Vapor and Light Responsive Biocompatible Soft Actuator.

Bioinspired smart polymeric materials that undergo three-dimensional shape deformation in response to specific stimuli have gained significant attention in the field of soft robotics and intelligent devices. Despite the substantial advancements in soft robotics, there is a growing demand for the design of multistimuli-responsive soft actuators using a single layer of material due to its reduced complexity and ease of manufacturing and durability. Here, we report the actuation characteristics of a single-layer, dual-responsive soft actuator that overcomes the commonly encountered delamination issues often associated with bilayer systems by incorporating PEDOT:PSS with cassava starch. This soft actuator exhibits deformations in response to various solvent vapors, such as water, alcohol, and acetone. Remarkably, it demonstrates opposite deformations upon exposure to water and alcohol vapors. Additionally, the actuator responds to light triggers and folds upon exposure to sunlight and infrared light. The degree of folding can be precisely controlled by adjusting the intensity of the light source. Furthermore, the periodic geometric patterns imposed on the surface of the actuator provide an additional handle to control the bending axis. For proof of concept, we leverage the actuation capabilities of our actuator to showcase a range of potential applications, including its usage in wearable textiles, crawler robots, smart curtains, push-and-pull machines, and smart lifts.

Determining the Characteristics of Ultrathin Polymer Films: A Spectroscopic Ellipsometry Approach.

Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.

Contact Angle Modulation: In Situ Polymer Deposition during Sessile Drop Evaporation.

The drying kinetics of a sessile drop on a solid surface are a widely studied phenomenon because of their relevance to various fields such as coating, printing, medical diagnostics, sensing, and microfluidic technology. Typically, the drop undergoes drying either at a constant contact radius (R) with a decrease in the three-phase contact angle or at a constant contact angle (θ) with a reduction in the radius with time. These two drying modes are referred to as CCR and CCA, respectively. It is not uncommon where both R and θ may decrease during drying, especially in the penultimate stage of drying. In this work, we report a scenario wherein the θ increases while R decreases during the drying process of an aqueous polymer solution on a high surface energy substrate. This behavior is observed across different polymer systems (such as poly(ethylene oxide) and polyvinyl pyrrolidine), varying molecular weights, and polymer concentrations. As the drop dries, the polymer gets deposited at the three-phase contact line, thus reducing the surface energy of the substrate and leading to an increase in the contact angle. The drop responds by attempting to reach a new equilibrium contact angle through slipping. The temporal increase in contact angle follows a power law scaling behavior. This study demonstrates an in situ modulation of contact angle facilitated by evaporation and polymer deposition, showcasing unconventional drying dynamics.

Responsive soft actuator: harnessing multi-vapor, light, and magnetic field stimuli.

Bioinspired soft actuators, capable of undergoing shape deformation in response to external triggers, hold great potential in fields such as soft robotics, artificial muscles, drug delivery, and smart switches. However, their widespread application is hindered by limitations in responsiveness, durability, and complex fabrication processes. In this study, we propose a new approach to tackle these challenges by developing a single-layer soft actuator that responds to multiple stimuli using a straightforward solution-casting method. This actuator comprises bio-polymer gelatin, bio-compatible PEDOT:PSS, and iron oxide (Fe3O4) nanoparticles. Our actuator exhibits responsiveness to a range of organic solvent vapors, including water vapor, light, and magnetic fields. Notably, it exhibits rapid and reversible bending in distinct directions in response to different vapors, bending upwards in the presence of water vapor and downwards in the presence of alcohol vapor. Moreover, exposure to infrared (IR) light induces a bending toward the light source. The incorporation of magnet-responsive Fe3O4 nanoparticles induces multi-functionality in the actuator. The actuation characteristics of the actuator are controlled by leveraging its responsiveness to dual stimuli, such as water vapor and magnetic fields, as well as light and magnetic fields. For the proof of concept, we showcase several potential applications of our multi-stimuli responsive soft actuator, including magnet-triggered electrical switches, cargo transportation, soft grippers, targeted drug delivery, energy harvesting, and bio-mimicry.

Probing the thermoelectric properties of aluminium-doped copper iodide.

