Air-water Interface Research Articles

Modulating outcomes of oil drops bursting at a water–air interface

Recent studies have shown that capillary waves generated by the bursting of an oil drop at the water–air interface produces a daughter droplet inside the bath, while a part of it floats above it. Successive bursting events produce next generations of daughter droplets, gradually diminishing in size until the entire volume of oil rests atop the water–air interface. In this work, we demonstrate two different ways to modulate this process by modifying the constitution of the drop. First, we introduce hydrophilic clay particles inside the parent oil drop and show that it arrests the cascade of daughter droplet generation preventing it from floating over the water–air interface. Second, we show that bursting behavior can be modified by a compound water–oil–air interface made of a film of oil with finite thickness and design a regime map, which displays each of these outcomes. We underpin both of these demonstrations by theoretical arguments providing criteria to predict outcomes resulting therein. Finally, all our scenarios have a direct relation to control of oil–water separation and stability of emulsified solutions in a wide variety of applications, which include drug delivery, enhanced oil recovery, oil spills, and food processing, where a dispersed oil phase tries to separate from a continuous phase.

Tailoring of Polypyrrole Wrapped Cotton Fiber Sponge for Simultaneous Solar Steam and Solar Thermoelectric Generation

AbstractSolar steam generation (SSG) using floatable evaporators to absorb solar energy and generate heat at the water–air interface has attracted increasing interest in achieving water purification and desalination. Using biodegradable and porous biomass materials as evaporators to fabricate high‐performance SSG devices is a promising route, but the poor efficiency and fussy and energy‐intensive manufacturing process for biomass material‐based evaporators will restrict their practical application. Here, an old commercial cotton quilt is used to prepare porous cotton fiber sponges (CFS) via a simple and scalable mechanical foaming strategy. After being decorated by the polypyrrole (PPy), the CFS@PPy sponge with a hierarchical porous structure shows broadband light absorption capacity, good hydrophilicity, and excellent photothermal capacity. The obtained sponge can be directly used as an evaporator floating on the seawater and shows a high steam‐generation efficiency of 85.07% under 1 sun irradiation. Additionally, it can be used as a photothermal material to construct a solar thermoelectric generation (STG) device and achieve an enhanced open‐circuit voltage (Vout) of 0.4 V and output current (Iout) of ≈59.6 mA under 5 sun irradiations. With the help of a boost converter, the power generation from the STG device can continuously charge the electric bulb and wristband.

Evolution of water wave packets by wind in shallow water

We use the Korteweg–de Vries (KdV) equation, supplemented with several forcing/friction terms, to describe the evolution of wind-driven water wave packets in shallow water. The forcing/friction terms describe wind-wave growth due to critical level instability in the air, wave decay due to laminar friction in the water at the air–water interface, wave stress in the air near the interface induced by a turbulent wind and wave decay due to a turbulent bottom boundary layer. The outcome is a modified KdV–Burgers equation that can be a stable or unstable model depending on the forcing/friction parameters. To analyse the evolution of water wave packets, we adapt the Whitham modulation theory for a slowly varying periodic wave train with an emphasis on the solitary wave train limit. The main outcome is the predicted growth and decay rates due to the forcing/friction terms. Numerical simulations using a Fourier spectral method are performed to validate the theory for various cases of initial wave amplitudes and growth and/or decay parameter ranges. The results from the modulation theory agree well with these simulations. In most cases we examined, many solitary waves are generated, suggesting the formation of a soliton gas.

Effect of Hofmeister Anions Series on the Langmuir Film of Tetronic 90R4 and Tetronic 701 Block Copolymers.

The air-water interfacial behavior of Tetronic 90R4 and Tetronic 701 was studied in the presence of sodium salts with different anions namely , , , , Cl-, Br-, , I-, and SCN-. Their presence in the subphase altered the arrangement of both tetronic molecules at the air-water interface. The limiting mean molecular area of the Langmuir film for both tetronics was found to be ion specific; it increased following the series < < < < Cl-< Br-< < I-< SCN-, which was found to be aligned with the Hofmeister series of anions. Furthermore, the study explored the effects of the hydration enthalpy, free energy, viscosity BJD coefficient, and polarizability of these anions on the interfacial behavior of tetronics. The Langmuir-Blodgett film morphology was also examined in the presence of these salt species using SEM. Morphologies were explained considering kosmotropic and chaotropic nature of these anions.

Spontaneous Molecular Bromine Production in Sea-Salt Aerosols.

Bromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea-salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br-) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br-, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton-electron transfer process at the air-water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br- and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm-2 ⋅ s-1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br- activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br- activation. Our findings highlight the crucial role of the proton-electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br- by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Coalescence time of ellipsoidal-wobbling bubbles at surfactant-free interface: Experimental analysis and collision criteria

This work experimentally analyzed bubbles' coalescence with an air-water interface in the ellipsoidal-wobbling regime for different bubble approach velocities, encompassing the ranges of Eötvös, Weber, and Reynolds numbers of 2-3, 1-4, and 500-1100, respectively. We employed high-speed imaging to measure the bubbles' size, shape, velocity, coalescence time, and number of bounces at the interface. We investigated two criteria to determine the beginning of bubble-interface interaction (“collision”): the physical criterion, based on the distance between the bubble top surface and the interface, and the hydrodynamic criterion, based on the bubble velocity. Gamma distributions represented the coalescence times of bubbles at their terminal velocities well. We found a linear relationship between the coalescence time and the number of bounces. The hydrodynamic criterion was more consistent in representing our data on coalescence time.

Modification of protein concentrate from carioca bean (Phaseolus vulgaris L.) by dynamic high-pressure technology: Structural and techno-functional properties

Bean protein has emerged as a promising alternative for the food industry. To explore its potential applications, this study aims to investigate the effects of dynamic high-pressure (DHP) on the structural, techno-functional, and color properties of carioca bean protein concentrate (CBPC). CBPCs were processed at 50, 100, and 180 MPa for a single cycle. The results revealed that DHP in different levels induced alterations in CBPC protein structure, which consequently affected its techno-functional properties. Surface hydrophobicity (H0), intrinsic fluorescence, and Fourier transform infrared spectroscopy analyses confirmed that the protein underwent modification due to pressure or mechanical forces generated during treatment. Treatment at 180 MPa exposed hydrophobic groups of the protein, leading to an increase in H0. The samples processed at this pressure exhibited higher solubility, foaming capacity (FC), and emulsifying properties than the untreated CBPC. The solubility of non-processed CBPC (67 %) increased with increasing pressure (68 % to 79 %). FC improved from 79 % to 87 % likely due to faster protein adsorption at the air-water interface, as a result of its higher H0. Emulsion properties also enhanced after the DHP, probably due to the increased solubility and H0. Additionally, all CBPC samples maintained a white appearance with low-coloring components, making them visually appealing compared to other pulse proteins. The findings suggest that DHP treatment is an effective technique for enhancing the techno-functional properties of CBPC, expanding its application as an ingredient in food systems.

Formation of Conjugated Polymer Monolayer Networks on Water Surface and Nonlinear Charge Transport

AbstractMaterial‐networked conduction paths provide nonlinear electronic properties, which are essential components of computing and physically mimic the brain. In this study, the formation of conjugated polymer monolayer networks and their nonlinear charge transport is demonstrated. Poly(3‐hexylthiophene) (P3HT) monolayer networks doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) is fabricated using the co‐spread method with an amphiphilic liquid crystal molecule at the air–water interface. Atomic force microscopy and Ultraviolet–visible–near‐infrared absorption spectroscopy measurements reveal the network surface morphologies and doped electronic states. The correlation between the nonlinear electronic characteristics and network structures of the P3HT/F4TCNQ monolayer networks is further systematically investigated through current–voltage and voltage–time measurements for various doping levels, network densities, and numbers of transferred layers. The current–voltage characteristics of the P3HT/F4TCNQ monolayer network device with a simple two‐terminal structure exhibit nonlinear and ohmic conduction behavior, which depend strongly on the network density and geometric dimension (number of transferred layers). It is concluded that the nonlinear properties arise from the limited and unique network of 2D conduction passes. This study highlights the unique features of conducting polymer monolayer networks, paving the way for neuromorphic device applications including conjugated semiconducting polymer‐based material reservoirs with controllable nanostructures.

Synthesis and Characterization of Monolayer Colloidal Sheets.

