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We report a facile approach to prepare ultrathin physical hydrogel films based on the Marangoni effect, which drives an ethanol solution of poly(stearyl acrylate-co-acrylic acid) (P(SA-co-AAc)) to rapidly spread on the water surface. The subsequent solvent exchange leads to sol-gel transition, where the long alkyl chains of SA units segregate to form physical cross-linking junctions. The resultant disk-shaped single-network (SN) gel films are uniform with tunable thickness (40-80 μm) and diameter (5-12 cm) and possess robust mechanical properties with tensile breaking stress, σ, and breaking strain, ε, being 0.3-1.1 MPa and 30-290%, respectively. The mechanical properties of SN gel films can be further improved by introducing ductile poly(N-isopropylacrylamide) (PNIPAm) into the preformed gel matrix, which forms strong hydrogen bonds with the first network. The obtained physical double-network (DN) hydrogel films are transparent and show excellent mechanical performances with σ of 3-5 MPa and ε of 100-500%. Due to the ultrathin thickness of gel films and response of PNIPAm to saline solutions, the tough DN gel films exhibit fast response (≤60 s) and large stroke force (0.5 MPa) after switching the environment from water bath to saline solution, making them an ideal material to design artificial muscles, soft actuators, and chemomechanical devices.

Constructing dual or multiple noncovalent crosslinks is highly effective to improve the mechanical and stimuli-responsive properties of supramolecular physical hydrogels, due to the synergistic effects of different noncovalent bonds. Herein, a series of tough physical hydrogels are prepared by solution casting and subsequently swelling the films of poly(ureidopyrimidone methacrylate-co-stearyl acrylate-co-acrylic acid). The hydrophobic interactions between crystallizable alkyl chains and the quadruple hydrogen bonds between ureidopyrimidone (UPy) motifs serve as the dual crosslinks of hydrogels. Synergistic effects between the hydrophobic interactions and hydrogen bonds render the hydrogels excellent mechanical properties, with tensile breaking stress up to 4.6 MPa and breaking strain up to 680%. The UPy motifs promote the crystallization of alkyl chains and the hydrophobic alkyl chains also stabilize UPy-UPy hydrogen bonding. The resultant hydrogels are responsive to multiple external stimuli, such as temperature, pH, and ion; therefore, they show the thermal-induced dual and metal ion-induced triple shape memory behaviors.

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Hydrogel films have promising applications in medical dressings, flexible electronics, etc. However, it is challenging to fabricate ultrathin hydrogel films with high toughness and controllable thickness. Here, we report a facile approach to prepare tough physical hydrogel films by spin-coating of a poly(acrylic acid-co-acrylamide) (P(AAc-co-AAm)) solution and subsequent gelation in FeCl3 solution to form carboxyl-Fe3+ coordination complexes. The thickness of the obtained gel films, ranging from several to hundreds of micrometers, was easily tunable by adjusting the spin conditions and polymer concentration. The thus obtained hydrogel films showed excellent mechanical properties, with tensile breaking strengths of 0.6-14.5 MPa, breaking strains of 140-840%, Young's moduli of 0.1-61.7 MPa, and tearing fracture energies of 300-1300 J m-2. Based on this approach, responsive tough hydrogel films can also be prepared by spin-coating of a poly(acrylic acid-co-N-isopropylacrylamide) (P(AAc-co-NIPAm)) solution. The obtained gel films showed a fast response (<60 s) and a large output force (∼0.2 MPa) triggered by a concentrated saline solution, making them an ideal material in the design of chemomechanical devices. Furthermore, a bilayer hydrogel film was fabricated by two-step spin-coating of P(AAc-co-NIPAm) and P(AAc-co-AAm) solutions, which showed reversible bending deformation under external stimuli. This simple yet effective approach should be applicable to other systems to prepare versatile hydrogel films with tunable thickness and promising applications in diverse areas.

Ultrasonic sonotrodes play an essential role in transmitting power ultrasound into the large-scale metallic casting. However, cavitation erosion considerably impairs the in-service performance of ultrasonic sonotrodes, leading to marginal microstructural refinement. In this work, the cavitation erosion behaviour of ultrasonic sonotrodes in large-scale castings was explored using the industry-level experiments of Al alloy cylindrical ingots (i.e. 630 mm in diameter and 6000 mm in length). When introducing power ultrasound, severe cavitation erosion was found to reproducibly occur at some specific positions on ultrasonic sonotrodes. However, there is no cavitation erosion present on the ultrasonic sonotrodes that were not driven by electric generator. Vibratory examination showed cavitation erosion depended on the vibration state of ultrasonic sonotrodes. Moreover, a finite element (FE) model was developed to simulate the evolution and distribution of acoustic pressure in 3-D solidification volume. FE simulation results confirmed that significant dynamic interaction between sonotrodes and melts only happened at some specific positions corresponding to severe cavitation erosion. This work will allow for developing more advanced ultrasonic sonotrodes with better cavitation erosion-resistance, in particular for large-scale castings, from the perspectives of ultrasonic physics and mechanical design.

Stretchable and biocompatible hydrogel-based strain sensors have considered as promising candidates for flexible wearable electronics in the light of their properties are similar to those of biological tissues. To date,...

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