Copper iodide, an environmentally friendly material abundant in nature, holds great significance for room temperature thermoelectric (TE) applications owing to its high Seebeck coefficient and optical transparency. However, to fully unlock its thermoelectric potential and match the performance of conventional TE materials, there is a need to further enhance its electrical conductivity. In this study, we have successfully synthesized nano-crystalline powders of both undoped and aluminium-doped CuI at room temperature using the chemical precipitation method in an ethanol medium. The concentration of aluminium dopant has been optimized to maximize TE performance. At 400 K, the highest TE power factor and figure of merit achieved are 79 µW m-1 K-2 and 0.08, respectively, for CuI doped with 0.1 mol% Al. This enhancement in TE properties can be attributed to the increased carrier density resulting from aluminium doping. The impact of aluminium doping on the temperature-dependent thermal conductivity has been investigated, and the findings are explained by the decay mechanism of optical phonons, supported by the anharmonic phonon coupling theory. Our work delves into the evolution of structural, thermal, optical, and TE properties of CuI upon aluminium (Al) doping. The results provide valuable insights into the future application of CuI in transparent thermoelectric and optoelectronic fields.

Effect of Colloidal Surface Charge on Desiccation Cracks.

We report the effect of polarity and surface charge density on the nucleation and growth kinetics of desiccation cracks in deposits of colloids formed by drying. We show that the average spacing between desiccation cracks and crack opening are higher for the deposit of positively charged colloids than that of negatively charged colloids. The temporal evolution of crack growth is found to be faster for positively charged particle deposits. The distinct crack patterns and their kinetics are understood by considering the spatial arrangement of particles in the deposit, which is strongly influenced by the substrate-particle and particle-particle interactions. Interestingly, the crack spacing, the crack opening, and the rate at which the crack widens are found to increase upon decreasing the surface charge of the colloids.

Triple-line dynamics of a soft colloid-laden drop on a hydrophobic surface.

Evaporation of fluid from a pinned drop placed on solid surface proceeds via constant contact radius (CCR) mode, with a continuous reduction in the contact angle. The reduction of contact angle leads to an imbalance of interfacial tensions at the three-phase contact line. When the unbalanced force is sufficiently strong, the drop slips from the pinned contact line and slides inward. Depinning of the drop alters the mode of evaporation to constant contact angle (CCA) mode till it repins onto the surface. The change in evaporation mode from CCR to CCA is usually achieved by tuning the pinning energy barrier by controlling the surface properties of the substrate. Here, we demonstrate that the evaporation mode can be controlled by solely tailoring the surface tension of the drop, which is achieved in microgel particle-laden sessile drops that show spontaneous adsorption of microgels to the air/water interface, leading to a decrease in the interfacial tension. We show that droplets containing a sufficient number of microgels evaporate predominantly in CCR mode even on a hydrophobic surface, and the contact line remains pinned throughout the evaporation of the drop. Interestingly, the contact line dynamics can be controlled by tuning the softness of the microgels and the particle concentration in the drops.

Probing the tightly bound layer in poly(vinyl alcohol) thin films using swelling measurements.

A strongly adsorbed, tightly bound polymer layer can exist at the polymer/substrate interface in polymer thin films and polymer nanocomposites. The characteristics of the tightly bound layer have long been of interest because of its effect on physical properties. However, direct investigations are challenging as the layer is buried deep within the sample. A common approach to access the tightly bound layer is by rinsing or washing away the loosely bound polymer using a good solvent. While this enables direct investigations of the tightly bound layer, it is unclear if the layer remains unperturbed by the preparation process. Therefore, in situ techniques that can probe the tightly bound layer without strongly perturbing it are preferable. In previous work (P. D. Lairenjam, S. K. Sukumaran and D. K. Satapathy, Macromolecules, 2021, 54, 10931-10942), we introduced an approach to estimate the thickness of the tightly bound layer at the chitosan/silicon interface using swelling of nanoscale thin films when exposed to solvent vapour. To determine the general validity of the approach, in this work we investigated the swelling of poly(vinyl alcohol) (PVA) thin films using two independent techniques: spectroscopic ellipsometry and X-ray reflectivity. We found that the swelling kinetics for thin films of initial thickness in the range 18-215 nm could be described by a single time-dependent swelling ratio, c(t), provided we account for a tightly bound layer of thickness 15 nm at the polymer/substrate interface. Consistent with the conclusions from the swelling measurements, electron density profiles determined by modeling X-ray reflectivity data clearly indicated the existence at the polymer/substrate interface of a 15 nm thick layer of a slightly higher density than the rest of the film. The early-time diffusion coefficient of H2O in PVA determined from the temporal evolution of the mass uptake of the solvent vapour was found to decrease by 3-4 orders of magnitude when the film thickness decreased by approximately an order of magnitude.

