Deforming active droplets in viscoelastic solutions.

The motion of biological swimmers in typical bodily fluids is modelled using a system of micellar solubilization driven active droplets in a viscoelastic polymeric solution. The viscoelastic nature of the medium, as perceived by the moving droplet, characterized by the Deborah number (De), is tuned by varying the surfactant (fuel) and polymer concentration in the ambient medium. At moderate De, the droplet exhibits a steady deformed shape, markedly different from the spherical shape observed in Newtonian media. A theoretical analysis based on the normal stress balance at the interface is shown to accurately predict the droplet shape. With a further increase in De, time-periodic deformation accompanied by an oscillatory transition in swimming mode is observed. The study unveils the hitherto unexplored rich complexity in the motion of active droplets in viscoelastic fluids.

Polymer multilayer films regulate macroscopic fluid flow and power microfluidic devices via supramolecular interactions.

Self-powered supramolecular micropumps could potentially provide a solution for powerless microfluidic devices where the fluid flow can be manipulated via modulating non-covalent interactions. An attempt has been made to fabricate thin-film-based micropumps by depositing a ß-cyclodextrin ('host') functionalized polymer on a glass slide via layer-by-layer assembly. These supramolecular micropumps turned on the fluid flow upon addition of 'guest' molecules to the multilayer films. The flow velocity was tuned using the concentration of the guest molecules as well as the number of host layers inside the multilayer films. Numerical modelling reveals that the solutal buoyancy, which originates from host-guest complexation, is primarily responsible for the fluid flow. In view of its potential application in self-powered devices, the thin-film-based micropump was integrated into a microfluidic device to show molecular and colloidal transport over long distances.

Role of suction pressure in the stability of a gravity-driven thermoviscous liquid film flow down the interior surface of a cylinder.

This study aims to analyze the stability of a gravity-driven thin film flow in the heated/cooled interior surface of a vertical hollow cylinder. The model development involves simplifying the flow and energy equations using the usual thin-film approximation, where the average film thickness is considered to be much smaller than the radius of cylinder. A dispersion relation is then derived to study the temporal stability of the system in order to quantify the effect of various non-dimensional parameters present in the model, such as the thermoviscous number, Marangoni number, Biot number, and Bond number. Another non-dimensional parameter is introduced by considering an opposing suction pressure in the annulus region. The thermocapillary stress and the thermoviscous effect are shown to strongly affect the temporal stability of the flow. It is shown that although the suction pressure affects the velocity profile of the flow, it does not affect the temporal stability results. The suction pressure is then shown to have some effect on the spatiotemporal stability. Critical condition is presented for the transition between absolutely and convectively unstable systems, and parameter regimes are presented to quantify the effect of the above-mentioned parameters.

Stability analysis of a gravity-driven thermoviscous liquid film flowing over a heated flat substrate.

Two-dimensional steady-state solutions and their stability analysis are presented for a gravity-driven thin film of a thermoviscous liquid. The governing equations and boundary conditions are simplified using the lubrication approximation. The analytically obtained film thickness evolution equation consists of various dimensionless parameters such as the Marangoni number, Biot number and thermoviscosity number. The viscosity of the liquid is assumed as an exponential function of temperature. The viscosity decreases within the liquid film as the temperature increases. Due to localized heating interfacial temperature gradients generate surface tension gradient which results into thermocapillary or Marangoni stress. The Marangoni stress opposes the fluid flow at the leading edge of heater leading to an increase in the film thickness locally. This locally thick structure becomes unstable beyond critical values of the parameters that leads to formation of rivulets in the transverse direction. Using the linear stability analysis it is found that the Marangoni stress and the thermoviscous effect have a destabilizing effect on the thin-film flow. At much higher values of the thermoviscosity number another mode of instability appears which is known as thermocapillary instability which leads to oscillating film profiles. For streamwise perturbations, the destabilizing effect of the thermoviscosity number for localized and uniform heating remains consistent.

Highly Transparent and Integrable Surface Texture Change Device for Localized Tactile Feedback.

