Water-assisted strong underwater adhesion via interfacial water removal and self-adaptive gelation.

In nearly all cases of underwater adhesion, water molecules typically act as a destroyer. Thus, removing interfacial water from the substrate surfaces is essential for forming super-strong underwater adhesion. However, current methods mainly rely on physical means to dislodge interfacial water, such as absorption, hydrophobic repulsion, or extrusion, which are inefficient in removing obstinate hydrated water at contact interface, resulting in poor adhesion. Herein, we present a unique means of reversing the role of water to assist in realizing a self-strengthening liquid underwater adhesive (SLU-adhesive) that can effectively remove water at contact interface. This is achieved through multiscale physical-chemical coupling methods across millimeter to molecular levels and self-adaptive strengthening of the cohesion during underwater operations. As a result, strong adhesion over 1,600 kPa (compared to ~100 to 1,000 kPa in current state of the art) can be achieved on various materials, including inorganic metal and organic plastic materials, without preloading in different environments such as pure water, a wide range of pH solutions (pH = 3 to 11), and seawater. Intriguingly, SLU-adhesive/photothermal nanoparticles (carbon nanotubes) hybrid materials can significantly reduce the time required for complete curing from 24 h to 40 min using near-infrared laser radiation due to unique thermal-response of the chemical reaction rate. The excellent adhesion property and self-adaptive adhesion procedure allow SLU-adhesive materials to demonstrate great potential for broad applications in underwater sand stabilization, underwater repair, and even adhesion failure detection as a self-reporting adhesive. This concept of "water helper" has potential to advance underwater adhesion and manufacturing strategies.

Design of large-span stick-slip freely switchable hydrogels via dynamic multiscale contact synergy.

Solid matter that can rapidly and reversibly switch between adhesive and non-adhesive states is desired in many technological domains including climbing robotics, actuators, wound dressings, and bioelectronics due to the ability for on-demand attachment and detachment. For most types of smart adhesive materials, however, reversible switching occurs only at narrow scales (nanoscale or microscale), which limits the realization of interchangeable surfaces with distinct adhesive states. Here, we report the design of a switchable adhesive hydrogel via dynamic multiscale contact synergy, termed as DMCS-hydrogel. The hydrogel rapidly switches between slippery (friction ~0.04 N/cm2) and sticky (adhesion ~3 N/cm2) states in the solid-solid contact process, exhibits large span, is switchable and dynamic, and features rapid adhesive switching. The design strategy of this material has wide applications ranging from programmable adhesive materials to intelligent devices.

Repeatedly Regenerating Mechanically Robust Polymer Brushes from Persistent Initiator Coating (PIC).

In this study, a novel surface initiated polymerization (SIP) method was developed from organic-inorganic hybrid persistent initiator coating (PIC) that embeds initiator molecules into inorganic silica sol-gel layer. Comparing with traditional silane initiator surface that prepared by chemical vapor deposition (CVD) method, the PIC can effectively improve the mechanical stability of initiator that was able to endure ten-thousand times of friction cycles. Besides, it allows polymer grafting from sub-surface and so the grafted brushes, poly 3-sulfopropyl methacrylate potassium salt (pSPMA) on the PIC were also much more wear-resisting than those prepared by the traditional ways. More importantly, the PIC could still trigger new polymerization reaction when the grafted brushes were worn off. In addition, the PIC is universal and can be covered on different substrates including glass, metals and plastics, etc. to realize functionalization of these materials. The approach may pave technological way for the application of surface grafted polymer brushes.

"Brush-like" Amphiphilic Polymer for Environmental Adaptive Coating.

Multiple functional coating is urgently needed in complex service surroundings to meet various requirements. In this work, a brush-like amphiphilic copolymer of poly methacryloxyethyl dimethyl butyl ammonium bromide-polydimethylsiloxane (pMDBAB-PDMS) was synthesized to construct an environment-adaptive multifunctional coating based on the copolymer via the UV-curing method. The special molecule chains of the copolymer assembled predominately on the coating surface in different surroundings, which rendered the surface with various functions. In water-rich surroundings, the hydrophilic quaternary ammonium groups in the coating endow the coating surface with antifogging, oleophobicity underwater, self-cleaning, antibacteria, triboelectric resistance, and super lubrication properties. In dry air surroundings, the long, flexible, low surface energy molecular PDMS chains tend to distribute on the top of the coating surface, which gives a low friction coefficient and antioil properties. This work presents a strategy to construct environmental adaptive coating that has an important application prospect in the field of optical lens.

