Switchable Surface Research Articles

Water drop transportation on wettability switchable surface via anisotropic molecules

Active control of the transportation of liquid drops on a horizontal surface is achieved using surfaces with switchable wettability via remote stimuli. However, the mechanism how the dynamic wettability influences drop dynamics is rarely reported. In this paper, we demonstrate that a surface with switchable wettability induces depinning of the contact line through re-orientation of anisotropic molecules. We investigated the dynamics of contact lines and contact angles during the initiation of drop movement by the advancing and receding angles of the surface. We found that imbalance between advancing and receding angles with respect to the dynamic contact angle provides the force needed to overcome the energy barrier due to contact angle hysteresis on the surface. We discovered that the driving energy is accumulated with oscillations in contact angle until it breaks the pinning energy barrier. Understanding the role of dynamic contact angles in drop movement on switchable surfaces paves the way for designing effective fluid manipulation devices, such as water harvesters, biosensors, and oil–water separators.

Four-Mode Switchable Frequency-Selective Surface Featuring Wideband and Angular Stability

Four-Mode Switchable Frequency-Selective Surface Featuring Wideband and Angular Stability

Intelligent Wearable Graphene Nano‐Electronics with Switchable Surface Wettability Capabilities for Autonomous Sweat Enrichment‐Purification‐Analysis (Adv. Funct. Mater. 41/2024)

Intelligent Wearable Graphene Nano‐Electronics with Switchable Surface Wettability Capabilities for Autonomous Sweat Enrichment‐Purification‐Analysis (Adv. Funct. Mater. 41/2024)

A PDMS/Silicon Adhesion Control Method at Millimeter‐Scale Based on Microvibration

Switchable surface adhesion at a small scale is crucial for robot end‐effector design, allowing the manipulation of small objects such as semiconductors, optical lenses, and precision mechanical parts. In this work, a detailed characterization of a millimeter‐scale (1–5 mm) adhesion modulation method is performed, demonstrating its effectiveness for switching adhesion on small, lightweight objects with smooth surfaces. This modulation phenomenon arises from the viscoelastic behavior when PDMS interacts with a rigid surface and is controlled via microvibration. A maximum apparent adhesion enhancement of 2400% and a reduction of 50% are achieved with a 1 mm‐diameter PDMS hemisphere vibrating at a 30 μm amplitude and a 700 Hz frequency. The effects of different parameters, including size, actuation amplitude/frequency, surface roughness, and material properties, on adhesion performance are carefully measured and analyzed. A monotonic increase in maximum adhesion is observed with increased device size and surface smoothness, while nonlinear relationships of other factors are generalized with a numerical model. A long working lifespan and high endurance are also observed during the characterization. This work serves as a practical reference for the further design of small‐scale soft grippers, highlighting its continuous, large modulation range, simple structure, and flexible control.

Real-time monitoring of voltage-responsive biomolecular binding onto electro-switchable surfaces.

Voltage-responsive biosensors capable of monitoring real-time adsorption behavior of biological analytes onto electroactive surfaces offer attractive strategies for disease detection, separations, and other adsorption-dependent analytical techniques. Adsorption of biological analytes onto electrically switchable surfaces can be modelled using neutravidin and biotin. Here, we report self-assembled monolayers formed from voltage-switchable biotinylated molecules on gold surfaces with tunable sensitivity to neutravidin in response to applied voltages. By using electrochemical quartz crystal microbalance (EQCM), we demonstrated real-time switchable behavior of these bio-surfaces and investigate the range of sensitivity by varying the potential of the same surfaces from -400 mV to open circuit potential (+155 mV) to +300 mV. We compared the tunability of the mixed surfaces to bare Au surfaces, voltage inert surfaces, and switchable biotinylated surfaces. Our results indicate that quartz crystal microbalance allows real-time changes in analyte binding behavior, which enabled observing the evolution of neutravidin sensitivity as the applied voltage was shifted. EQCM could in principle be used in kinetic studies or to optimize voltage-switchable surfaces in adsorption-based diagnostics.