Human-machine haptic interaction is typically detected by variations in friction, roughness, hardness, and temperature, which combines to create sensation of surface texture change. Most of the current technologies work to simulate changes in tactile perception (via electrostatic, lateral force fields, vibration motors, etc.) without creating actual topographical transformations. This makes it challenging to provide localized feedback. Here, a new concept for on-demand surface texture augmentation that is capable of physically forming local topographic features in any predesigned pattern is demonstrated. The transparent, flexible, integrable device comprises of a hybrid electrode system with conductive hydrogel, silver nanowires, and conductive polymers with acrylic elastomer as the dielectric layer. Desired surface textures can be controlled by a predesigned pattern of electrodes, which operates as independent or interconnected actuators. Surface features with up to a height of 0.155 mm can be achieved with a transformation time of less than a second for a device area of 18 cm2 . High transparency levels of 76% are achieved due to the judicious choice of the electrode and the active elastomer layer. The capability of localized and controlled deformations makes this system highly useful for applications such as display touchscreens, touchpads, braille displays, on-demand buttons, and microfluidic devices.

Effect of an insoluble surfactant on the dynamics of a thin liquid film flowing over a non-uniformly heated substrate.

The stability analysis of a gravity-driven thin liquid film with an insoluble surfactant flowing over a surface with embedded, regularly spaced heaters is investigated. At the leading edge of a heater, the presence of a temperature gradient induces an opposing Marangoni stress at the interface leading to the formation of a capillary ridge. This ridge has been shown to be susceptible to thermocapillary (oscillating in the flow direction) and rivulet (spanwise periodic pattern) instabilities. The presence of an insoluble surfactant is shown to have a stabilizing effect on this system. The governing equations for the evolution of the film thickness and surfactant concentration are obtained within the lubrication approximation. The coupled two-dimensional base solutions for the film thickness and surfactant concentration show that there is no significant change in the height of the capillary ridge at the subsequent heaters downstream. The height of the capillary ridge is reduced by the presence of the surfactant. For very small Peclet number, the presence of multiple heaters has almost no significant effect on the film stability as compared to a single heater and similar trends are observed between the two configurations in the presence of the surfactant as for the case of a clean interface. However, for large Peclet number, the effect was observed on both types of instabilities for certain heater configurations. The Biot number is shown to have a strong effect on the stability results wherein the dominant mode of instability is altered (from rivulet to thermocapillary instability) for a passive or no surfactant case with increase in the Biot number. For an active surfactant thermocapillary instability is found to remain the dominant mode of instability for all the values of the Biot number. It is shown that increasing the number of heaters beyond a couple does not further affect the stability results.

Flexible Ionic-Electronic Hybrid Oxide Synaptic TFTs with Programmable Dynamic Plasticity for Brain-Inspired Neuromorphic Computing.

Emulation of biological synapses is necessary for future brain-inspired neuromorphic computational systems that could look beyond the standard von Neuman architecture. Here, artificial synapses based on ionic-electronic hybrid oxide-based transistors on rigid and flexible substrates are demonstrated. The flexible transistors reported here depict a high field-effect mobility of ≈9 cm2 V-1 s-1 with good mechanical performance. Comprehensive learning abilities/synaptic rules like paired-pulse facilitation, excitatory and inhibitory postsynaptic currents, spike-time-dependent plasticity, consolidation, superlinear amplification, and dynamic logic are successfully established depicting concurrent processing and memory functionalities with spatiotemporal correlation. The results present a fully solution processable approach to fabricate artificial synapses for next-generation transparent neural circuits.

Dip coating in the presence of an opposing Marangoni stress.

Theoretical analysis of the dip coating process of uniformly wetting and selectively wetting substrates with a shear-thinning power-law fluid is presented. The viscosity is modeled by a three-constant Ellis model that allows for Newtonian and power-law fluids at low and high shear, respectively. The effect of an opposing Marangoni stress is studied on the entrained film thickness. Ellis number and Marangoni number are the important parameters in the analysis that control the film thickness. Numerical results show that the film thickness is a monotonically decreasing function of these two parameters. A correlation between the parameters is found for both the surfaces to obtain an apparent "dry" substrate with a microscopically thin film attached to the surfaces.

Stability of a gravity-driven thin electrolyte film flowing over a heated substrate.

The stability of a thin electrolyte liquid film driven by gravity over a vertical substrate is presented. A film thickness evolution equation is derived and solved numerically. The substrate is non-uniformly heated from below which is modeled by imposing a temperature profile at the liquid-solid interface. The electrohydrodynamics is included in the model with Maxwell's stress tensor. The governing flow and energy equations are simplified using the lubrication approximation. The Poisson-Boltzmann equation with Debye-Hückel approximation is used for the potential which is generated inside the film due to a charged layer at the liquid-solid interface. The positive temperature gradient at the substrate leads to the formation of a thermocapillary ridge due to an opposing Marangoni stress. This thermocapillary ridge becomes unstable beyond critical parameters related to Marangoni stress and convective energy loss at the free surface. The electroosmotic flow has no effect on the base profile of the film, but enhances its instability. A parameter space is presented delineating the stable and unstable regimes for the film dynamics.

