paper, we show that secretion of biomolecules from aquatic cells and subsequent changes in the interfacial tension of the surrounding media can trigger dewetting of the lubricant, ultimately exposing the surface to the outer solution and therefore becoming prone to fouling. By observing LIS immersed in Nannochloropsis oculata algae solutions at various stages of population growth, we establish a correlation between the decrease in interfacial tension and wetting states of the surface. We also visualize dewetting of the lubricant through confocal imaging performed in situ. Finally, we establish a diagram providing fundamental insights to design sturdy LIS circumventing such dewetting, therefore ensuring long-term protection against biofouling upon extended immersion in living cell solutions.

In recent years, marine carbon removal technologies have gained attention as a means of reducing greenhouse gas concentrations. One family of these technologies is electrochemical systems, which employ Faradaic reactions to drive alkalinity-swings and enable dissolved inorganic carbon (DIC) removal as gaseous CO 2 or as solid minerals. In this work, we develop a thermodynamic framework to estimate upper bounds on performance for Faradaic DIC removal systems. To assess the fundamental mass balances of these systems, we first define unit operations in the DIC/total alkalinity (TA) space. By coupling a seawater speciation model to an electrochemical framework, we provide a generalized comparison of gas evolution and mineralization DIC removal routes, focusing on asymmetric charge/discharge systems. We then show how this framework can be extended to other processes, such as those employing dilution schemes. Finally, we provide a minimum energetic assessment of mCDR pathways relative to direct air capture. Overall, this thermodynamic framework aims to guide system and process design and to drive material discovery and engineering for future electrochemical marine DIC removal systems.

Dust accumulation on solar panels is a mjor operational challenge faced by the photovoltaic industry. Removing dust using water-based cleaning is expensive and unsustainable. Dust repulsion via charge induction is an efficient way to clean solar panels and recover power output without consuming any water. However, it is still challenging to remove particles of ≈30 µm and smaller because Van der Waals force of adhesion dominates electrostatic force of repulsion. Here, the study proposes nano-textured, transparent, electrically conductive glass surfaces to significantly enhance electrostatic dust removal for particles smaller than ≈30 µm. We perform atomic force microscopy pull-off force experiments and demonstrates that nano-textured surfaces reduce the force of adhesion of silica micro-particles by up to 2 orders of magnitude compared to un-textured surfaces from 460 to 8.6 nN. We show that reduced adhesion on nano-textured surfaces results in significantly better dust removal of small particles compared to non-textured or micro-textured surfaces, reducing the surface coverage from 35% to 10%. We fabricate transparent, electrically conductive, nano-textured glass that can be retrofitted on solar panel surfaces using copper nano-mask based scalable nano-fabrication technique and shows that 90% of lost power output for particles smaller than ≈10 µm can be recovered.

Electrochemical CO2 reduction has emerged as a promising CO2 utilization technology, with Gas Diffusion Electrodes becoming the predominant architecture to maximize performance. Such electrodes must maintain robust hydrophobicity to prevent flooding, while also ensuring high conductivity to minimize ohmic losses. Intrinsic material tradeoffs have led to two main architectures: carbon paper is highly conductive but floods easily; while expanded Polytetrafluoroethylene is flooding resistant but non-conductive, limiting electrode sizes to just 5 cm2. Here we demonstrate a hierarchically conductive electrode architecture which overcomes these scaling limitations by employing inter-woven microscale conductors within a hydrophobic expanded Polytetrafluoroethylene membrane. We develop a model which captures the spatial variability in voltage and product distribution on electrodes due to ohmic losses and use it to rationally design the hierarchical architecture which can be applied independent of catalyst chemistry or morphology. We demonstrate C2+ Faradaic efficiencies of ~75% and reduce cell voltage by as much as 0.9 V for electrodes as large as 50 cm2 by employing our hierarchically conductive electrode architecture. Conventional electrochemical CO2 conversion electrodes are bound by a tradeoff which prevents electrodes from being both stable and scalable. Here the authors develop a composite electrode which achieves both, enabling scaling to a 50 cm2 electrode with low ohmic losses.

Fluid instabilities can be harnessed for facile self-assembly of patterned structures on the nano- and microscale. Evaporative self-assembly from drops is one simple technique that enables a range of patterning behaviors due to the multitude of fluid instabilities that arise due to the simultaneous existence of temperature and solutal gradients. However, the method suffers from limited controllability over patterns that can arise and their morphology. Here, we demonstrate that a range of distinct crystalline patterns including hexagonal arrays, branches, and sawtooth structures emerge from evaporation of water drops containing calcium sulfate on hydrophilic and superhydrophilic substrates. Different pattern regimes emerge as a function of contact line dynamics and evaporation rates, which dictate which fluid instabilities are most likely to emerge. The underlying physical mechanisms behind instability for controlled self-assembly involve Marangoni flows and forced wetting/dewetting. We also demonstrate that these patterns composed of water-soluble inorganic crystals can serve as sustainable and easily removable masks for applications in microscale fabrication.