Recent advances in shape memory scaffolds and regenerative outcomes.

The advent of tissue engineering (TE) technologies has revolutionized human medicine over the last few decades. Despite splendid advances in the fabricating and development of different substrates for regenerative purposes, non-responsive static composites have been used to heal injured tissues. After being transplanted into the target sites, grafts will lose their original features, leading to a reduction in regenerative potential. Along with these statements, the use of shape memory polymers (SMPs), smart substrates with unique physicochemical properties, has been extended in different disciplines of regenerative medicine in recent years. These substrates are intelligent and they can easily change physicogeometry features such as stiffness, strain size, shape, etc. in response to external stimuli. It has been proposed that SMPs can easily acquire their original properties after deformation, even in the presence or absence of certain stimuli. It has been indicated that the application of distinct synthesis protocols is required to fabricate dynamically switchable surfaces with prominent cell-to-substrate interaction, resulting in better regulation of cell function, dynamic growth, and reparative mechanisms. Here, we aimed to scrutinize the prominent regenerative properties of SMPs in the TE and regenerative medicine fields. Whether and how SMPs can orchestrate certain cell behavior, with reconfigurable features and adaptability were discussed in detail.

Uniformity improvement of microwave heating with switchable frequency selective surface

Microwave heating has attracted interests in many industrial applications, yet heating inhomogeneity remains a critical issue. This study proposes a novel approach to enhance microwave heating uniformity employing a switchable frequency selective surface (FSS). The PIN diodes in each FSS unit cell can be toggled ON or OFF by controlling voltage biases, enabling the FSS to either reflect or transmit incident electromagnetic waves. Placing this switchable FSS in front of a heating cavity wall creates a dynamic reflecting boundary, which, through periodic alterations of the FSS and metallic wall, effectively improves the heating uniformity. To validate this method, we simulated the heating of a potato sample at 2450 MHz using COMSOL Multiphysics and conducted experimental tests. Both simulation and experimental results demonstrate that the proposed method significantly enhances microwave heating uniformity across various materials and sample sizes. This work provides a promising solution for designing microwave heating reactors.

Redox-mediated ion separation and exchange by tailored design of immobilized metallopolymers

Mining and harvesting of valuable components from water or organic phases are urgent challenges. Metallopolymers can show excellent control of the binding and release of target molecules or ions without additional external chemical inputs. Here, we report the covalent functionalization of porous polystyrene microparticles with poly(hydroxyethyl methacrylate) (PHEMA) and poly(ferrocenylmethyl methacrylate) (PFMMA) copolymers and block copolymers via surface-initiated living anionic polymerization to tailor the functionality and polarity of the surface. The metallopolymer-decorated particles were characterized by infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, and the redox-responsiveness was analyzed in different media using cyclic voltammetry. The variation in polarity enabled the investigation of the influence of polarity on the electrochemical performance. The metallopolymer particles were used as efficient ion-adsorbers and ion-exchange materials in both static and flow measurements, and the resulting findings were compared to homopolymer-functionalized particles. The adsorption of fluoride, nitrate, and sulfate and their adsorption selectivity were investigated. The capacity of sulfate was increased for hydrophilic particles and remained more constant over time. The results demonstrate the importance of the controlled design of surfaces for tailoring the efficiency of ion-adsorption materials. The insights on the structure-property relation of the metallopolymer immobilization pave the way for new adsorption technologies and future design of electrochemically switchable surfaces.

A low cost, acrylic acid based low swelling hydrogel

Low swelling hydrogels have attracted much attention because of good mechanical stability, whereas they suffer the shortcoming of high cost. Here, we demonstrate a low-cost low swelling hydrogel which produced using acrylic acid as the main precursor. This hydrogel has a switchable hydrophobic or hydrophilic surface depending on the copolymerized monomers and adjuvants. The gel is moldable and flexible. In fresh water, swelling ratio of the gel is around 0.2 g/g. In saline solutions, it has adjustable swelling ratio while maintaining low swelling properties. This gel has the characteristics of a low BET surface area of 3.9758 m2/g and an average pore diameter of 20 nm. The gel could be used as a smart adaptive material and shape-stable adsorption material in saline environments.