Organic-Inorganic Hybrid Polysiloxane Brushes with Improved Lubrication and Load-Bearing Capacity.

With the development of microelectromechanical systems (MEMS), ultrathin dry lubrication coatings have received significant attention. In this study, a nanoscale organic-inorganic hybrid lubricative coating (OHL) with a low friction coefficient and wear resistance was developed by grafting polysiloxane brushes on an inorganic silica sol layer. Friction evaluations, including the friction coefficient, load-bearing capacity, abrasion, and durability, were conducted. Compared with the surface of polysiloxane brushes without a silica sol layer, the introduction of a silica sol interlayer can effectively improve the mechanical stability of polysiloxane brushes; namely, the friction coefficient under high load pressure was able to remain low for a long time. In addition, the lubrication performance can also further improve by modifying the upper friction pair surface with the OHL. More importantly, the OHL has an excellent stability and general applicability. The OHL coating can be applied to various solid surfaces that provide a similar lubrication performance, which may provide a new vision for reducing the friction coefficient and enhancing the wear resistance, especially under dry friction conditions.

Esophagus-Inspired Actuator for Solid Transportation via the Synergy of Lubrication and Contractile Deformation.

Directional transportation of objects has important applications from energy transfer and intelligent robots to biomedical devices. Although breakthroughs in liquid migration on 2D surfaces or 3D tubular devices have been achieved, realizing smooth/on-demand transportation of constrained solids within a 3D cavity environment under harsh pressurized environment still remains a daunting challenge, where strong interface friction force becomes the main obstacle restricting the movement of solids. Inspired by typical feeding mechanism in natural esophagus system which synergistically couples a lubricating mucosa surface with the peristaltic contraction deformation of the cavity, herein, this challenge is addressed by constructing an esophagus-inspired layered tubular actuator with a slippery inner surface and responsive hydrogel matrix to realize spherical solid propulsion by photo(thermo)-induced cavity deformation. The as-constructed tubular actuator containing Fe3 O4 nanoparticles exhibits local volumetric shrinkage upon NIR-irradiation, which can generate large hydrodynamic pressure and considerable mechanical extrusion force (Fdriving force ≈ 0.18 N) to overcome low interface friction force (ffriction force ≈ 0.03 N), enabling on-demand transportation of constrained (pressure: 0.103 MPa) spherical solids over a long distance in an arbitrary direction. This actuator is anticipated to be used as bionic medicine transportation devices or artificial in vitro esophagus simulation systems, for example, to help formula eating-related physiotherapy plans for patients and astronauts.

Bioinspired high-power-density strong contractile hydrogel by programmable elastic recoil.

Stimuli-responsive hydrogels have large deformability but-when applied as actuators, smart switch, and artificial muscles-suffer from low work density due to low deliverable forces (~2 kPa) and speed through the osmotic pressure-driven actuation. Inspired by the energy conversion mechanism of many creatures during jumping, we designed an elastic-driven strong contractile hydrogel through storing and releasing elastic potential energy in polymer network. It can generate high contractile force (40 kPa) rapidly at ultrahigh work density (15.3 kJ/m3), outperforming current hydrogels (~0.01 kJ/m3) and even biological muscles (~8 kJ/m3). This demonstrated elastic energy storing and releasing method endows hydrogels with elasticity-plasticity switchability, multi-stable deformability in fully reversible and programmable manners, and anisotropic or isotropic deformation. With the high power density and programmability via this customizable modular design, these hydrogels demonstrated potential for broad applications in artificial muscles, contractile wound dressing, and high-power actuators.

Cartilage Mimics Adaptive Lubrication.

The natural cartilage layer exhibits excellent interface low friction and good load-bearing properties based on the mechanically controlled adaptive lubrication mechanism. Understanding and imitating such a mechanism is important for developing high-load-bearing water-lubrication materials. Here, we report the successful preparation of thermoresponsive layered materials by grafting a poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) polyelectrolyte brush onto the subsurface of an initiator-embedded high strength hydrogel [poly(N-isopropylacrylamide-co-acrylic acid-co-initiator/Fe3+)] [P(NIPAAm-AA-iBr/Fe3+)]. The top soft hydrogel/brush composite layer provides aqueous lubrication, while the bottom thermoresponsive hydrogel layer exhibits adaptive load-bearing capacity that shows tunable stiff or modulus in response to the temperature above and below the lower critical solution temperature (LCST, 32.5 °C). An obvious friction-reduction feature is realized above the LCST, resulting from the dynamic increase of the bottom layer mechanical modulus. Furthermore, in situ lubrication-improvement behavior is achieved upon applying a near-infrared (NIR) laser onto the surface of Fe3O4 nanoparticle (NP)-integrated layered materials. Such a typical lubrication-regulated behavior can be attributed to the synergy effect of the improved load-bearing capacity of the bottom layer and the enhanced lubrication behavior of the top layer with an increase in the polyelectrolyte brush chain density, which is similar to the mechanically controlled adaptive lubrication mechanism of the natural cartilage layer. Current research results provide an inspiration for developing novel biomimetic lubrication materials with considerable load-bearing capacity and also propose a strategy for designing intelligent/stable friction-actuation devices.