The adverse effects of electrochemical bubbles on the performance of gas-evolving electrodes are well known, but studies on the degree of adhered bubble-caused inactivation, and how inactivation changes during bubble evolution are limited. We study electrode inactivation caused by oxygen evolution while using surface engineering to control bubble formation. We find that the inactivation of the entire projected area, as is currently believed, is a poor approximation which leads to non-physical results. Using a machine learning-based image-based bubble detection method to analyze large quantities of experimental data, we show that bubble impacts are small for surface engineered electrodes which promote high bubble projected areas while maintaining low direct bubble contact. We thus propose a simple methodology for more accurately estimating the true extent of bubble inactivation, which is closer to the area which is directly in contact with the bubbles.

International audience

Electrochemical CO2 reduction has emerged as a promising CO2 utilization technology, with Gas Diffusion Electrodes (GDEs) becoming the predominant architecture to maximize performance. GDEs must maintain robust hydrophobicity to prevent flooding, while also ensuring high conductivity to minimize ohmic losses. Intrinsic material tradeoffs have led to two main GDE architectures: carbon paper is highly conductive but floods easily; ePTFE is flooding resistant but non-conductive, limiting electrode sizes to just 5cm2. Here we demonstrate a Hierarchically Conductive GDE architecture (HCGDE) which overcomes these limitations by employing inter-woven microscale conductors within a hydrophobic ePTFE membrane. We develop a model which captures the spatial variability in voltage and product distribution on electrodes due to ohmic losses and use it to rationally design the HCGDE. The HCGDE architecture overcomes scaling limitations, achieving C2+ Faradaic efficiencies of ~75% for electrodes as large as 50cm2. Our approach can be broadly applied to scale any electrode, independent of catalyst chemistry and morphology.

The undesirable, yet inevitable, presence of bacterial biofilms in spacecraft poses a risk to the proper functioning of systems and to astronauts' health. To mitigate the risks that arise from them, it is important to understand biofilms' behavior in microgravity. As part of the Space Biofilms project, biofilms of Pseudomonas aeruginosa were grown in spaceflight over material surfaces. Stainless Steel 316 (SS316) and passivated SS316 were tested for their relevance as spaceflight hardware components, while a lubricant impregnated surface (LIS) was tested as potential biofilm control strategy. The morphology and gene expression of biofilms were characterized. Biofilms in microgravity are less robust than on Earth. LIS strongly inhibits biofilm formation compared to SS. Furthermore, this effect is even greater in spaceflight than on Earth, making LIS a promising option for spacecraft use. Transcriptomic profiles for the different conditions are presented, and potential mechanisms of biofilm reduction on LIS are discussed.

Abstract Controlling cell adhesion to surfaces is an important, but difficult, problem. Current methods to control adhesion rely on surface functionalization, which have limited material choice to avoid cell toxicity and are typically cell specific. Herein, cell adhesion is modulated by using nanometric high‐k dielectric films. Voltage is applied across the dielectric film, changing the film surface's zeta potential, ζ. High performance dielectrics, HfO 2 and SiO 2 , enables a change in the ζ polarity and magnitude over large, 100 mV, ranges by applying ≈1 V across the dielectrics with ≈1nW power draw. Freshwater Chlorella vulgaris and saltwater Nannochloropsis oculata, which have a negative ζ, are used as model cells. Cell adhesion is observed to be inhibited when both surface and cell ζ are negative and enhanced when surface ζ is positive and cell ζ are negative using microfluidic experiments. Finally, millimetric scale cell patterning is demonstrated by spatially modulating ζ with no observed toxicity to cells over 4 weeks.

A recently discovered phenomenon in which crystalline structures grown from evaporating drops of saline water self-eject from superhydrophobic materials has introduced new possibilities for the design of anti-fouling materials and sustainable processes. Some of these possibilities include evaporative heat exchange systems using drops of saline water and new strategies for handling/processing waste brines. However, the practical limits of this effect using realistic, non-ideal source waters have yet to be explored. Here, we explore how the presence of various model aquatic contaminants (colloids, surfactants, and calcium salt) influences the self-ejection phenomena. Counterintuitively, we find that the addition of "contaminant" chemistries can enable ejection under conditions where ejection was not observed for waters containing only sodium chloride salt (e.g., from smooth hydrophobic surfaces), and that increased concentrations of both surfactants and colloids lead to longer ejection lengths. This result can be attributed to decreased crystallization nucleation time caused by the presence of other species in water.

Although protein crystallization offers a promising alternative to chromatography for lower-cost protein purification, slow nucleation kinetics and high protein concentration requirements are major barriers for using crystallization as a viable strategy in downstream protein purification. Here, we demonstrate that nanoparticles functionalized with bioconjugates can result in an in situ template for inducing rapid crystallization of proteins at low protein concentration conditions. We use a microbatch crystallization setup to show that the range of successful crystallization conditions is expanded by the presence of functionalized nanoparticles. Furthermore, we use a custom machine learning-enabled emulsion crystallization setup to rigorously quantify nucleation parameters. We show that bioconjugate-functionalized nanoparticles can result in up to a 7-fold decrease in the induction time and a 3-fold increase in the nucleation rate of model proteins compared to those in control environments. We thus provide foundational insight that could enable crystallization to be used in protein manufacturing by reducing both the protein concentration and the time required to nucleate protein crystals.

CO 2 is removed from oceanwater acidified during chloride-mediated electrochemically modulated reaction of bismuth electrodes.