The emergence of two-dimensional (2D) materials as functional surfaces for sensing, electronics, mechanics, and other myriad applications underscores the importance of understanding 2D material-liquid interactions. The thinness and environmental sensitivity of 2D materials induce novel surface forces that drive liquid interactions. This complexity makes fundamental 2D material-liquid interactions variable. In this review, we discuss the (1) wettability, (2) electrical double layer (EDL) structure, and (3) frictional interactions originating from 2D material-liquid interactions. While many 2D materials are inherently hydrophilic, their wettability is perturbed by their substrate and contaminants, which can shift the contact angle. This modulation of the wetting behavior enables templating, filtration, and actuation. Similarly, the inherent EDL at 2D material-liquid interfaces is easily perturbed. This EDL modulation partially explains the wettability modulation and enables distinctive electrofluidic systems, including supercapacitors, energy harvesters, microfluidic sensors, and nanojunction gating devices. Furthermore, nanoconfinement of liquid molecules at 2D material surfaces arising from a perturbed liquid structure results in distinctive hydrofrictional behavior, influencing the use of 2D materials in microchannels. We expect 2D material-liquid interactions to inform future fields of study, including modulation of the chemical reactivity of 2D materials via tuning 2D material-liquid interactions. Overall, 2D material-liquid interactions are a rich area for research that enables the unique tuning of surface properties, electrical and mechanical interactions, and chemistry. Origin of 2D material interactions with liquids The ideal interaction between a surface and a liquid is dictated by surface forces. Surface forces can be subdivided into three components: van der Waals forces, which exist at any interface between solids and liquids; electrostatic interactions, which exist between charged or polar surfaces and fluids; and structural forces, principally hydrogen bonding 1. These interactions influence the wetting behavior and determine whether the interaction with water is hydrophilic or hydrophobic, control the electronic structure at the liquid-solid interface, influence the frictional interaction, and modulate the chemical activity. However, this ideal picture of the surface interactions is complicated when considering surface heterogeneities, including roughness 2 and contamination 3 , which interfere with the formation of an equilibrium configuration between the liquid and the surface, leading to eccentric behaviors, including superwetting, superslipping, and superhydrophobicity. These surface heterogeneities also allow the surface liquid interactions to be tuned, enabling an array of applications in sensing, chemical transport, and actuation. Two-dimensional (2D) materials, which are extremely thin, have unique electrical properties, and have a tendency to deform and accumulate contamination during proce...

Flexible, architectured, photonic nanostructures such as colloidal photonic crystals (CPCs) can serve as colorimetric strain sensors, where external applied strain leads to a noticeable color change. However, CPCs' response to strain is difficult to quantify without the use of optical spectroscopy. Integration of flexible electrical readout of CPCs' color change is a challenge due to a lack of flexible/stretchable electrical transducers. This work details a colorimetric strain sensor with optoelectrical quantification based on an integrated system of CPCs over a crumpled graphene phototransducer, which optoelectrically quantifies CPCs, response to strain. The hybrid system enables direct visual perception of strain, while strain quantification via electrical measurement of the hybrid system outperforms that of crumpled graphene strain sensors by more than 100 times. The unique combination of a photonic sensing element with a deformable transducer will allow for the development of novel, electrically quantifiable colorimetric sensors with high sensitivity.