Sheet-like colloidal assemblies represent model systems to investigate the structure and properties of two-dimensional materials. Here, we report a simple yet versatile method for the preparation of colloidal monolayer sheet-like assemblies that affords control over the size, crystalline order, flexibility, and defect density. The protocol that we report relies on self-assembly of colloids as a sessile drop of dispersion is evaporated on an oil-covered substrate. In this case, the contact line continually moves as the drop shrinks. Polyethyleneimine polymer-covered micrometer-sized colloidal particles are transported to the air-water interface and assemble to form a monolayer sheet as the drop dries. Cross-linking the polymer renders the colloidal assembly permanent. Interestingly, monodisperse colloidal particles form disordered assemblies when dried from low concentration dispersions, while polycrystalline ordered assemblies form at higher concentrations. We demonstrate that increasing the cross-linker to polymer ratio decreases the flexibility of the assembly. Introduction of different-sized colloidal particles in a sheet leads to increased disorder. Removal of sacrificial particles from the sheet allowed the introduction of "holes" in the sheets. Thus, these colloidal sheets are models for probing the effects of disorder, doping, and vacancies in two-dimensional systems.

High spatiotemporal resolution free surface detection using cost-effective video equipment and computer vision techniques in nearly stationary flow along a transparent wall in the laboratory

The identification of the air–water interface in free surface flows traditionally involves intrusive techniques or costly equipment. Non-intrusive alternatives, such as computer vision, are emerging as highly effective substitutes or supplements for more invasive techniques in laboratory measurements, thanks to their straightforward implementation and cost efficiency. This research specifically delves in the conjunction of various naive techniques, exploring their collective precision in detecting the air–water interface along transparent walls in laboratory. A detection technique based on the double gradient of the image is applied and thoroughly examined. The study progresses through multiple refinement stages, culminating in a method that is both cost effective and easy to implement. This methodology allows for large-scale, high resolution measurements (200 mm × 1800 frames per video at a 0.25 mm, 50 Hz resolution), offering both spatial and temporal measurements by adeptly detecting the free surface along transparent walls.

Wettability of a Polymethylmethacrylate Surface by Fluorocarbon Surfactant Solutions

To clarify the adsorption behavior of fluorocarbon surfactants on PMMA surfaces, the contact angles of two nonionic fluorocarbon surfactants (FNS-1 and FNS-2) and an anionic fluorocarbon surfactant (FAS) on polymethylmethacrylate (PMMA) surface were determined using the sessile drop method. Moreover, the effects of molecular structures on the surface tension, adhesion tension, solid–liquid interfacial tension, and adhesion work of the three fluorocarbon surfactants were investigated. The results demonstrate that the adsorption amounts for three fluorocarbon surfactants at the air–water interface are 4~5 times higher than those at the PMMA–solution interface. The three fluorocarbon surfactants adsorb on the PMMA surface by polar groups before CMC and by hydrophobic chains after CMC. Before CMC, FNS-2 with the smallest molecular size owns the highest adsorption amount, while FAS with large-branched chains and electrostatic repulsion has the smallest adsorption amount. After CMC, the three fluorocarbon surfactants form aggregates at the PMMA-liquid interface. FAS possesses the smallest adsorption amount after CMC. Besides, FNS-1 possesses a higher adsorption amount than FNS-2 due to the longer fluorocarbon chain and the lower CMC value of FNS-1. The adsorption behaviors of nonionic and anionic fluorocarbon surfactants on the PMMA surface are different. FAS forms interfacial aggregates before CMC, which may be attributed to the electrostatic interaction between the anionic head of FAS and the PMMA surface.

PH Affects the Spontaneous Formation of H2O2 at the Air-Water Interfaces.

Recent studies have shown that the air-water interface of aqueous microdroplets is a source of OH radicals and hydrogen peroxide in the atmosphere. Several parameters such as droplet size, salt, and organic content have been suggested to play key roles in the formation of these oxidants. In this study, we focus on the effect of acidity on the spontaneous interfacial hydrogen peroxide formation of salt-containing droplets. Na2SO4, NaCl, and NaBr bulk solutions, at the range of pH 4 to 9.5, were nebulized, using ultra high-purity N2/O2 (80%/20%), and H2O2 was measured in the collected droplets. All of the experiments were performed in T = 292 ± 1 K and humidity levels of 90 ± 2%. For Na2SO4 and NaCl, the H2O2 concentration was increased by ∼40% under alkaline conditions, suggesting that OH- enriched environments promote its production. When CO2 was added in the ultrapure air, H2O2 was observed to be lower at higher pH. This suggests that dissolved CO2 can initiate reactions with OH radicals and electrons, impacting the interfacial H2O2 production. H2O2 formation in NaBr droplets did not display any dependence on the pH or the bath gas, showing that secondary reactions occur at the interface in the presence of Br-, which acts as an efficient interfacial source of electrons.

