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1. Adhesion of flat-ended pillars with non-circular contacts

   https://doi.org/10.1039/D0SM01105C  

   Luo, Aoyi ; Mohammadi Nasab, Amir ; Tatari, Milad ; Chen, Shuai ; Shan, Wanliang ; Turner, Kevin T. ( October 2020 , Soft Matter)

   null (Ed.)

   Fibrillar adhesives composed of fibers with non-circular cross-sections and contacts, including squares and rectangles, offer advantages that include a larger real contact area when arranged in arrays and simplicity in fabrication. However, they typically have a lower adhesion strength compared to circular pillars due to a stress concentration at the corner of the non-circular contact. We investigate the adhesion of composite pillars with circular, square and rectangular cross-sections each consisting of a stiff pillar terminated by a thin compliant layer at the tip. Finite element mechanics modeling is used to assess differences in the stress distribution at the interface for the different geometries and the adhesion strength of different shape pillars is measured in experiments. The composite fibrillar structure results in a favorable stress distribution on the adhered interface that shifts the crack initiation site away from the edge for all of the cross-sectional contact shapes studied. The highest adhesion strength achieved among the square and rectangular composite pillars with various tip layer thicknesses is approximately 65 kPa. This is comparable to the highest strength measured for circular composite pillars and is about 6.5× higher than the adhesion strength of a homogenous square or rectangular pillar. The results suggest that a composite fibrillar adhesive structure with a local stress concentration at a corner can achieve comparable adhesion strength to a fibrillar structure without such local stress concentrations if the magnitude of the corner stress concentrations are sufficiently small such that failure does not initiate near the corners, and the magnitude of the peak interface stress away from the edge and the tip layer thickness are comparable. 

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2. Mutually Reinforced Polymer–Graphene Bilayer Membranes for Energy‐Efficient Acoustic Transduction

   https://doi.org/10.1002/adma.202004053  

   Khan, Assad U. ; Zeltzer, Gabriel ; Speyer, Gavriel ; Croft, Zacary L. ; Guo, Yichen ; Nagar, Yehiel ; Artel, Vlada ; Levi, Adi ; Stern, Chen ; Naveh, Doron ; et al ( November 2020 , Advanced Materials)

   Graphene holds promise for thin, ultralightweight, and high‐performance nanoelectromechanical transducers. However, graphene‐only devices are limited in size due to fatigue and fracture of suspended graphene membranes. Here, a lightweight, flexible, transparent, and conductive bilayer composite of polyetherimide and single‐layer graphene is prepared and suspended on the centimeter scale with an unprecedentedly high aspect ratio of 10⁵. The coupling of the two components leads to mutual reinforcement and creates an ultrastrong membrane that supports 30 000 times its own weight. Upon electromechanical actuation, the membrane pushes a massive amount of air and generates high‐quality acoustic sound. The energy efficiency is≈10–100 times better than state‐of‐the‐art electrodynamic speakers. The bilayer membrane's combined properties of electrical conductivity, mechanical strength, optical transparency, thermal stability, and chemical resistance will promote applications in electronics, mechanics, and optics.

    

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3. Investigation of the rate-mediated form-function relationship in biological puncture

   https://doi.org/10.1038/s41598-023-39092-8  

   Zhang, Bingyang ; Anderson, Philip S. ( December 2023 , Scientific Reports)