Generalized linear stability of non-inertial rimming flow in a rotating horizontal cylinder.

The stability of a thin film of viscous liquid inside a horizontally rotating cylinder is studied using modal and non-modal analysis. The equation governing the film thickness is derived within lubrication approximation and up to first order in aspect ratio (average film thickness to radius of the cylinder). Effect of gravity, viscous stress and capillary pressure are considered in the model. Steady base profiles are computed in the parameter space of interest that are uniform in the axial direction. A linear stability analysis is performed on these base profiles to study their stability to axial perturbations. The destabilizing behavior of aspect ratio and surface tension is demonstrated which is attributed to capillary instability. The transient growth that gives maximum amplification of any initial disturbance and the pseudospectra of the stability operator are computed. These computations reveal weak effect of non-normality of the operator and the results of eigenvalue analysis are recovered after a brief transient period. Results from nonlinear simulations are also presented which also confirm the validity of the modal analysis for the flow considered in this study.

Stability analysis of a thermocapillary spreading film with slip-model.

Thin liquid films spreading on a solid substrate due to thermocapillary stresses are susceptible to rivulet instability at the advancing solid-liquid-vapor contact line. The unstable front is related to the presence of a capillary ridge at the contact line. In this work, the dynamics and stability of thermocapillary-driven films are analyzed using a detailed slip-model to alleviate the stress singularity at the moving contact line. The slip-model is well suited to model partially wetting fluids due to the possibility of defining the contact angle explicitly. The effect of motion of the contact line on the dynamic contact angle and subsequently on the dynamics and stability of the film is explored. The apparent contact angle is a result of the static contact angle and motion of the contact line. It is shown that one can obtain exactly the same base profile with and without taking into account the effect of motion on the contact angle with suitable change of parameters but the linear stability of the two profiles is different. Further the transient growth is found to be somewhat different but small for both configurations. Analysis of the ε -pseudospectra indicates a highly non-normal system for the case of dynamic contact angle.

Theoretical analysis of dip-coating of uniformly wetting and chemically micropatterned surfaces with an Ellis fluid.

Dip-coating of a chemically micropatterned surface is an important technique for selective material deposition in a confined region for various applications. An analysis of the dip-coating of a micropatterned surface with a pure Newtonian liquid was performed by Davis (Phys. Fluids 17, 03852 (2005)). In this study, the analysis of dip-coating of the heterogeneous substrate is extended to the deposition of an Ellis Fluid. Governing equations are derived using lubrication theory to determine the thickness of a liquid film deposited on the O(10 µm) stripe at small capillary number. A uniformly wetting surface is also considered using a consistent treatment of the governing equations for comparison. The effect of the Ellis model parameters on the thickness of the entrained liquid film is found to be much less for the micropatterned surfaces because of the dominant effect of the lateral fluid confinement due to heterogeneous wettability. This confinement imposes a geometric length scale in case of the patterned surface that replaces the dynamic capillary length used for the analysis of the dip-coating of a uniform surface. A composite equation is further developed for the Ellis fluid to include the effect of gravity on the dip-coated film thickness on both types of surfaces. The film thinning due to gravity drainage is found to be negligible for the case of patterned substrate.

Closely Packed Stretchable Ultrasound Array Fabricated with Surface Charge Engineering for Contactless Gesture and Materials Detection.

Communication with hand gestures plays a significant role in human-computer interaction by providing an intuitive and natural way for humans to communicate with machines. Ultrasound-based devices have shown promising results in contactless hand gesture recognition without requiring physical contact. However, it is challenging to fabricate a densely packed wearable ultrasound array. Here, a stretchable ultrasound array is demonstrated with closely packed transducer elements fabricated using surface charge engineering between pre-charged 1-3 Lead Zirconate Titanate (PZT) composite and thin polyimide film without using a microscope. The array exhibits excellent ultrasound properties with a wide bandwidth (≈57.1%) and high electromechanical coefficient (≈0.75). The ultrasound array can decipher gestures up to 10 cm in distance by using a contactless triboelectric module and identify materials from the time constant of the exponentially decaying impedance based on their triboelectric properties by utilizing the electrostatic induction phase. The newly proposed metric of the areal-time constant is material-specific and decreases monotonically from a highly positive human body (1.13 m2 s) to negatively charged polydimethylsiloxane (PDMS) (0.02 m2 s) in the triboelectric series. The capability of the closely packed ultrasound array to detect material along with hand gesture interpretation provides an additional dimension in the next-generation human-robot interaction.