Dilatational rheological studies on the surface micelles of Poly(styrene)-[formula omitted]-Poly(4-vinyl pyridine) block copolymer at the air-water interface

A thin film of an amphiphilic block copolymer, poly(styrene)-b-poly(4-vinyl pyridine), is investigated at the air–water interface. The surface pressure - area per molecule isotherm and the static compressional modulus provide evidence of two phases, assigned as L1 and L2, respectively. The surface topography and the phase images of these phases obtained using atomic force microscope reveal the presence of surface micelles of different sizes and densities. The dilatational rheology of these phases was investigated using the oscillatory barrier method for different strain amplitudes (1−5%) and qualitatively assessed using Lissajous–Bowditch curves. It points out the emergence of nonlinearity (asymmetric shape and distorted ellipse) with an increase in strain amplitude. Using a fast Fourier transform, the amplitudes and the phases of different harmonics of the stress–strain curves were extracted, and the total harmonic distortion was determined. In contrast to L1 phase, the total harmonic distortion for the L2 phase increases monotonically with strain amplitude. Fixing the strain amplitude at 1% (small amplitude regime), the temperature as well as frequency dependence studies of the dilatational moduli were carried out. The dynamic storage moduli of the L1 phase is found to be insensitive to temperature while it decreases for the L2 phase suggesting softening. Further, evidence for solid-like viscoelastic response emerges (explained using the Kelvin–Voigt model) for the L1 phase, whereas the L2 phase show a cross-over behaviour.

ASLdC3: A Derivative of Acidic Sophorolipid Disrupts Mitochondrial Function, Induces ROS Generation, and Inhibits Biofilm Formation in Candida albicans.

Fungal infections account for more than 140 million cases of severe and life-threatening conditions each year, causing approximately 1.7 million deaths annually. Candida albicans and related species are the most common human fungal pathogens, causing both superficial (mucosal and cutaneous) and life-threatening invasive infections (candidemia) with a 40-75% mortality rate. Among many virulence factors of Candida albicans, morphological transition from yeast to hyphae, secretion of hydrolytic enzymes, and formation of biofilms are considered to be crucial for pathogenicity. However, the arsenals for the treatment against these pathogens are restricted to only a few classes of approved drugs, the efficacy of which is being compromised by host toxicity, fungistatic activity, and the emergence of drug resistance. In this study, we have described the development of a molecule, exhibiting excellent antifungal activity (MIC 8 μg/mL), by tailoring acidic sophorolipids with aryl alcohols via enzyme catalysis. This novel derivative, ASLdC3, is a surface-active compound that lowers the surface tension of the air-water interface up to 2-fold before reaching the critical micelle concentration of 25 μg/mL. ASLdC3 exhibits excellent antibiofilm properties against Candida albicans and other nonalbicans Candida species. The molecule primarily exhibits its antifungal activity by perturbing mitochondrial function through the alteration of the mitochondrial membrane potential (MMP) and generation of reactive oxygen species (ROS). The ROS damages fungal cell membrane function and cell wall integrity, eventually leading to cell death. ASLdC3 was found to be nontoxic in in vitro assay and nonhemolytic. Besides, it does not cause toxicity in the C. elegans model. Our study provides a valuable foundation for the potential of acidic sophorolipid as a nontoxic, biodegradable precursor for the design and synthesis of novel molecules for use as antimicrobial drugs as well as for other clinical applications.

Achieving highly interfacial evaporation rate and continuous salt resistance simultaneously via multi-dimensional composite biomimetic evaporator

In response to the global freshwater crisis, various measures have been proposed. Solar-driven interfacial photoevaporation is gaining attention because its environmentally friendly and energy-efficient. However, challenges such as low solar-thermal conversion efficiency and low evaporation rates continue to hinder the advancement of solar photoevaporation technology. Inspired by Tamarisk, we proposed a multistage reflective structure using a hydrothermal method, increasing the carbon cloth (CC) surface area. Numerical simulations indicate that the plasmon resonance effect of the composites could effectively enhance the light absorption properties of the structures. Subsequently, a polyvinyl alcohol-phytic acid (PVA-PA) hydrogel was integrated with the composites to achieve effective separation of the evaporation layer from the water transport layer, thereby minimizing the interfacial light evaporation material. The PVA-PA/MnO2@CC evaporation system demonstrates excellent thermal management performance. Both experimental and simulation results confirm that the system can localize the heat obtained from photothermal conversion at the water–air interface. Additionally, the Marangoni effect, induced by the temperature gradient in the evaporation system, facilitates the transport of water in the PVA-PA hydrogel to the interface through microscopic holes. The evaporator exhibited an evaporation rate of 3.190 kg m-2h−1 in deionized water, with an evaporation efficiency of 94.1 %. Even under high-concentration brine conditions, the evaporation rate remained high, and during continuous evaporation in high-concentration brine, the surface showed no significant salt accumulation. In summary, this work combines two-dimensional carbon cloth with three-dimensional hydrogel, and incorporates metal oxides onto the carbon cloth surface to further enhance its light-trapping ability. By synthesizing the advantages of both materials, the performance of the evaporator is significantly improved.