Multifunctional piezoelectric materials with controllable mechanical and thermal properties could enable the next generation of soft interconnect materials, energy harvesters, and health monitors. [1] Boron nitride nanotubes (BNNTs) possess unique properties that have piqued the interest of the broader research community as the basis for these multifunctional materials. [2] BNNTs exhibit high mechanical strength, possessing a Young's modulus (Y) as high as 1.3 TPa [3] which outperforms most structural materials. BNNTs also show enhanced thermal properties including extreme thermal stability in air (>800 °C) [4] and thermal conductivity competitive with carbon nanotubes (CNT) [5] which, when combined with their electrically insulating nature, may enable the development of novel thermally conductive, electrically insulating materials. [6] Of particular interest are BNNTs' piezoelectric properties, [7] arising from polarization due to the electronegativity difference between boron and nitrogen atoms, which is predicted to result in the generation of a coupled electric dipole in response to deformation. [8] With the advent of techniques to produce large volumes of BNNTs such as the high temperature pressure (HTP) method, [9,10] interest has shifted to production of ensembles of BNNTs which translate the nanoscale properties of BNNTs to macroscale systems. The most widely employed technique to produce functional structures of BNNTs is suspension in a matrix material to form a functional composite. For instance, the dispersion of BNNTs in soft polymers has been shown to augment mechanical properties, nearly doubling the compressive modulus of polyurethane. [11] Similarly, BNNTs can be added to thermally insulating materials to enhance thermal conductivity, with addition of BNNTs to polymer-derived ceramic (PDC) enhancing thermal conductivity by 2100%. [12] Most saliently, BNNT/polymer composites represent the first realization of BNNT piezoelectricity at the macroscale. Polyimide composites containing strainaligned BNNTs demonstrate piezoelectric coefficients up to 4.81 pm V −1 , [13] and nanoporous BNNT thin films are predicted to possess piezoelectric responses competitive with commercial piezoelectric polymers. [14] Of particular interest are stretchable BNNT composites which are optimal for use as flexible thermal interconnects [15] and platforms for strain aligning nanotubes [16] Boron nitride nanotubes (BNNT) uniformly dispersed in stretchable materials, such as poly(dimethylsiloxane) (PDMS), could create the next generation of composites with augmented mechanical, thermal, and piezoelectric characteristics. This work reports tunable piezoelectricity of multifunctional BNNT/PDMS stretchable composites prepared via co-solvent blending with tetrahydrofuran (THF) to disperse BNNTs in PDMS while avoiding sonication or functionalization. The resultant stretchable BNNT/PDMS composites demonstrate augmented Young's modulus (200% increase at 9 wt% BNNT) and thermal conductivity (120% increase at 9 wt% BNNT) without losin...

In article number 1902216, SungWoo Nam and co‐workers report a colorimetric strain sensor with electrical quantification based on an integrated system of colloidal photonic crystals and a crumpled graphene photo‐transducer. The developed sensor enables direct visual readout and a 100‐fold improved strain sensing over plain crumpled graphene strain sensors in applications including body motion monitoring.

We propose surface plasmon resonance biosensors based on crumpled graphene and molybdenum disulphide (MoS2) flakes supported on stretchable polydimethylsiloxane (PDMS) or silicon substrates. Accumulation of specific biomarkers resulting in measurable shifts in the resonance wavelength of the plasmon modes of two-dimensional (2D) material structures, with crumpled structures demonstrating large refractive index shifts. Using theoretical calculations based on the semiclassical Drude model, combined with the finite element method, we demonstrate that the interaction between the surface plasmons of crumpled graphene/MoS2 layers and the surrounding analyte results in high sensitivity to biomarker driven refractive index shifts, up to 7499 nm/RIU for structures supported on silicon substrates. We can achieve a high figure of merit (FOM), defined as the ratio of the refractive index sensitivity to the full width at half maximum of the resonant peak, of approximately 62.5 RIU-1. Furthermore, the sensing properties of the device can be tuned by varying crumple period and aspect ratio through simple stretching and integrating material interlayers. By stacking multiple 2D materials in heterostructures supported on the PDMS layer, we produced hybrid plasmon resonances detuned from the PDMS absorbance region allowing higher sensitivity and FOM compared to pure crumpled graphene structures on the PDMS substrates. The high sensitivity and broad mechanical tunability of these crumpled 2D material biosensors considerable advantages over traditional refractive index sensors, providing a new platform for ultrasensitive biosensing.

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