Lanthanide-Doped ZnO Nanoparticles: Unraveling Their Role in Cytotoxicity, Antioxidant Capacity, and Nanotoxicology.

This study used a sonochemical synthesis method to prepare (La, Sm)-doped ZnO nanoparticles (NPs). The effect of incorporating these lanthanide elements on the structural, optical, and morphological properties of ZnO-NPs was analyzed. The cytotoxicity and the reactive oxygen species (ROS) generation capacity of ZnO-NPs were evaluated against breast (MCF7) and colon (HT29) cancer cell lines. Their antioxidant activity was analyzed using a DPPH assay, and their toxicity towards Artemia salina nauplii was also evaluated. The results revealed that treatment with NPs resulted in the death of 10.559-42.546% and 18.230-38.643% of MCF7 and HT29 cells, respectively. This effect was attributed to the ability of NPs to downregulate ROS formation within the two cell lines in a dose-dependent manner. In the DPPH assay, treatment with (La, Sm)-doped ZnO-NPs inhibited the generation of free radicals at IC50 values ranging from 3.898 to 126.948 µg/mL. Against A. salina nauplii, the synthesized NPs did not cause death nor induce morphological changes at the tested concentrations. A series of machine learning (ML) models were used to predict the biological performance of (La, Sm)-doped ZnO-NPs. Among the designed ML models, the gradient boosting model resulted in the greatest mean absolute error (MAE) (MAE 9.027, R2 = 0.86). The data generated in this work provide innovative insights into the influence of La and Sm on the structural arrangement and chemical features of ZnO-NPs, together with their cytotoxicity, antioxidant activity, and in vivo toxicity.

Enhancing antioxidant properties of CeO2 nanoparticles with Nd3+ doping: structural, biological, and machine learning insights.

The antioxidant capabilities of nanoparticles are contingent upon various factors, including their shape, size, and chemical composition. Herein, novel Nd-doped CeO2 nanoparticles were synthesized and the neodymium content was varied to investigate the synergistic impact on the antioxidant properties of CeO2 nanoparticles. Incorporating Nd3+ induced changes in lattice parameters and significantly altered the morphology from nanoparticles to nanorods. The biological activity of Nd-doped CeO2 was examined against pathogenic bacterial strains, breast cancer cell lines, and antioxidant models. The antibacterial and anticancer activities of nanoparticles were not observed, which could be associated with the Ce3+/Ce4+ ratio. Notably, the incorporation of neodymium improved the antioxidant capacity of CeO2. Machine learning techniques were employed to forecast the antioxidant activity to enhance understanding and predictive capabilities. Among these models, the random forest model exhibited the highest accuracy at 96.35%, establishing it as a robust computational tool for elucidating the biological behavior of Nd-doped CeO2 nanoparticles. This study presents the first exploration of the influence of Nd3+ on the structural, optical, and biological attributes of CeO2, contributing valuable insights and extending the application of machine learning in predicting the therapeutic efficacy of inorganic nanomaterials.

Unraveling the Complex Interactions: Machine Learning Approaches to Predict Bacterial Survival against ZnO and Lanthanum-Doped ZnO Nanoparticles.

The rise in antibiotic-resistant bacteria is a global health challenge. Due to their unique properties, metal oxide nanoparticles show promise in addressing this issue. However, optimizing these properties requires a deep understanding of complex interactions. This study incorporated data-driven machine learning to predict bacterial survival against lanthanum-doped ZnO nanoparticles. The effect of incorporation of lanthanum ions on ZnO was analyzed. Even with high lanthanum concentration, no significant variations in structural, morphological, and optical properties were observed. The antibacterial activity of La-doped ZnO nanoparticles against Gram-positive and Gram-negative bacteria was qualitatively and quantitatively evaluated. Nanoparticles induce 60%, 95%, and 55% bacterial death against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus, respectively. Algorithms such as Multilayer Perceptron, K-Nearest Neighbors, Gradient Boosting, and Extremely Random Trees were used to predict the bacterial survival percentage. Extremely Random Trees performed the best among these models with 95.08% accuracy. A feature relevance analysis extracted the most significant attributes to predict the bacterial survival percentage. Lanthanum content and particle size were irrelevant, despite what can be assumed. This approach offers a promising avenue for developing effective and tailored strategies to reduce the time and cost of developing antimicrobial nanoparticles.

