Rapid, Tough, and Trigger-Detachable Hydrogel Adhesion Enabled by Formation of Nanoparticles In Situ.

Integrating hydrogel with other materials is always challenging due to the low mass content of hydrogels and the abundance of water at the interfaces. Adhesion through nanoparticles offers characteristics such as ease of use, reversibility, and universality, but still grapples with challenges like weak bonding. Here, a simple yet powerful strategy using the formation of nanoparticles in situ is reported, establishing strong interfacial adhesion between various hydrogels and substrates including elastomers, plastics, and biological tissue, even under wet conditions. The strong interfacial bonding can be formed in a short time (60 s), and gradually strengthened to 902 J m-2 adhesion energy within an hour. The interfacial layer's construction involves chain entanglement and other non-covalent interactions like coordination and hydrogen bonding. Unlike the permanent bonding seen in most synthetic adhesives, these nanoparticle adhesives can be efficiently triggered for removal by acidic solutions. The simplicity of the precursor diffusion and precipitation process in creating the interfacial layer ensures broad applicability to different substrates and nanoparticle adhesives without compromising robustness. The tough adhesion provided by nanoparticles allows the hydrogel-elastomer hybrid to function as a triboelectric nanogenerator (TENG), facilitating reliable electrical signal generation and output performance due to the robust interface.

A Comparative Study on the Effects of Spray Coating Methods and Substrates on Polyurethane/Carbon Nanofiber Sensors.

Thermoplastic polyurethane (TPU) has been widely used as the elastic polymer substrate to be combined with conductive nanomaterials to develop stretchable strain sensors for a variety of applications such as health monitoring, smart robotics, and e-skins. However, little research has been reported on the effects of deposition methods and the form of TPU on their sensing performance. This study intends to design and fabricate a durable, stretchable sensor based on composites of thermoplastic polyurethane and carbon nanofibers (CNFs) by systematically investigating the influences of TPU substrates (i.e., either electrospun nanofibers or solid thin film) and spray coating methods (i.e., either air-spray or electro-spray). It is found that the sensors with electro-sprayed CNFs conductive sensing layers generally show a higher sensitivity, while the influence of the substrate is not significant and there is no clear and consistent trend. The sensor composed of a TPU solid thin film with electro-sprayed CNFs exhibits an optimal performance with a high sensitivity (gauge factor ~28.2) in a strain range of 0-80%, a high stretchability of up to 184%, and excellent durability. The potential application of these sensors in detecting body motions has been demonstrated, including finger and wrist-joint movements, by using a wooden hand.

Liquid Metal Droplet and Graphene Co-Fillers for Electrically Conductive Flexible Composites.

Colloidal liquid metal alloys of gallium, with melting points below room temperature, are potential candidates for creating electrically conductive and flexible composites. However, inclusion of liquid metal micro- and nanodroplets into soft polymeric matrices requires a harsh auxiliary mechanical pressing to rupture the droplets to establish continuous pathways for high electrical conductivity. However, such a destructive strategy reduces the integrity of the composites. Here, this problem is solved by incorporating small loading of nonfunctionalized graphene flakes into the composites. The flakes introduce cavities that are filled with liquid metal after only relatively mild press-rolling (<0.1 MPa) to form electrically conductive continuous pathways within the polymeric matrix, while maintaining the integrity and flexibility of the composites. The composites are characterized to show that even very low graphene loadings (≈0.6 wt%) can achieve high electrical conductivity. The electrical conductance remains nearly constant, with changes less than 0.5%, even under a relatively high applied pressure of >30 kPa. The composites are used for forming flexible electrically-conductive tracks in electronic circuits with a self-healing property. The demonstrated application of co-fillers, together with liquid metal droplets, can be used for establishing electrically-conductive printable-composite tracks for future large-area flexible electronics.

A Novel pH- and Salt-Responsive N-Succinyl-Chitosan Hydrogel via a One-Step Hydrothermal Process.