Intelligent Wearable Graphene Nano‐Electronics with Switchable Surface Wettability Capabilities for Autonomous Sweat Enrichment‐Purification‐Analysis

AbstractSweat wearable biosensors facilitate continuous monitoring of individuals’ in‐depth body physiological state with real‐time and molecular‐level insight. However, limited detection accuracy and sensitivity resulted from insufficient amount of sweat sampling and impurities interferences still hinder their practical applications. Here, a miniature wearable skin‐interfaced intelligent graphene nano‐electronic (SIGN) patch employing a novel Janus membrane integrated surface wettability switchable microfluidic module with autonomous sweat sampling and purification capabilities is presented for in situ analysis of sweat biomarkers. Due to the asymmetric surface energy distribution characteristics of the microfluidic surfaces, rapid, directional transport of sufficient amount of sweat to the Janus membrane is achieved. The Janus membrane purifies the sweat sample and transport the sample to the sensing surface autonomously, thus eliminating impurities interferences and enhancing the sensing performance. An ultra‐flexible bio‐receptor functionalized graphene transistor for accurately monitoring sweat biomarkers such as lactate, with outstanding signal reproducibility and good long‐term (over 1 month) stability, and a signal processing unit are employed incorporating with the microfluidic module. In practical wearing tests, the SIGN patch enables the continuous measuring of sweat lactate levels for volunteers during exercises and intelligently providing a preliminary diagnostic assessment on their exercise intensity successfully, suggesting its potential commercialization prospects.

Switchable Slippery Surfaces Controlled by a Humidity‐Induced Glass Transition of Polyelectrolyte‐Grafted Brushes

AbstractPolymer brushes have found extensive applications as nano‐scale surface coatings with responsive properties, particularly in achieving tunable friction in solvent environments. Here, a special property of hygroscopic polyelectrolyte‐grafted brushes, where the friction forces change by over two orders of magnitude within a narrow range in humidity is reported. Using mechanical measurements of nano‐scale modulus and water absorption coupled with friction and surface‐sensitive spectroscopy, this sharp change in friction is controlled by a humidity‐induced glass transition that abruptly shifts the mode of sliding is demonstrated. Contrary to expectations based on conventional thinking regarding brush lubrication, friction remains large and humidity‐independent below the glass transition even for systems that absorb as much as 30–40% water by volume. This results in an abrupt change in friction past the glass transition humidity. Tuning the chemistry of brushes and their humidity‐induced glass transition offers the tunability to control the on/off friction (or slipperiness) for nanoactuators, ratchets, and catheters, without the need for externally applied lubricating liquids.

Ceramic Bowl‐Supported Nanofibrous Membrane with Fluorinated Silsesquioxane‐inspired Switchable Surfaces for Successive Crude Oil/Water Separation and Secondary Water Treatment

A simple, facile and economic approach was developed to transform a 2-tier hydrophilic biased electrospun hybrid PEBAX-FPOSS or Cabon/silica-FPOSS deposited on top of silica aerogel-exfoliated kaolinite ceramic, into hydrophobicity via a wetting switching mechanism subject to %FPOSS variance. Under the same wetting switching mechanism, we did transform originally hydrophobic expanded polystyrene (EP)-FPOSS into super-hydrophobicity with overriding good oil sorption and recovery capacity. Within 5%−10% FPOSS incorporation into PEBAX or Carbon/Silica, the consolidated hybrid membrane scaffold exhibited hydrophilic wettings with the capability not only to separate crude oil/water mixture but also to remove bacteria from both oil/water and polluted water. However, with 30–70% FPOSS blends via electrospinning, the system switched into hydrophobic/oleophilic wetting with the capability to separate crude oil/water in good performance. The hydrophilic electrospun hybrid membranes exhibited good performances in terms of water flux rate, organic rejection, and mechanical strength, which however is an index of %FPOSS incorporation. However, it became impossible to engage in gravity-driven filtration as FPOSS incorporation exceeded 70%.