Natural Product Inspired Environmentally Friendly Strategy Based on Dopamine Chemistry toward Sustainable Marine Antifouling.

The combination of natural active antifouling composition and intelligent coatings has been regarded as a prospective approach to avoid marine biofouling. However, the relatively complex coating structure and the excessive rapid release of antifoulants maintain unresolved issues in their practical application. In this work, a novel environmentally friendly natural antifoulant (Stellera chamaejasme, SC) encapsulated in the polydopamine (PDA) microcapsule (SC@PDA) is prepared by emulsion interfacial polymerization and blended with the acrylate polymer to prepare a sustainable antifouling coating. Herein, the PDA shell acting as an "intelligent capsule" ensures the controlled release of the antifoulant SC, and the corresponding SC loading amount could be high up to 75.40%. As a model antifoulant, the impact of SC on the adsorption activity of the protein bovine serum albumin (BSA) is studied, as well as the settlement of presentative fouling communities (diatom Navicula sp. and red algae Porphyridium sp.) on the constructed coating. The experimental results demonstrate that the natural product SC integrated eco-friendly antifouling coating occupies the superior capacity of impeding the adsorption of both protein BSA and algae. Such antifoulant (SC) integrating with controlled release character is a great advance in terms of marine antifouling applications. It is, therefore, expected that this innovation will provide guiding significance for developing the next generation of antifouling techniques, especially in the field of marine antifouling.

Embedded polyzwitterionic brush-modified nanofibrous membrane through subsurface-initiated polymerization for highly efficient and durable oil/water separation.

HYPOTHESIS: Developing separation membranes functionalized by polymer brushes with high separation efficiency and good cycling stability is of great importance for oil/water separation, yet is still challenged. EXPERIMENTS: In this work, the covalently embedded polyzwitterionic brush-functionalized nanofibrous membrane was developed for efficient and durable oil/water separation. The nanofibrous membrane was prepared by the electrospinning method using initiator-embedded polyacrylonitrile (PAN) resin, followed by novel subsurface-initiated atom transfer radical polymerization (SSI-ATRP) to graft embedded poly(sulfobetaine methacrylate) brushes (PSBMA). The hydration ability, underwater oil adhesion, oil/water separation performance as well as self-cleaning properties of the as prepared membrane (PAN-sg-PSBMA) were systematically studied. FINDINGS: The PAN-sg-PSBMA membrane exhibited extraordinary hydration ability and underwater superoleophobicity with extremely low oil adhesion, which outperformed conventional polymer brush-modified membrane (PAN-g-PSBMA). The PAN-sg-PSBMA membrane was able to separate both oil/water mixture and surfactant-stabilized emulsions with ultrahigh permeation flux and separation efficiency. Moreover, compared with PAN-g-PSBMA, PAN-sg-PSBMA membrane exhibited unprecedented recycling stability in both permeation flux and separation efficiency, which is attributed to mechanical robustness of embedded polymer brushes and outstanding antifouling ability. The current findings revealed that embedded polymer brushes from SSI-ATRP could offer a promising design of functionalized nanofibrous membrane for highly efficient and durable oil/water separation.

In Situ Grafting Hydrophilic Polymeric Layer for Stable Drag Reduction.

Developing drag reduction techniques has attracted great attention because of their need in practical applications. However, many of the proposed strategies exhibit some inevitable limitations, especially for long period of adhibition. In this work, the dynamic but stable drag reduction effect of superhydrophilic hydrogel-coated iron sphere falling freely in a cylindrical water tank was investigated. The absolute instantaneous velocities and displacements of either the hydrogel-encapsulated or unmodified iron sphere falling freely in water were monitored via a high-speed video. It was revealed that, in the range of Reynolds number from 104 to 106, the optimized hydrogel-coated iron sphere with uniform stability could reduce the resistance by up to 40%, which was mainly due to the boundary slip of water and the delayed boundary separation that resulted from the coated hydrogel. Besides, the deliberate experiments and analysis further indicated that the superhydrophilic hydrogel layer accompanied by the emergence of the drag crisis has largely effected the distribution of flow field at the boundary around the sphere. More importantly, the drag reduction behavior based on the proposed method was thermodynamically stable and resistant to external stimulus, including fluidic oscillator and hydrodynamic pressure. The effective long-term drag reduction performance of the hydrophilic substrate can be expected, correspondingly, and also provides a novel preliminary protocol and avenues for the development of durable drag reduction technologies.