In this study, we synthesized a series of pH-sensitive and salt-sensitive N-succinyl-chitosan hydrogels with N-succinyl-chitosan (NSCS) and the crosslinker glycidoxypropyltrimethoxysilane (GPTMS) via a one-step hydrothermal process. The structure and morphology analysis of the NSCS and glycidoxypropyltrimethoxysilane-N-succinyl chitosan hydrogel (GNCH) revealed the close relation between the swelling behavior of hydrogels and the content of crosslinker GPTMS. The high GPTMS content could weaken the swelling capacity of hydrogels and improve their mechanical properties. The hydrogels show high pH sensitivity and reversibility in the range of pH 1.0 to 9.0, and exhibit on-off switching behavior between acidic and alkaline environments. In addition, the hydrogels perform smart swelling behaviors in NaCl, CaCl2, and FeCl3 solutions. These hydrogels may have great potential in medical applications.

Silicone Microemulsion Structures Are Maintained During Polymerization with Reactive Surfactants.

Bicontinuous microemulsions exhibit domain structures on the nanoscale (<20 nm). Normally, such fine details are lost during the conversion from a fluid microemulsion to solid elastomeric materials, as a consequence of interfacial destabilization via polymerization of either the oil phase or monomers in the aqueous phase. Very little is known about the polymerization of silicone microemulsions and the morphological changes that occur upon transition from a nanostructured liquid to a solid matrix. Silicone microemulsions polymerized by free radical (aqueous phase) and condensation (silicone phase) processes, respectively, were characterized by small-angle X-ray scattering and transmission electron microscopy. It was found that cross-linking of the silicone phase alone led, over time, to large increase of the size of the microemulsion nanodomains. By contrast, photoinduced polymerization of a reactive surfactant and acrylic monomers in the aqueous phase was effective at retaining bicontinuous nanomorphology, irrespective of the degree of cross-linking of the silicone phase.

Crystallization of Femtoliter Surface Droplet Arrays Revealed by Synchrotron Small-Angle X-ray Scattering.

The crystallization of oil droplets is critical in the processing and storage of lipid-based food and pharmaceutical products. Arrays of femtoliter droplets on a surface offer a unique opportunity to study surfactant-free colloidlike systems. In this work, the crystal growth process in these confined droplets was followed by cooling a model lipid (trimyristin) from a liquid state utilizing synchrotron small-angle X-ray scattering (SAXS). The measurements by SAXS demonstrated a reduced crystallization rate and a greater degree of supercooling required to trigger lipid crystallization in droplets compared to those of bulk lipids. These results suggest that surface droplets crystallize in a stochastic manner. Interestingly, the crystallization rate is slower for larger femtoliter droplets, which may be explained by the onset of crystallization from the three-phase contact line. The larger surface nanodroplets exhibit a smaller ratio of droplet volume to the length of three-phase contact line and hence a slower crystallization rate.

Formation, growth and applications of femtoliter droplets on a microlens.

Formation and growth of femtoliter droplets on surface microstructures are important in many fundamental and practical interfacial processes, such as water collection, vapour condensation in cooling devices, drop self-removal on anti-icing surfaces and fabrication of droplet-templated functional microstructures. In this work, we experimentally and theoretically investigate the growth of femtoliter oil-like liquid on the microlens surrounded by a hydrophilic planar area. The droplets were produced by solvent exchange, a process where the droplets nucleate and grow from an oversaturation created by displacing a good solvent by a poor solvent of the droplet liquid. Our results showed that the droplet fully coats the lens surface and the contact angle of the droplet relative to the flat surface is finely tuned over a large range by the droplet volume. The growth of the droplet on a microlens is largely described by the constant contact radius model. To demonstrate the new opportunities provided by the controlled formation of the droplet situated on a microlens, we will show a simple and effective approach for production of arrays of composite microlenses consisting of two types of polymers with different refractive indices. A high curvature of the composite microlens results in desirable diffraction patterns with potential application for enhanced light harvesting. Moreover, we demonstrate that extraction of traces of a hydrophobic solute from the flow is much faster as the droplet is lifted up from the channel wall by the microlens, promising a time effective in situ detection process in narrow channels.

Collective interactions in the nucleation and growth of surface droplets.

In the process of solvent exchange, oil droplets nucleate and grow on a solid substrate in response to the oversaturation generated through the displacement of a good oil solvent by a poor one. The mean size of the droplets depends on flow rate, flow geometry and solution conditions. In this work, we investigate the surface coverage of the droplets and the correlation between the base area of the droplets and of the bare zone surrounding the droplets for various flow and solution conditions during the solvent exchange. The surface coverage increases with the increase in the flow rate, channel height and the oil concentration, and reaches a plateau between 35% and 50%. The spatial correlation is analysed with the help of the radial distribution function g(r) and a Voronoi tessellation. When the surface coverage reaches ∼25-30%, the number density of the droplets starts to drop, reflecting the mutual interaction and merging of the droplets. With further decrease in the droplet spacing and increase in surface coverage, the Voronoi analysis shows that the base area of the droplets increases linearly with the area size of the depleted zone. The collective interaction in the growth of surface nanodroplets is universal, independent of the specific conditions that control the droplet growth.

