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Most granular packings possess shear moduli (G) that increase with the applied external pressure, and bulk moduli (B) that increase or remain constant with pressure. This paper presents “tessellated” granular metamaterials for which both Gand the ratio G/Bdecrease with increasing pressure. The granular metamaterials are made from flexible tessellations forming a ring of closed cells, each containing a small number of solid particles. For under-constrained tessellations, the dominant contributions to Gand Bare the particle–particle and particle–cell interactions. With specific particle configurations in the cells, we limit the number of possible particle rearrangements to achieve decreasing Gas we increase the pressure difference between the inside and outside of the tessellation, leading to G/B≪1at large pressures. We further study tessellated granular metamaterials with cells containing a single particle and many particles to determine the variables that control the mechanical response of particle-filled tessellations as a function of pressure.

We studied the evolution of capillary bridges between nominally flat plates undergoing multiple cycles of compression and stretching in experiments and simulations. We varied the distance between the plates in small increments to study the full evolution of the bridge shape. Experiments show that contact angle hysteresis determines the shape of the bridge. In sliding drops, hysteresis can be modeled using a contact angle-dependent resistive force F̃Rapplied at the contact line. We developed a model that accurately captures the evolution of the bridge shape by combining F̃Rand constrained energy minimization. Unlike previous work, this allows for both complete and partial contact line pinning. We also explored the effect of using nonparallel plates. The asymmetry in the bridge shape causes the movement of the center of mass of the bridge and can be explained by contact angle hysteresis. We find that even a slight misalignment between the flat plates can have a measurable effect.

The spongy mesophyll is a complex, porous tissue found in plant leaves that enables carbon capture and provides mechanical stability. Unlike many other biological tissues, which remain confluent throughout development, the spongy mesophyll must develop from an initially confluent tissue into a tortuous network of cells with a large proportion of intercellular airspace. How the airspace in the spongy mesophyll develops while the tissue remains mechanically stable is unknown. Here, we use computer simulations of deformable polygons to develop a purely mechanical model for the development of the spongy mesophyll tissue. By stipulating that cell wall growth and remodelling occurs only near void space, our computational model is able to recapitulate spongy mesophyll development observed in Arabidopsis thalianaleaves. We find that robust generation of pore space in the spongy mesophyll requires a balance of cell growth, adhesion, stiffness and tissue pressure to ensure cell networks become porous yet maintain mechanical stability. The success of this mechanical model of morphogenesis suggests that simple physical principles can coordinate and drive the development of complex plant tissues like the spongy mesophyll.

The magnetization in hyperpolarized gas (HP) MRI is generated by laser polarization that is independent of the magnet and imaging process. As a consequence, there is no equilibrium magnetization during the image acquisition. The competing processes of gas inflow and depolarization of the spins lead to large changes in signal as one samples k‐space. A model is developed of dynamic changes in polarization of hyperpolarized 3He during infusion and in vivoimaging of the lung and verified experimentally in a live guinea pig. Projection encoding is used to measure the view‐to‐view variation with temporal resolution <4 ms. Large excitation angles effectively sample the magnetization in the early stages of inflow, highlighting larger airways, while smaller excitation angles produce images of the more distal spaces. The work provides a basis for pulse sequences designed to effectively exploit HP MRI in the lung.

Magnetic resonance (MR) microscopy was used to noninvasively investigate the development of live rat embryos in utero.The difficulty in making sequential observations of a developing mammalian embryo has frustrated developmental biologists for many years. Most current technologies analyze normal and abnormal development by observing end point phenotypes (in fixed specimens) rather than investigating the live embryo. MR microscopy was adapted to allow rat litters to be scanned three times each (at 1‐ to 3‐day intervals) and has produced images of live developing embryos. It was demonstrated that repeated anesthesia and imaging protocols produced no gross malformations in the rat pups that were subsequently delivered and observed. Three‐dimensional projection encoding with phase re‐winders produced isotropic (2563) image data sets in about 30 minutes with excellent tissue contrast arising from steady‐state effects in the amniotic fluid.

Radial acquisition (RA) techniques have been extended to produce isotropic, three‐dimensional images of lung in live laboratory animals at spatial resolution down to 0.013 mm3with a signal‐to‐noise ratio of 30:1. The pulse sequence and reconstruction algorithm have been adapted to allow acquisition of image matrices of up to 2563in less than 15 min. Scan‐synchronous ventilation has been incorporated to limit breathing motion artifacts. The imaging sequence permits randomizing and/or discarding selected views to minimize the consequences of breathing motion. The signal in lung parenchyma was measured as a function of flip angle (α) for different repetition times and found to follow the predictions for which there is an optimum excitation (Ernst) angle. A single T1, relaxation value of 780 ± 54 ms fits all data from six guinea pigs at 2.0 T. This T1, value parameterizes the signal and allows for a priorioptimization, such as calculation of the Ernst angle appropriate for lung imaging.