Metabolic processes drive spatio-temporal variations of carbon sink/source in a karst river

Riverine carbon dioxide (CO2) exchange is a crucial component of the global carbon cycle. However, the changes in the CO2 sink/source in karst rivers caused by differences in lithological features and climate, hindered the resolution of the spatio-temporal heterogeneity of global inland water carbon emissions. Here, we use hydrochemical data and CO2 gas isotopic data to reveal the spatio-temporal variations of CO2 sink/source in karst rivers and their controlling mechanisms. Fifty-two monitoring transects were set up in the subtropical Lijiang River in southwest China in June and December 2019. Our results indicated that the CO2 flux across the water-air interface (FCO2) in the Lijiang River basin ranged from −43.77 to 519.67 mmol/(m2·d). In June, the Lijiang River acted as an atmospheric carbon source due to higher water temperatures (Twater). However, driven by hydrodynamic conditions and the metabolism of aquatic photosynthesis, the river shifts from being an atmospheric carbon source in June to an atmospheric carbon sink in December. The stable isotopes of CO2 (δ13C-CO2) show significant differences in the spatio-temporal variations of CO2 sink/source. In December, the transects of the Lijiang River basin with a negative CO2 flux are significantly negatively correlated with dissolved oxygen (DO) and chlorophyll-a (Chl-a) concentration (p < 0.05). This confirms that the enhancement of aquatic photosynthesis efficiency increased water DO concentrations, which resulted in the positive movement of water δ13C-CO2 and a decrease in the partial pressure of CO2 (pCO2) and FCO2. Comparative analysis with global river FCO2 indicates that under the combined driving forces of metabolic processes of aquatic photosynthetic organisms and hydrodynamic conditions, rivers tend to act more frequently as CO2 sinks, particularly in subtropical and temperate rivers. In conclusion, this study represents a new example focusing on CO2 dynamics to address the spatio-temporal heterogeneity of carbon emissions in inland waters on a global scale.

De novo Cu-MOF@CNS nanocomposite coated on a cotton fibrils framework for sustainable solar-driven desalination.

Environmental researchers are extremely concerned about addressing the declining availability of drinking water, which is a critical issue in many nations. Solar-driven desalination is an emerging and pioneering renewable approach to reduce potable water scarcity that is suitable for remote locations, developing countries, and disaster zones as it does not require additional energy supply. However, there are still issues with the scalable preparation of photothermal materials, such as achieving low cost and widening the assortment of useful applications. Conventional carbon- and metal-based absorbers are intricate and fragile, which make them difficult to install and transport in places with minimal infrastructure. Thus, a universal process for creating adaptable solar evaporators is sorely required. Herein, we have come up with a holistic approach using a solar absorber (GJ-01(Cal)) derived from a Cu-MOF (HKUST-1) and carbon nanosheets (CNSs) for generating potable water from saline water using solar radiation. The as-synthesized material provides high-performance photothermal water evaporation when illuminated under solar irradiation at the air-water interface. Moreover, its porous structure, high photothermal conversion efficiency, rapid water flow, and heat insulation make it appropriate for saline water desalination. CNS play a pivotal role in improving the photothermal features of the solar absorber (GJ-01(Cal)) in terms of conjugation to promote Cu(0) species and pyrrolic nitrogen (P-N) amplification and thereby enrich the p-type nature of the absorber's triphasic configuration. With these photothermal factors, the localised surface plasmon resonance (LSPR) of electrons increases and achieves high solar spectrum absorption. The GJ-01(Cal) was further coated on porous cotton fibrils (CF) that regulate photothermal interfacial evaporation (PTIE) by allowing water transportation via capillary action. This assemblage of the nanocomposite on CF efficiently evaporates water at a higher surface temperature of ∼47 °C under one solar illumination, achieving 4.23 kg m-2 h-1 of evaporation flux and 96.5% light-to-heat conversion efficiency. Interestingly, the GJ-01(Cal) coated over CF can be recycled at least 10 times. Additionally, it offers scalable production for higher photothermal efficiency with a flexible substrate as a solar evaporator and is beneficial for society paving new horizons towards a sustainable environment.