Influence of Solution Composition on the Formation of Surface Nanodroplets by Solvent Exchange.

Solvent exchange is a simple process of forming surface nanodroplets on an immersed substrate. In this process, the droplets nucleate and grow in response to transient oversaturation when a good solvent of the droplet liquid is displaced by a poor solvent. Here we will show how the final droplet size is influenced by solution composition in the solvent exchange. To do this, we produced water droplets on a hydrophilic substrate and cyclohexane droplets on a hydrophobic substrate by using a tertiary system of cyclohexane, ethanol, and water. The composition of the good solvent was varied systematically in the one-phase region on the phase diagram. We found that the key feature closely related to the droplet size is the area (A) in the phase diagram defined by the phase boundary and the concentration ratio between the good solvent and the droplet liquid. This area reflects the excessive amount of droplet liquid in the tertiary mixture, which can be complicated by bulk droplet formation during solvent exchange. We will also show that the droplet volume per unit surface area also increases with A. The findings from this work will provide guideline for the selection of solution conditions to achieve a desirable droplet size and number density on the surface.

How a Surface Nanodroplet Sits on the Rim of a Microcap.

The location and morphology of femtoliter nanodroplets that nucleate and grow on a microcap-decorated substrate in contact with a liquid phase were investigated. We experimentally examined four different wetting combinations of the flat area and the microcaps. The results show that depending on the relative wettability, the droplets sit either on the plain surface or on the top of the microcap or on the rim of the microcap. The contact angle and, for the last case, the radial positions of the nanodroplets relative to the microcap center were characterized, in reasonable agreement with our theoretical analysis, which is based on an interfacial energy minimization argument. However, the experimental data show considerable scatter around the theoretical equilibrium curves, reflecting pinning and thus nonequilibrium effects. We also provide the theoretical phase diagram in parameter space of the contact angles, revealing under which conditions the nanodroplet will nucleate on the rim of the microcap.

Collective Effects in Microbubble Growth by Solvent Exchange.

Regulating the formation and growth of microscopic bubbles at solid-liquid interfaces is essential in many physical, chemical, and catalytic processes, such as the electrolysis of water or a dry-wet transition of a superhydrophobic surface. The growth of bubbles in a group is influenced by the neighboring bubbles as well as the overall gas concentration in the system. In this work, we have investigated the growth of multiple microbubbles on highly ordered hydrophobic microcavity arrays, seeded by pre-existing gas pockets trapped inside the cavities. A pulse of gas oversaturation at an extremely low level was supplied in a process we call solvent exchange. Our results show that the distance between the seeding air pockets has significant effects on the location, number density, and size of bubbles on the array. With closely spaced microcavities, growing microbubbles self-organized into symmetric patterns. Their growth rate was enhanced at the corners and edges of the array, and interior bubbles dissolved because of the competitive growth. By contrast, no symmetric patterns were observed when the space between the microcavities was large. The findings reported in this work provide important insights into solvent exchange and collective interactions in the formation of surface nanobubbles.

Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets.

The formation and morphology of microscopic droplets on a chemically modified surface are important for many droplet-related applications. In this study, we examined the formation and morphological characteristics of nanodroplets produced in the same process of solvent exchange on a gold surface coated with a methyl-terminated alkanethiol monolayer. From atomic force microscopy images, we obtained the contact angles of polymerized nanodroplets in 12 combinations of the length of a straight alkyl chain and the type of droplet liquid. Our results show a significant decrease in the number density of the droplets as the number of methyl groups extends from 8 to 12 or 14. The contact angle of the droplets on octanethiol is significantly larger than that on dodecanethiol or tetradecanethiol, possibly because of the screening effect from the monolayer. Our results demonstrate that under the same solution conditions the morphology of surface nanodroplets is sensitive to the detailed molecular structures of the monolayer on the substrate. This finding has important implications for understanding static wetting on the microscopic scale and the origin of three-phase contact line pinning.