Remote Control over Underwater Dynamic Attachment/Detachment and Locomotion.

Despite extensive efforts to mimic the fascinating adhesion capability of geckos, the development of reversible adhesives underwater has long been lagging. The appearance of mussels-inspired dopamine chemistry has provided the feasibility to fabricate underwater adhesives; however, for such a system, imitating the reversible and fast dynamic attachment/detachment mechanism of gecko feet still remains unsolved. Here, by synthesizing a thermoresponsive copolymer of poly(dopamine methacrylamide-co-methoxyethyl-acrylate-co-N-isopropyl acrylamide) and then decorating it onto mushroom-shaped poly(dimethylsiloxane) pillar arrays, a novel underwater thermoresponsive gecko-like adhesive (TRGA) can be fabricated, yielding high adhesion during the attachment state above the lower critical solution temperature (LCST) of the copolymer, yet low adhesion during the detachment state below the LCST of the copolymer. By integrating the Fe3 O4 nanoparticles into the TRGA, TRGAs responsive to near-infrared laser radiation can be engineered, which can be successfully used for rapid and reversible remote control over adhesion so as to capture and release heavy objects underwater because of the contrast force change of both the normal adhesion force and the lateral friction force. It is also demonstrated that the material can be assembled on the tracks of an underwater mobile device to realize controllable movement. This opens up the door for developing intelligent underwater gecko-like locomotion with dynamic attachment/detachment ability.

Polymer brushes on structural surfaces: a novel synergistic strategy for perfectly resisting algae settlement.

The current paper reports a novel model of a marine antibiofouling surface based on polymer brushes on a wrinkled silicone elastomer. Polymer brushes (POEGMA and PSPMA) were grafted via surface-initiated atom transfer radical polymerization (SI-ATRP). Successful grafting was verified with various characterization techniques including infrared spectroscopy, X-ray photoelectron spectroscopy and contact angle measurements. A series of laboratory static and dynamic bioassays as well as field immersion tests were carried out to systematically investigate the relationship between surface chemistry, surface topography and antifouling properties. The results indicated that the adhesion of marine organisms was strongly influenced by the surface chemistry composition and surface topography structure. The synergistic effect of the surface chemistry, surface topography and bulk properties of the substrates endowed the new marine coatings with excellent antifouling properties.

Contribution of Charges in Polyvinyl Alcohol Networks to Marine Antifouling.

Semi-interpenetrated polyvinyl alcohol polymer networks (SIPNs) were prepared by integrating various charged components into polyvinyl alcohol polymer. Contact angle measurement, attenuated total reflection Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and tensile tests were used to characterize the physicochemical properties of the prepared SIPNs. To investigate the contribution of charges to marine antifouling, the adhesion behaviors of green algae Dunaliella tertiolecta and diatoms Navicula sp. in the laboratory and of the actual marine animals in field test were studied for biofouling assays. The results suggest that less algae accumulation densities are observed for neutral-, anionic-, and zwitterionic-component-integrated SIPNs. However, for the cationic SIPNs, despite the hydration shell induced by the ion-dipole interaction, the resistance to biofouling largely depends on the amount of cationic component because of the possible favorable electrostatic attraction between the cationic groups in SIPNs and the negatively charged algae. Considering that the preparation of novel nontoxic antifouling coating is a long-standing and cosmopolitan industrial challenge, the SIPNs may provide a useful reference for marine antifouling and some other relevant fields.

Contribution of Surface Chemistry to the Shear Thickening of Silica Nanoparticle Suspensions.

Shear thickening is a general process crucial for many processed products ranging from food and personal care to pharmaceuticals. Theoretical calculations and mathematical simulations of hydrodynamic interactions and granular-like contacts have proved that contact forces between suspended particles dominate the rheological characteristic of colloidal suspensions. However, relevant experimental studies are very rare. This study was conducted to reveal the influence of nanoparticle (NP) interactions on the rheological behavior of shear-thickening fluids (STFs) by changing the colloidal surface chemistries. Silica NPs with various surface chemical compositions are fabricated and used to prepare dense suspensions. Rheological experiments are conducted to determine the influence of NP interactions on corresponding dense suspension systems. The results suggest that the surface chemistries of silica NPs determine the rheological behavior of dense suspensions, including shear-thickening behavior, onset stress, critical volume fraction, and jamming volume fraction. This study provides useful reference for designing effective STFs and regulating their characteristics.