Effect of the Shape of the Confining Boundary and Particle Shape Anisotropy on the Morphology of Desiccation Cracks.

The control of the morphology of desiccation cracks is fascinating not only from the application point of view but also from the rich physics behind it. Here, we present a seemingly simple method to tailor the morphology of desiccation cracks by exploitation of the combined effect of particle shape anisotropy and the shape of the confining boundary. This allows us to make circular, square, and triangular-shaped desiccation cracks in the vicinity of the confining boundaries. As the colloidal dispersion dries in confined wells, a drying front appears at the center of the well. With further evaporation, the drying front recedes toward the boundary from the center of the well. We show that the temporal evolution of the drying front is strongly influenced by the shape of the well. Subsequently, desiccation cracks appear in the penultimate stage of drying, and the morphology of the cracks is governed by the shape of the drying front and hence by the shape of the wells. The spatial evolution of the crack pattern is quantified by estimation of the curvature of the cracks, which suggests that the influence of the confining boundary on crack formation is long-ranged. However, the cracks in the dried deposit consisting of spherical particles remain unaffected by the shape of the well, and the cracks are always radial. We establish a one-to-one correspondence between the shape of the drying front and the morphology of the crack pattern in the final dried deposit of ellipsoids.

Bidirectional Actuation of Silk Fibroin Films: Role of Water and Alcohol Vapors.

Three-dimensional (3D) shape morphism observed in nature inspires the development of stimuli-responsive soft actuators. Vapor-responsive actuators are promising among the different stimuli-responsive materials due to their capability to produce macroscale movements in response to a minuscule amount of specific chemical vapor. Here, we report unusual multiple vapor-responsive bidirectional macroscale actuation behaviors of single-layer regenerated silk fibroin films. The vapor-responsive silk fibroin actuator exhibits antagonistic actuation characteristics in a reversible manner to both water and ethanol vapors. For instance, it produces an upward bending in the presence of water vapor and downward bending in ethanol vapor, which demonstrates the chemical vapor-specific actuation. However, the actuation characteristics remain largely invariant upon changing the polarity of alcohol molecules. The silk fibroin actuators effectively utilize the vapor-induced minuscule expansion and contraction of the film surface to produce large-scale actuation, which is fully reversible. The intrinsic water content of the films and the vapor pressure of the stimulants are exploited to control the actuation performance. Further, we demonstrated the 3D shape morphing ability of the actuator by generating an undulating wavelike motion via preprogrammed water and ethanol vapor exposure conditions. The change in the actuation direction is instantaneous, which ensures the sensitivity and rapid response of the fabricated actuators.

Jamming of Nano-Ellipsoids in a Microsphere: A Quantitative Analysis of Packing Fraction by Small-Angle Scattering.

The packing of particles is ubiquitous, and it is of fundamental importance, particularly in materials science in the nanometric length scale. It becomes more intriguing when constituent particles deviate from spherical symmetry owing to the inherent complexity in quantifying their positional and rotational correlation. For quantitative estimation of packing fraction, it requires a thorough analysis of the positional correlation of jammed particles. This article adopts a novel approach for determination of the packing fraction of strongly correlated nano-ellipsoids in a microsphere using small-angle scattering. The method has been elucidated through a quantitative analysis of structural correlation of nano-hematite ellipsoids in 3D micrometric granules, which are realized using rapid evaporative assembly. Owing to the deviation from spherical symmetry, the conventional analysis of scattering data fails to interpret the actual packing fraction of the anisotropic particles. The structural correlation gets smeared out because of orientation distribution among the packed anisotropic particles, which leads to an anomaly in the estimation of packing fraction using the conventional analysis approach. It is illustrated that consideration of an interparticle distance distribution function of the correlated nano-ellipsoids becomes indispensable in determining their packing fraction.

Intrinsic-water desorption induced thermomechanical response of hydrogels.

We report an interplay between the desorption of intrinsic water and relaxation of polymer chains resulting in an unusual thermomechanical response of a hydrogel, wherein the elastic modulus increases in a certain temperature range followed by a sharp decrease with a further increase in temperature. We establish that, in a hydrogel, the desorption of disparate water types having distinct binding energy affects the consolidation and relaxation behaviour of the matrix, which in turn affects the mechanical properties at different temperature ranges. Using temperature-dependent dielectric relaxation spectroscopy and nanoindentation techniques, the chain dynamics and mechanical properties are investigated.