Microwetting of pH-Sensitive Surface and Anisotropic MoS2 Surfaces Revealed by Femtoliter Sessile Droplets.

Understanding the microwettability of anisotropic molybdenum disulfide crystal is critically important in separation and processing of this material in liquid. In this work, static microwetting properties of MoS2 face (MF) and MoS2 edge (ME) surfaces in water are revealed by the morphology of femtoliter interfacial droplets. The oil droplets with different size distribution were produced from heterogeneous nucleation and growth of nanodroplets during the solvent exchange under controlled flow and solution conditions, and were polymerized for droplet morphology characterization to reveal the relative wettability of the droplets on surfaces. We first demonstrate that the shape of the nanodroplets is responsive to the surface charges on a model pH sensitive substrate of gold coated with a self-assembled monolayer of two types of thiol. The experimental results on MoS2 substrates indicate that (1) oil contact angle of the droplets on ME surface is much larger than that on MF surface at pH 3.0, suggesting that the ME surface is more hydrophilic than MF; (2) the droplets are pinned by the layered nanostructure on MoS2 edge. The fundamental understanding of microwettability elucidated in this study may allow for an improved control of the interaction between anisotropic MoS2 surfaces and the surrounding liquid environment, which is critically important for many industrial applications such as flotation and catalysis systems.

3D spherical-cap fitting procedure for (truncated) sessile nano- and micro-droplets & -bubbles.

In the study of nanobubbles, nanodroplets or nanolenses immobilised on a substrate, a cross-section of a spherical cap is widely applied to extract geometrical information from atomic force microscopy (AFM) topographic images. In this paper, we have developed a comprehensive 3D spherical-cap fitting procedure (3D-SCFP) to extract morphologic characteristics of complete or truncated spherical caps from AFM images. Our procedure integrates several advanced digital image analysis techniques to construct a 3D spherical-cap model, from which the geometrical parameters of the nanostructures are extracted automatically by a simple algorithm. The procedure takes into account all valid data points in the construction of the 3D spherical-cap model to achieve high fidelity in morphology analysis. We compare our 3D fitting procedure with the commonly used 2D cross-sectional profile fitting method to determine the contact angle of a complete spherical cap and a truncated spherical cap. The results from 3D-SCFP are consistent and accurate, while 2D fitting is unavoidably arbitrary in the selection of the cross-section and has a much lower number of data points on which the fitting can be based, which in addition is biased to the top of the spherical cap. We expect that the developed 3D spherical-cap fitting procedure will find many applications in imaging analysis.

Enhancing thermal stability and mechanical properties of lyotropic liquid crystals through incorporation of a polymerizable surfactant.

We present a facile method to prepare thermally stable and mechanically robust crosslinked lyotropic liquid crystals (LLCs) through incorporation of a polymerizable amphiphile into a binary LLC system comprising commercially available surfactant Brij 97 and water. Thermal stability and mechanical properties of the polymerized LLCs were significantly enhanced after polymerization of the incorporated polymerizable surfactant. The effect of incorporating a polymerizable amphiphile on the phase behavior of the LLC system was studied in detail. In situ photo-rheology was used to monitor the change in the mechanical properties of the LLCs, namely the storage modulus, loss modulus, and viscosity, upon polymerization. The retention of the LLC nanostructures was evaluated by small angle X-ray scattering (SAXS). The ability to control the thermal stability and mechanical strength of LLCs simply by adding a polymerizable amphiphile, without tedious organic synthesis or harsh polymerization conditions, could prove highly advantageous in the preparation of robust nanomaterials with well-defined periodic structures.

Mixed mode of dissolving immersed nanodroplets at a solid-water interface.

The dissolution dynamics of microscopic oil droplets (less than 1 µm in height, i.e. nanodroplets) on a hydrophobilized silicon surface in water was experimentally studied. The lateral diameter was monitored using confocal microscopy, whereas the contact angle was measured by (disruptive) droplet polymerisation of the droplet. In general, we observed the droplets to dissolve in a mixed mode, i.e., neither in the constant contact angle mode nor in the constant contact radius mode. This means that both the lateral diameter and the contact angle of the nanodroplets decrease during the dissolution process. On average, the dissolution rate is faster for droplets with larger initial size. Droplets with the same initial size can, however, possess different dissolution rates. We ascribe the non-universal dissolution rates to chemical and geometric surface heterogeneities (that lead to contact line pinning) and cooperative effects from the mass exchange among neighbouring droplets.