   Puncture is a vital mechanism for survival in a wide range of organisms across phyla, serving biological functions such as prey capture, defense, and reproduction. Understanding how the shape of the puncture tool affects its functional performance is crucial to uncovering the mechanics underlying the diversity and evolution of puncture-based systems. However, such form-function relationships are often complicated by the dynamic nature of living systems. Puncture systems in particular operate over a wide range of speeds to penetrate biological tissues. Current studies on puncture biomechanics lack systematic characterization of the complex, rate-mediated, interaction between tool and material across this dynamic range. To fill this knowledge gap, we establish a highly controlled experimental framework for dynamic puncture to investigate the relationship between the puncture performance (characterized by the depth of puncture) and the tool sharpness (characterized by the cusp angle) across a wide range of bio-relevant puncture speeds (from quasi-static to$$\sim$$$\sim$50 m/s). Our results show that the sensitivity of puncture performance to variations in tool sharpness reduces at higher puncture speeds. This trend is likely due to rate-based viscoelastic and inertial effects arising from how materials respond to dynamic loads. The rate-dependent form-function relationship has important biological implications: While passive/low-speed puncture organisms likely rely heavily on sharp puncture tools to successfully penetrate and maintain functionalities, higher-speed puncture systems may allow for greater variability in puncture tool shape due to the relatively geometric-insensitive puncture performance, allowing for higher adaptability during the evolutionary process to other mechanical factors.

    

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4. Forces to pierce cuticle of tarsi and material properties determined by nanoindentation: The Achilles’ heel of bed bugs

   https://doi.org/10.1242/bio.028381  

   Bustamante, Jr, Jorge ; Panzarino, Jason F. ; Rupert, Timothy J. ; Loudon, Catherine ( January 2017 , Biology Open)

   The mechanical properties of bed bug (Cimex lectularius L.) tarsi and pretarsi were investigated in order to evaluate their vulnerability to piercing by plant trichomes (sharp microscopic hairs). Nanoindentation was used to measure the force required to insert a sharp probe into the cuticle of these different regions, as well as to determine creep and reduced elastic moduli for the cuticle. Scanning electron microscopy was used to visualize the indents that had been generated by nanoindentation. The force required to insert a cube corner nanoindenter probe into the cuticle was determined for a range of displacements (1 to 9 µm) and strain rates (0.003 to 0.5 s−1). Greater force was required to insert this sharp probe at greater depth or at faster strain rates. A specific region of the pretarsus (membrane with microtrichia), more frequently pierced by trichomes during bed bug locomotion, required approximately 20-30% less force, exhibited more creep, and had a lower reduced elastic modulus for the first micron of indentation, compared to the other regions, although this pattern was not consistent for greater displacements. These mechanical attributes, which will facilitate the initial stage of puncture, in addition to the presence of natural infoldings in the cuticle of this area, may make that area of the pretarsus particularly vulnerable to piercing. This information will help inform development of physical methods for control of insect pests such as bed bugs.

    

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5. Porous hydrophobic-hydrophilic composite membranes for direct contact membrane distillation

   https://doi.org/10.1016/j.memsci.2019.117225  

   Puranik, A.A. ; Rodrigues, L.N. ; Chau, J. ; Li, L. ; Sirkar, K.K. ( July 2019 , Journal of membrane science)

   Direct contact membrane distillation (DCMD) for desalination is attractive for high salt concentrations if low cost steam/waste heat is available and waste brine disposal cost for inland desalination is factored in. A number of innovations have taken place in DCMD in terms of the structure of the porous hydrophobic membrane. Composite membranes are of increasing interest. Composite membrane structures of great interest include a thin hydrophobic porous layer over a porous hydrophilic layer of polyvinylidene fluoride (PVDF) or a thin porous hydrophobic layer over a more conventional hydrophobic porous membrane. These membranes can be in the form of an integral composite or a stacked composite or a laminated composite. A facile method of fabricating such integral composite membranes is plasma polymerization under vacuum. A class of such membranes yielding quite high water vapor fluxes have been characterized using a variety of characterization techniques: Contact angle, liquid entry pressure (LEP), bubble-point pressure, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM). Stacked composites of a hydrophobic ePTFE membrane over a hydrophilic PVDF membrane or a hydrophobic PVDF membrane over another hydrophobic PVDF membrane were also studied. Novel conditions created lead to very high water vapor fluxes compared to those from conventional hydrophobic membranes supported on a mesh support. 

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