Thermoelectric properties of Ag-doped CuI: a temperature dependent optical phonon study.

Due to the natural abundance and non-toxicity of copper (Cu) and iodine (I), γ-CuI has been widely explored as a potential transparent thermoelectric material for near room temperature applications. Here, we report the effect of doping of an heavy atom such as silver (Ag) on the evolution of temperature-dependent optical phonon modes and thermoelectric properties of chemically synthesized single-phase nanocrystalline γ-CuI. We found that Ag doping reduces the lattice parameters of CuI and thereby confirms the occupancy of Ag atoms at the vacancy sites of CuI. The decrease in phonon lifetime with the increase in temperature, which strongly influences the lattice thermal conductivity, is established from temperature-dependent optical phonon vibrations study. The four-phonon/Umklapp scattering is found to be more prominent in undoped CuI, whereas three-phonon scattering is prominent in Ag-doped CuI. At low temperatures, an almost 90% increase in the Seebeck coefficient is observed for Ag-doped CuI compared to undoped CuI, which can be understood by taking into account a net decrease in the hole carrier concentration in doped CuI.

Evaporative self-assembly of soft colloidal monolayers: the role of particle softness.

We investigate the sessile drop evaporation aided self-assembly of microgel particles by varying their softness. Evaporation of sessile drops containing amphiphilic microgel particles at suitable concentrations results in uniform monolayer deposits that span the entire drop area. At lower concentrations, the deposits are in the form of monolayer coffee rings whose width scales with particle concentration. Using softer microgels synthesised with a lower quantity of crosslinker, we show that the monolayer coffee rings do not form at low particle concentrations. The microgels adsorbed at the interface deform, and the extent of deformation depends on the softness of the microgels as well as their concentration at the interface. Upon complete evaporation of the solvent, the microgel-laden interface is transferred to the substrate. The final deposit shows hexagonal particle arrays where the interparticle separation increases with increasing microgel softness and decreases with particle concentration in the drop. Further insight into the role of microgel softness in the microstructure of the particulate deposits is obtained by measuring the viscoelasticity of the particle-laden interface. Interestingly, the interface loaded with lesser crosslinked microgels exhibits viscoelastic nature even at lower particle concentrations, whereas the higher crosslinked microgels show viscous behaviour.

Evaporative self-assembly of the binary mixture of soft colloids.

We have reported experimental studies on the self-assembly and degree of ordering of a binary mixture of soft colloids in monolayer deposits obtained by controlled evaporation. A sessile drop containing soft colloids is evaporated on a solid surface to achieve a loosely-packed two-dimensional deposit with a hexagonal arrangement. The soft microgel particles possess a hard core with a compliant corona, which plays a crucial role in retaining the crystallinity of the binary particle monolayer. The ordered arrangement of the binary mixture is observed even when the bulk diameter of one type of particle is 25% higher than the other, irrespective of their mixing ratio (1 : 3, 1 : 1, and 3 : 1). The microgel particles of both sizes are found to be homogeneously distributed throughout the deposit, completely suppressing the size-dependent particle segregation. Furthermore, in contrast to the self-assembly of bidisperse hard colloids, wherein the lattice distorts to accommodate particles of disparate sizes, in soft colloids, the particles deform at the interface to preserve the crystalline lattice. Moreover, unlike the gradual order-to-disorder transition observed in the deposits consisting of monodisperse microgel particles, the deposits of a binary mixture of microgels exhibit no noticeable trend. The areal disorder parameter, pair correlation function and the shape factor which quantifies the local ordering of particles in the deposit indicate the absence of a distinct order-to-disorder transition for the binary mixtures.

Effect of iodine doping on the electrical, thermal and mechanical properties of SnSe for thermoelectric applications.

We report the evolution of the thermoelectric and mechanical properties of n-type SnSe obtained by iodine doping at the Se site. The thermoelectric performance of n-type SnSe is detailed in the temperature range starting from 150 K ≤ T ≤ 700 K. The power factor of 0.25% iodine doped SnSe is found to be 0.33 mW m-1 K-2 at 700 K, comparable to that of the other monovalent doped n-type SnSe. The temperature-dependent electrical conductivity of the undoped and iodine doped SnSe samples is corroborated by using the adiabatic small polaron hopping model. A very low value of thermal conductivity, 0.62 W m-1 K-1, is obtained at 300 K and is comparable to that of SnSe single crystals. The low thermal conductivity of n-type polycrystalline SnSe is understood by taking into account the anharmonic phonon vibrations induced by the incorporation of heavy iodine atoms at the Se sites as well as the structural hierarchy of the compound. Besides, iodine doping is found to improve the reduced Young's modulus and hardness values of SnSe, which is highly desirable for thermoelectric device applications.