Switchable superlyophobic CoAl-layered double hydroxide (LDH)-based mesh membranes for separation of various oil/water systems and removal of soluble organic pollutants

Switchable surface wettability has attracted tremendous attention owing to its potential applications, but it is still thorny to construct under-liquid dual superlyophobic surfaces for reversible separation of emulsified oil/water mixtures and treatment of complex oily wastewater. In this work, a CoAl-LDH-based mesh membrane was prepared for switchable separation of emulsified oily wastewater. The interpenetrating CoAl-LDH nanosheets were grown on the stainless steel mesh (SSM) forming a micro-/nano hierarchical structure via a hydrothermal method, which endows the membrane with under-liquid dual superlyophobicity. The obtained CoAl-LDH membrane exhibited switchable separation capability for both oil/water mixtures and oil/water emulsions with high fluxes and efficiencies (99.7%). In addition, the CoAl-LDH membrane could not only separate but also photodegrade the oil-in-water (O/W) emulsions containing soluble organic pollutants. Interestingly, after introducing a ZnO nanorod layer, the LDH/ZnO membrane exhibited progressive removal efficiency (99%) and photodegradation efficiency (＞99%) for methylene blue (MB). The cycled and harsh condition tests also confirmed the stability and robustness of the prepared membranes. The excellent separation performance and photocatalytic property endow the membranes appealing to practical oil/water separation and oily wastewater treatment.

A Superhydrophobicity-Slipperiness Switchable Surface with Magneto- and Thermo-responsive Wires for Repelling Complex Droplets.

The inefficacy of repelling water droplets laden with macromolecules (complex droplets or diluted polymer solution) is a long-standing shortcoming of superhydrophobic surfaces, which severely limits their reliability in practical applications. Here, we design a surface termed the superhydrophobicity-slipperiness switchable surface (3S surface), which demonstrates superhydrophobicity at room temperature and slipperiness when heated. The 3S surface is composed of magneto-responsive wires coated with superhydrophobic nanoparticles and impregnated with thermoresponsive paraffin, exhibiting lotus leaf-inspired passive water repellency and respiratory cilia-inspired active water repellency at room temperature. When heated, the impregnated paraffin melts and forms a lubricant layer atop the surface structures, exhibiting the pitcher-plant-inspired removal of complex droplets that remain pinned on conventional superhydrophobic surfaces. The counterintuitive integration of superhydrophobicity (a liquid-solid-gas composite system) and slipperiness (a liquid-lubricant-gas system) into a surface and the on-demand switch between them are not only important to the applicability of self-cleaning surfaces to real-world environments, where complex liquids are inevitable, but also provide insights into various interface-related applications.

Construction of C8/threonine-functionalized mesoporous silica aerogel based on mixed-mode and its adsorption properties for both anionic and cationic dyes

High adsorption and desorption efficiencies for both anionic and cationic dyes are still a challenge for adsorbents to treat dyestuffs wastewater. In this study, the C8/threonine-functionalized mesoporous silica aerogel based on mixed-mode was constructed to overcome the challenge, with properties of hydrophobicity and switchable surface charge of n-octyl and threonine groups. With methylene blue and azophloxine as cationic and anionic dye models, batch tests were conducted. The consequences proofed the adsorption process was efficient and fast, and the highest adsorption quantities for methylene blue and azophloxine were estimated as 274.30 mg/g and 152.43 mg/g, respectively. Additionally, the adsorbent showed good removal efficiency and reusability. The desorption efficiencies exceeded 97% using low concentration HCl or NaOH as eluents. Mechanism investigation proved that adsorption process relied on hydrophobicity and electrostatic interactions. These results instructed that SG-C8/Thr was a promising adsorbent with excellent adsorption and desorption efficiencies for both anionic and cationic dyes.

Switchable surface and loading/release of target molecules in hierarchically porous PLA nonwovens based on shape memory effect.