Microwetting of supported graphene on hydrophobic surfaces revealed by polymerized interfacial femtodroplets.

Understanding the wettability of graphene is the crucial step toward the design and control of graphene-based surface in contact with liquids. In this work, the static microwettability of a supported single layer graphene (SLG) immersed in water or alcoholic aqueous solutions is revealed by the morphological characterization of the polymerized interfacial femtoliter droplets. As expected, the contact angle of the femtoliter droplets on the SLG in water is in between that on the underlying silanized silicon and that on graphite (HOPG). However, the wettability of femtoliter droplets on the SLG demonstrates a unique dependence on the compositions of the surrounding liquid medium: Their contact angle on SLG becomes much larger than that on both graphite and on silanized silicon, once short-chain alcohol molecules are present in the surrounding medium. To account for this finding, we hypothesize two scenarios to rationalize the effect of alcohol on the microwettability on SLG. The understanding elucidated in this study may allow for improved control of the interaction between graphene and the surrounding liquid environment and facilitate applications in which graphene is in contact with liquids, such as in microfluidics and in lab-on-chip systems.

From nanodroplets by the ouzo effect to interfacial nanolenses.

Polymerizing nanodroplets at solid-liquid interfaces is a facile solution-based approach to the functionalization of large surface areas with polymeric lens-shaped nanostructures. In this work, we have applied a one-pot approach to obtain polymeric nanolenses with controlled sizes and densities. We take advantage of the formation mechanism by the direct adsorption of nanodroplets from a surfactant-free microemulsion onto an immersed hydrophobic substrate. The interfacial nanodroplets were photopolymerized to produce polymeric nanolenses on the substrate surface. The surfactant-free microemulsion of the monomer nanodroplets was obtained through the spontaneous emulsification (i.e., ouzo effect) in the tertiary system of ethanol, water, and precusor monomer. The size of nanolenses on the surface was adjusted by the nanodroplet size, following a linear relationship with the ratio of the components in the microemulsion. This simple approach is applicable to produce nanolenses over the entire surface area or on any specific area at will by depositing a drop of the microemulsion. Possessing high optical transparency, the resulting substrates may have potential application as functional biomedical supporting materials or effective light-harvesting coatings.

Reversible photorheological lyotropic liquid crystals.

We describe novel lyotropic liquid-crystalline (LLC) materials based on photoresponsive amphiphiles that exhibit rapid photoswitchable rheological properties of unprecedented magnitude between solidlike and liquidlike states. This was achieved through the synthesis of a novel azobenzene-containing surfactant (azo-surfactant) that actuates the transition between different LLC forms depending on illumination conditions. Initially, the azo-surfactant/water mixtures formed highly ordered and viscous LLC phases at 20-55 wt % water content. Spectroscopic, microscopic, and rheological analysis confirmed that UV irradiation induced the trans to cis isomerization of the azo-surfactant, leading to the disruption of the ordered LLC phases and a dramatic, rapid decrease in the viscosity and modulus resulting in a 3 orders of magnitude change from a solid (20,000 Pa) to a liquid (50 Pa) at rate of 13,500 Pa/s. Subsequent exposure to visible light reverses the transition, returning the viscosity essentially to its initial state. Such large, rapid, and reversible changes in rheological properties within this LLC system may open a door to new applications for photorheological fluids.

Multistimuli responsive organogels based on a reactive azobenzene gelator.

A reactive azobenzene based super organogelator was found to rapidly and reversibly transform a range of hydrophobic solvents from gels to solutions upon changes in temperature, light and shear force. More specifically they formed gels at concentrations as low as 0.08 wt%. Upon heating, exposure to UV light, or application of shear, the π-π bonding was disrupted which resulted in a rapid drop of both modulus and viscosity. This was confirmed by (1)H NMR, SAXS, and rheological measurements. Although many examples of organogelators are known in the literature, this is the first time that a reactive group, a benzoyl chloride, has been incorporated in a supramolecular organogel structure. Moreover, this group is available for subsequent synthetic modifications. The presence of benzoyl chloride groups showed a remarkable effect on the formation and properties of the gels. Compared with other approaches, this strategy is advantageous in terms of structural design since it not only produces a multi-responsive soft material but also allows facile modifications which may expand the applications of organogels to other fields.