Modulation of Central Depletion Zone in Evaporated Sessile Drops via Substrate Heating.

In this article, we report the influence of substrate temperature (Tsub) on the evaporation driven patterning of colloids on solid substrates. When the drops are dried in an environment maintained at temperature, Tenv, lower than Tsub, the temperature difference between the drop apex and the three-phase contact line leads to thermal Marangoni flow. We show that the interplay between the radial capillary flow, the thermal Marangoni flow, and the descending rate of the drop surface can be tuned to modulate the spatial distribution of colloids in the dried deposits. At ΔT (=Tsub - Tenv) ≥ 45 °C, the distribution of particles in the interior region of the pattern is nearly uniform with a significant decrease in concentration of particles in the ring-like deposit at the edge. The deposits formed at 15 °C ≤ ΔT ≤ 40 °C are accompanied by a particle depleted zone in the center, which has not been reported to date. The formation of the central depletion zone arises from the suppression of the thermal Marangoni flow at the penultimate stage of drying and the interplay between the radial capillary flow and the descending rate of the drop surface. At ΔT < 15 °C, the dried deposits are found to exhibit coffee-ring-like stains.

On the origin and evolution of the depletion zone in coffee stains.

The variation in the concentration of surfactant molecules along the air-water interface of a drying sessile drop containing colloidal particle-surfactant mixtures is known to inhibit the formation of coffee stains. This also leads to the formation of particulate deposits with a region almost deprived of the particles, often called the depletion zone. The molecular size of the surface-active species used in such experiments poses limitations on the direct visualization of the build-up of surfactant molecules at the interface and how it correlates with the nucleation and growth kinetics of the depletion zone. We report a quantitative analysis of the origin and evolution of the depletion zone that forms in a drying sessile drop. By evaporating an aqueous sessile droplet containing a mixture of surface-active poly(N-isopropylacrylamide) (pNIPAM) microgels and polystyrene (PS) colloids, we establish that the concentration fluctuations of the PS particles along the air-water interface trigger a surface tension driven Marangoni flow along the interface. As a result of this, a depletion zone forms within the particulate deposit, which is analogous to the depletion zones observed for dried drops of particle- surfactant mixtures. The critical time at which the depletion zone forms correlates to the time at which surface tension-driven stress along the interface is maximum. Moreover, we show that the increase in the initial concentration of PS particles results in a decrease in the critical time. By using in situ video microscopy, we established that the rate at which the depletion zone grows is non-linear in time. The growth rate of the depletion zones for drops containing different concentrations of PS particles collapses into a master curve upon scaling with concentration and radius of the drop.

Viscoelastic Particle-Laden Interface Inhibits Coffee-Ring Formation.

We investigate the evaporation-driven pattern formation in drying drops containing mixtures of polystyrene and soft microgel particles. The well-known coffee-rings that form when drops containing polystyrene particles are dried can be completely undone in the presence of a small quantity of soft colloids. The addition of soft colloids facilitates the adsorption of polystyrene particles to the water-vapor interface leading to a steep increase in their concentration and also imparts viscoelasticity to the interface. Time-resolved video microscopy is used to conclusively show the formation of a gel-like particle-laden interface. The mean square displacement of the polystyrene particles adsorbed to the interface confirms their immobile nature at the interface. This viscoelastic interface almost prevents the bulk flow-assisted migration of polystyrene particles toward the drop edge, leading to the suppression of coffee-ring effect and the formation of uniform particulate deposits.

Water desorption from a confined biopolymer.

We study desorption of water from a confined biopolymer (chitosan thin films) by employing temperature dependent specular X-ray reflectivity and spectroscopic ellipsometry. The water desorption is found to occur via three distinct stages with significantly different desorption rates. The distinct rates of water desorption are attributed to the presence of different kinds of water with disparate mobilities inside the biopolymer film. We identify two characteristic temperatures (Tc1 and Tc2) at which the water desorption rate changes abruptly. Interestingly, the characteristic temperatures decrease with decreasing the film thickness. The thickness dependence of the characteristic temperature is interpreted in the context of a higher mobility of polymer chains at the free surface for polymers under one-dimensional confinement.