Environmental fate and transport of PFAS in wastewater treatment plant effluent discharged to rapid infiltration basins

Fate and transport of per- and polyfluoroalkyl substances (PFAS) in wastewater treatment plant (WWTP) effluent discharged to rapid infiltration basins (RIBs) is investigated using data from 26 WWTPs in Michigan, USA. PFAS were found to accumulate in groundwater downgradient from RIBs with median groundwater-effluent enrichment factors for ten commonly detected, terminal-form perfluoroalkyl acids (PFAAs) ranging from 1.3 to 5.2. Maximum contaminant levels for drinking water were exceeded in groundwater at all WWTPs with available PFAS data. Numerical models of unsaturated fluid flow and PFAS transport honoring RIB site properties, such as median vertical separation distance to the water table and a realistic range of area-normalized effluent fluxes, show long-chain PFAS undergo significant delays from air-water interface (AWI) adsorption, requiring up to 15 times longer to reach maximum mass flux to the saturated zone under low-flux conditions, where AWI area is 2.5 times greater. Short-chain PFAS commonly detected in effluent are only minimally affected by AWI adsorption and show little to no attenuation under high-flux conditions. The nonlinear inverse relationship between water content and AWI area highlights the important role of AWI adsorption in modulating unsaturated transport of long-chain PFAS to underlying groundwater due to the broad range of flux rates applied to RIB systems.

Crowding accelerates the rotation of a bacterial rotor

Understanding the propulsion of a swimmer in a large group of individuals holds the key to unravelling the intriguing dynamics of active matter collective motion. Here, we develop a two-dimensional (2-D) self-assembled rotor, powered by bacterial flagella. At a water–air interface, the average direction of rotation of a rotor is fixed. When the chiral rotor is put into a 2-D bacterial suspension, we examine the average and fluctuation of the angular velocity of the rotor. Remarkably, the average angular velocity of a rotor is found to increase up to 3 times when the density of surrounding bacterial suspension increases and the increase is nonlinear. In a dense suspension of bacteria, the existence of a rotor disrupts vortices in the surrounding active turbulence, and the acceleration of the rotor is independent of the activity level of the surrounding free bacteria. The nonlinear acceleration thus results from hydrodynamic interaction with surrounding crowdedness that can be quantitatively explained by hydrodynamic simulation. The simultaneity between the acceleration of rotor and free bacteria in active turbulence suggests that crowding-induced acceleration may promote the onset of instability. The result will inspire new active-matter-based microfluidic devices with improved transport properties.

PH-dependent self-assembly of an acid derivative of cholesterol at interfaces

Understanding and control over the self-assembly of molecules is desirable in many physico-chemical processes. Functional groups responsive to a pH change offer immense scope in driving selective molecular self-assembly. pH-dependent self-assembly of an aromatic acid derivative of cholesterol, 4-(6-(cholest-5-ene-3-yloxy)-5-(oxopentyloxy)) benzoic acid, ChBA, is investigated at the air-aqueous interface and characterized using different interface-sensitive techniques. Evidence for the presence of unstable films of ChBA at the air-water interface or for pH < 10 emerges from the surface pressure – area per molecule isotherm, surface pressure relaxation, and Brewster angle microscopy studies. In contrast, for pH > 10.6, ChBA forms a relatively stable film with a higher magnitude of the collapse pressure (45 mN/m). The compressional moduli and the textures obtained using Brewster angle microscopy reveal the presence of two phases (L1 and L2), which are also highly stable with temperature. The surface topography of ChBA films transferred from the air-water interface reveals the presence of multilayers of different thicknesses, for film transferred at pH 11 reveals primarily a uniform and homogenous monolayer. Based on the dependence of the collapse pressure with pH, the surface pKa of ChBA is estimated to be about 10.6. Our studies show that the undissociated ChBA spontaneously forms multilayers, whereas partially or fully dissociated ChBA forms distinct and ordered monolayer phases, highlighting the role of electrostatic interactions.