Thermo-responsive gels based on supramolecular assembly of an amidoamine and citric acid.

In this work, we report the formation of a novel, aqueous-based thermo-responsive, supramolecular gelling system prepared by a convenient and efficient self-assembly of a long-chain amino-amide and citric acid. To determine the viscosity behavior and to gain insights into the gelation mechanism, a complementary combination of techniques, including Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamic light scattering (DLS), and sinusoidal oscillatory tests, were used. The supramolecular gelator exhibited remarkably reversible sol-gel transitions induced by temperature at 76 °C. At a concentration of 5 wt%, the zero-frequency viscosity of the supramolecular system increased by about four orders of magnitude (from 4.2 to 12 563 Pa s) by changing the temperature from 23 °C to 76 °C. The viscous nature of the supramolecular gel could be preserved up to 90 °C. The synergistic combination of the hydrogen bonding between amino and carboxylic acid groups and the electrostatic interactions arising from the protonation of the amino-group and the deprotonation of carboxylic acid groups enhanced at higher temperatures is presumably responsible for the thermo-responsive behavior. We anticipate that these supramolecular gelators can be beneficial in various applications such as hydrogel scaffolds for regenerative medicine, personal care products and cosmetics, and enhanced oil recovery as viscosity modifiers.

Structural, tribological, and mechanical properties of the hind leg joint of a jumping insect: Using katydids to inform bioinspired lubrication systems.

This study investigates the structural properties of the hind leg femur-tibia joint in adult katydids (Orthoptera: Tettigoniidae), including its tribological and mechanical properties. It is of particular interest because the orthopteran (e.g., grasshoppers, crickets, and katydids) hind leg is highly specialized for jumping. We show that the katydid hind leg femur-tibia joint had unique surfaces and textures, with a friction coefficient (µ) at its coupling surface of 0.053±0.001. Importantly, the sheared surfaces at this joint showed no sign of wear or damage, even though it had undergone thousands of external shearing cycles. We attribute its resiliency to a synergistic interaction between the hierarchical surface texture/pattern on the femoral surfaces, a nanograded internal nanostructure of articulating joints, and the presence of lubricating lipids on the surface at the joint interface. The micro/nanopatterned surface of the katydid hind leg femur-tibia joint enables a reduction in the total contact area, and this significantly reduces the adhesive forces between the coupling surfaces. In our katydids, the femur and tibia joint surfaces had a maximum effective elastic modulus (Eeff) value of 2.6GPa and 3.9GPa, respectively. Presumably, the decreased adhesion through the reduction of van der Waals forces prevented adhesive wear, while the contact between the softer textured surface and harder smooth surface avoided abrasive wear. The results from our bioinspired study offer valuable insights that can inform the development of innovative coatings and lubrication systems that are both energy efficient and durable. STATEMENT OF SIGNIFICANCE: Relative to body length, insects can outjump most animals. They also accelerate their bodies at a much faster rate. Orthopterans (e.g., grasshoppers, crickets, and katydids) have hind legs that are specialized for jumping. Over an individual's lifetime, the hind leg joint endures repeated cycles of flexing and extending, including jumping, and its efficiency and durability easily surpass that of most mechanical devices. Although the efficient functioning of insect joints has long been recognized, the mechanism by which insect joints experience friction/adhesion/wear, and operate efficiently/reliably is still largely unknown. Our study on the structural, tribological, and mechanical properties of the orthopteran hind leg joints reveals the potential of katydid bioinspired research leading to more effective coatings and lubrication systems.

Metal-Organic-Inorganic Nanocomposite Thermal Interface Materials with Ultralow Thermal Resistances.

As electronic devices get smaller and more powerful, energy density of energy storage devices increases continuously, and moving components of machinery operate at higher speeds, the need for better thermal management strategies is becoming increasingly important. The removal of heat dissipated during the operation of electronic, electrochemical, and mechanical devices is facilitated by high-performance thermal interface materials (TIMs) which are utilized to couple devices to heat sinks. Herein, we report a new class of TIMs involving the chemical integration of boron nitride nanosheets (BNNS), soft organic linkers, and a copper matrix-which are prepared by the chemisorption-coupled electrodeposition approach. These hybrid nanocomposites demonstrate bulk thermal conductivities ranging from 211 to 277 W/(m K), which are very high considering their relatively low elastic modulus values on the order of 21.2-28.5 GPa. The synergistic combination of these properties led to the ultralow total thermal resistivity values in the range of 0.38-0.56 mm2 K/W for a typical bond-line thickness of 30-50 µm, advancing the current state-of-art transformatively. Moreover, its coefficient of thermal expansion (CTE) is 11 ppm/K, forming a mediation zone with a low thermally induced axial stress due to its close proximity to the CTE of most coupling surfaces needing thermal management.