Bio-Inspired Design and Fabrication of Micro/Nano-Brush Dual Structural Surfaces for Switchable Oil Adhesion and Antifouling.

The underwater superoleophobic surfaces play a significant role in anti-oil contamination, marine antifouling, etc. Inspired by the Gecko's feet and its self-cleaning property, a hierarchical structure composed of poly (acrylic acid) gel micro-brushes is designed by the liquid-infused method. This surface exhibits underwater superoleophobicity with very low oil adhesion. It is then modified with stimuli-responsive polymer nano-brushes via surface-initiated atom transfer radical polymerization from the embedded initiator. The micro/nano-brush dual structural surfaces can switch the underwater oil adhesion between low and high while keeping the superoleophobicity. The antifouling properties against algae attachment under different mediums are also investigated to show a strong link between oleophobicity and antibiofouling property. The model surface will be very useful in directing the design of marine self-cleaning coatings to both living and non-living species.

Bio-Inspired Renewable Surface-Initiated Polymerization from Permanently Embedded Initiators.

Herein, we describe a simple and robust approach to repeatedly modify surfaces with polymer brushes through surface-initiated atomic transfer radical polymerization (SI-ATRP), based on an initiator-embedded polystyrene sheet that does not rely on specific surface chemistries for initiator immobilization. The surface-grafted polymer brushes can be wiped away to expose fresh underlying initiator that re-initiates polymerization. This strategy provides a facile route for modification of molded or embossed surfaces, with possible applications in the preparation of fluidic devices and polymer-embedded circuits.

How solid-liquid adhesive property regulates liquid slippage on solid surfaces?

The influence of solid-liquid adhesive property on liquid slippage at solid surfaces has been investigated using experiment approach on well-defined model surfaces as well as theoretical analysis. Based on a classical molecular-kinetic description for molecular and hydrodynamic slip, we propose a simple theoretical model that directly relates the liquid slip length to the liquid adhesive force on solid surfaces, which yields an exponential decay function. Well-defined smooth surfaces with varied surface wettability/adhesion are fabricated by forming self-assembled monolayers on gold with different mole ratios of hydrophobic and hydrophilic thiols. The adhesive force of a water droplet and the molecular slippage on these surfaces are probed by surface force apparatus and quartz crystal microbalance measurements, respectively. The experiment results are well consistent with our theoretical prediction. Our finding benefits the understanding of the underlying mechanism of liquid slippage on solid surfaces at molecular level and the rational design of microfluidics with an aim to be frictionless or highly controllable.

Photoresponsive superhydrophobic coating for regulating boundary slippage.

A photoresponsive copolymer containing catechol and azobenzene derivatives was synthesized. The copolymer easily attached onto various substrates and showed a photoresponsive characteristic because of its catechol and azobenzene functional groups. The copolymer was successfully assembled on nanoparticles, plate mica, and rough anodized aluminum surface. The rough anodized aluminum sheet retained the Cassie-Baxter state after being modified with the copolymer. Moreover, surface adhesion can be interchanged by changing the UV exposure time. The sliding and adhesive states of water droplets were achieved by UV exposure and dark storage. Boundary slip on the rough sheet was measured using a commercial rheometer, and interchangeable slip length was also obtained after irradiation or storage. The versatile, substrate-independent approach may be significant in the development of new materials for smart fluid devices.

Switching fluid slippage on pH-responsive superhydrophobic surfaces.

Two stimuli-responsive polymer brushes, poly(dimethylaminoethyl methacrylate) and poly(methacrylic acid), were grafted from initiator-modified anodized alumina substrates to prepare two pH-responsive surfaces. By regulating the swelling states of the two polymers, water droplets can roll off or adhere onto the textured surface because of different adhesion forces. These forces also strongly affect boundary slippage. To determine the different slippage effects of fluid on our pH-responsive surfaces, a series of rheological experiments are carried out on two kinds of surfaces. A large slip length is obtained and reversibly regulated by changing the fluid pH. These responsive superhydrophobic surfaces with considerable slip length and pH-responsive properties have extensive potential applications in intelligent micro- and nanofluidic devices or biodevices, which can solve fluid flow problems.