In this work, hierarchically porous PLA (polylactic acid) shape memory nonwovens were prepared by electrospinning its blend solution with PEO (polyethylene oxide) and subsequent water etching. Based on shape memory effect resulting from tiny crystals and the amorphous matrix of PLA, the switch between compact and porous surfaces has been achieved via cyclical hot-pressing and recovery in a hot water bath. After hot-pressing, the disappearance of hierarchical pores contributes to compact surface, enabling embedding of the target molecule in PLA nonwoven (i.e., CLOSE state). Upon exposure to heat, PLA nonwoven recovers to its permanent shape and exhibits a porous surface, providing a penetrative diffusion pathway for small molecules (i.e., OPEN state). The hierarchically porous structure and shape memory effect endow PLA nonwoven with the capability of rapid release. Our results provide a good candidate for some potential applications, such as temperature-controlled quick-release of catalysts and drugs.

Channel-enhanced Transmission-reflection Switchable Reconfigurable Intelligent Surface with Arbitrary Source and Terminal Positions

Channel-enhanced Transmission-reflection Switchable Reconfigurable Intelligent Surface with Arbitrary Source and Terminal Positions

Switchable Frequency-Selective Surface for Multi-state Highly Selective Transmission in Ku-band

Switchable Frequency-Selective Surface for Multi-state Highly Selective Transmission in Ku-band

Fabrication of multifunction-integrated adsorbent based on zwitterion functionalized cellulose with high adsorption performance for anionic/cationic dyes and heavy metal ions

A multifunction-integrated adsorbent based on zwitterion functionalized cellulose (MCC@PGMA-IAA) was synthesized by surface grafting polymerization and then modification with polyethyleneimine and sodium chloroacetate in turn. The adsorbent exhibited high functional group contents, switchable surface charge and chelating structure of iminoacetic acid. With the methylene blue, azophloxine, Ni(II) and Cr(VI) as the models of cationic/anionic dyes and heavy metal ions, batch experiments were conducted. The material showed strong adsorption abilities for dyes and heavy metal ions, with maximum adsorption capacities up to 440.5 mg/g, 320.5 mg/g, 59.3 mg/g, and 127.9 mg/g for methylene blue, azophloxine, Ni(II) and Cr(VI), respectively. Especially, the adsorption capacities of methylene blue and Ni(II) after zwitterionic functionalization increased by 50 times and 95.5% than those after polyethyleneimine modification. The adsorption equilibria were achieved within 10–120 min, and the adsorbent exhibited excellent removal efficiency. Furthermore, the adsorbed pollutants were conveniently desorbed by low concentration of HCl or NaOH solution, and the adsorbent had good reusability. Mechanism studies proved that electrostatic attraction and chelation were main driving forces. These results showed that MCC@PGMA-IAA had the characteristics of extensive sources, simple preparation and multifunctional integration, which was poised to develop as universal adsorbent for removing heavy metal ions and dyes.

Anti-frosting characteristics of superhydrophobic-hydrophilic wettability switchable surfaces

Combining hydrophobicity with hydrophilicity on hybrid surfaces generates unique phenomena in anti-frosting. However, limited attention has been paid to investigate the anti-frosting effects of wettability switchable surfaces in response to temperature changes. Herein, frosting and defrosting characteristics of thermo-responsive, wettability switchable surfaces are investigated. Specifically, the wettability switchable surfaces are successfully fabricated by coating a poly(ε-caprolactone) (PCL) layer onto the microstructured aluminum substrates, resulting in the wettability transition from superhydrophobicity with an apparent contact angle of water droplet as 152° in the surface temperatures under 60 °C to hydrophilicity with an apparent contact angle of water droplet as 63° in the surface temperatures over 60 °C. The wettability transition capabilities of the surfaces not only enable excellent frost retardation comparable to superhydrophobic surfaces in frosting process, but also realize fast full drying capabilities slightly shorter than superhydrophilic surfaces in defrosting process. The unique feature of wettability switchable surfaces represents an effort to combine the distinct advantages of superhydrophobic and superhydrophilic surfaces to realize anti-frosting functionalities.