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Abstract Coherent structures are critical for controlling turbulent boundary layers due to their roles in momentum and heat transfer in the flow. Turbulent coherent structures can be detected by measuring wall shear stresses that are footprints of coherent structures. In this study, wall shear stress fluctuations were measured simultaneously in a zero pressure gradient turbulent boundary layer using two house-made wall shear stress probes aligned in the spanwise direction. The wall shear stress probe consisted of two hot-wires on the wall aligned in a V-shaped configuration for measuring streamwise and spanwise shear stresses, and their performance was validated in comparison with a direct numerical simulation result. Relationships between measured wall shear stress fluctuations and streamwise velocity fluctuations were analyzed using conditional sampling techniques. The peak detection method and the variable-interval time-averaging (VITA) method showed that quasi-streamwise vortices were inclined toward the streamwise direction. When events were simultaneously detected by the two probes, stronger fluctuations in streamwise velocity were detected, which suggests that stronger coherent structures were detected. In contrast to the former two methods, the hibernating event detection method detects events with lower wall shear stress fluctuations. The ensemble-averaged mean velocity profile of hibernating events was shifted upward compared to the law of the wall, which suggests low drag status of the coherent structures related with hibernating events. These methods suggest significant correlations between wall shear stress fluctuations and coherent structures, which could motivate flow control strategies to fully exploit these correlations. 

Lightweight and elastically deformable soft materials that are thermally conductive are critical for emerging applications in wearable computing, soft robotics, and thermoregulatory garments. To overcome the fundamental heat transport limitations in soft materials, room temperature liquid metal (LM) has been dispersed in elastomer that results in soft and deformable materials with unprecedented thermal conductivity. However, the high density of LMs (>6 g cm⁻³) and the typically high loading (⩾85 wt%) required to achieve the desired properties contribute to the high density of these elastomer composites, which can be problematic for large‐area, weight‐sensitive applications. Here, the relationship between the properties of the LM filler and elastomer composite is systematically studied. Experiments reveal that a multiphase LM inclusion with a low‐density phase can achieve independent control of the density and thermal conductivity of the elastomer composite. Quantitative design maps of composite density and thermal conductivity are constructed to rationally guide the selection of filler properties and material composition. This new multiphase material architecture provides a method to fine‐tune material composition to independently control material and functional properties of soft materials for large‐area and weight‐sensitive applications.

 

Human bone demonstrates superior mechanical properties due to its sophisticated hierarchical architecture spanning from the nano/microscopic level to the macroscopic. Bone grafts are in high demand due to the rising number of surgeries because of increasing incidence of orthopedic disorders, non‐union fractures, and injuries in the geriatric population. The bone scaffolds need to provide porous matrix with interconnected porosity for tissue growth as well as sufficient strength to withstand physiological loads, and be compatible with physiological remodeling by osteoclasts/osteoblasts. The‐state‐of‐art additive manufacturing (AM) technologies for bone tissue engineering enable the manipulation of gross geometries, for example, they rely on the gaps between printed materials to create interconnected pores in 3D scaffolds. Herein, the authors firstly print hierarchical and porous hydroxyapatite (HAP) structures with interconnected pores to mimic human bones from microscopic (below 10 µm) to macroscopic (submillimeter to millimeter level) by combining freeze casting and extrusion‐based 3D printing. The compression test of 3D printed scaffold demonstrates superior compressive stress (22 MPa) and strain (4.4%). The human mesenchymal stromal cells (MSCs) tests demonstrate the biocompatibility of printed scaffold.