High-performance one-dimensional halide perovskite crossbar memristors and synapses for neuromorphic computing.

Despite impressive demonstrations of memristive behavior with halide perovskites, no clear pathway for material and device design exists for their applications in neuromorphic computing. Present approaches are limited to single element structures, fall behind in terms of switching reliability and scalability, and fail to map out the analog programming window of such devices. Here, we systematically design and evaluate robust pyridinium-templated one-dimensional halide perovskites as crossbar memristive materials for artificial neural networks. We compare two halide perovskite 1D inorganic lattices, namely (propyl)pyridinium and (benzyl)pyridinium lead iodide. The absence of conjugated, electron-rich substituents in PrPyr+ prevents edge-to-face type π-stacking, leading to enhanced electronic isolation of the 1D iodoplumbate chains in (PrPyr)[PbI3], and hence, superior resistive switching performance compared to (BnzPyr)[PbI3]. We report outstanding resistive switching behaviours in (PrPyr)[PbI3] on the largest flexible crossbar implementation (16 × 16) to date - on/off ratio (>105), long term retention (105 s) and high endurance (2000 cycles). Finally, we put forth a universal approach to comprehensively map the analog programming window of halide perovskite memristive devices - a critical prerequisite for weighted synaptic connections in artificial neural networks. This consequently facilitates the demonstration of accurate handwritten digit recognition from the MNIST database based on spike-timing-dependent plasticity of halide perovskite memristive synapses.

Discrimination of enantiomers and constitutional isomers by self-generated macroscopic fluid flow.

The amplification of weak molecular signals to visible output could provide a gateway to the macroscopic world. In this context, supramolecular interfaces were designed by depositing macrocyclic "host" molecules in a multilayer film that can be utilized to discriminate isomers by their fluid flow response upon "host-guest" molecular recognition.

Iontronic pressure sensor with high sensitivity over ultra-broad linear range enabled by laser-induced gradient micro-pyramids.

Despite the extensive developments of flexible capacitive pressure sensors, it is still elusive to simultaneously achieve excellent linearity over a broad pressure range, high sensitivity, and ultrahigh pressure resolution under large pressure preloads. Here, we present a programmable fabrication method for microstructures to integrate an ultrathin ionic layer. The resulting optimized sensor exhibits a sensitivity of 33.7 kPa-1 over a linear range of 1700 kPa, a detection limit of 0.36 Pa, and a pressure resolution of 0.00725% under the pressure of 2000 kPa. Taken together with rapid response/recovery and excellent repeatability, the sensor is applied to subtle pulse detection, interactive robotic hand, and ultrahigh-resolution smart weight scale/chair. The proposed fabrication approaches and design toolkit from this work can also be leveraged to easily tune the pressure sensor performance for varying target applications and open up opportunities to create other iontronic sensors.

Triboelectric Nanosensor Integrated with Robotic Platform for Self-Powered Detection of Chemical Analytes.

Rapid on-site detection of hazardous chemicals is imperative for remote security and environmental monitoring applications. However, the implementation of current sensing technologies in real environments is limited due to an external high-power requirement, poor selectivity and sensitivity. Recent progress in triboelectric nanosensors and nanogenerators presents tremendous opportunities to address these issues. Here, we report an innovative self-powered triboelectric nanosensor for detection of Hg2+ ions, a harmful chemical pollutant, in a rapid single step on-site detection mechanism. Based on the mechanism of solid-liquid contact electrification, tellurium nanowire (Te NW) arrays serving as a solid triboelectric material as well as the sensing probe underwent periodic contact and separation with the Hg2+ solution, leading to the in situ formation of mercury telluride nanowire (HgTe NWs) owing to the selective binding affinity of Te NWs toward Hg2+ ions. To realize the on-site sensing potential, Te NW arrays were mounted onto the robotic hands equipped with additional wireless transmission functionality for rapid detection of Hg2+ ions in resource-limited settings by employing a simple "touch and sense" mechanism. Such a demonstration of direct integration of self-powered sensors with robotics would lead to the development of low-cost, automated chemical sensing machinery for the on-field detection of harmful analytes.