Active Turbulence

Active fluids exhibit spontaneous flows with complex spatiotemporal structure, which have been observed in bacterial suspensions, sperm cells, cytoskeletal suspensions, self-propelled colloids, and cell tissues. Despite occurring in the absence of inertia, chaotic active flows are reminiscent of inertial turbulence, and hence they are known as active turbulence. Here, we survey the field, providing a unified perspective over different classes of active turbulence. To this end, we divide our review in sections for systems with either polar or nematic order, and with or without momentum conservation (wet or dry). Comparing to inertial turbulence, we highlight the emergence of power-law scaling with either universal or nonuniversal exponents. We also contrast scenarios for the transition from steady to chaotic flows, and we discuss the absence of energy cascades. We link this feature to both the existence of intrinsic length scales and the self-organized nature of energy injection in active turbulence, which are fundamental differences with inertial turbulence. We close by outlining the emerging picture, remaining challenges, and future directions. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Flow transitions and length scales of a channel-confined active nematic

Different flow regimes realised by a channel-confined active nematic have a characteristic length same as channel width. Flow structures exhibit the intrinsic length scale of the fluid only in the fully developed active turbulence regime.

Geometry and thickness dependant anomalous mechanical behavior of fabricated SU-8 thin film micro-cantilevers

SU-8 micro-cantilever arrays consisting of V- and M-shaped structures fabricated using a simplified single hard mask step. Bending tests were performed under similar peak loads (ranging 2–10 µN), with thickness ranging between micron (3.5 µm) and sub-micron (0.2 µm) scales. Various mechanical properties such as stiffness and hysteresis are determined from the load versus deflection curves. When the thickness of the V-shaped beam is decreased from 2 µm to 0.2 µm, the stiffness increases by a factor of 2.7, which is in contradiction with the classical beam theory according to which the stiffness for 0.2 µm beam should be three orders of magnitude less than that of 2 µm beam. Micropolar elasticity theory with a variable-intrinsic length scale (thickness dependant) is used to explain such an anomalous response. Experimentally obtained stiffness of two M-shaped beams of thickness 2 µm and 0.2 µm are almost identical. Reason behind this contradictory result is that the thicker beam has a residual strain with a large plastic deformation which usually increases the cross-linking network density, leading to increase in elastic modulus, hardness and thus stiffness of polymers. But the thinner beam has undergone an elastic deformation. The size effect of V- and M-shaped cantilever beams is discussed.

Temperature, Silicon Thickness and Intrinsic Length Influence on the Operation of Lateral SOI PIN Photodiodes

This work presents an analysis of the influence of the intrinsic length region (Li) and the thickness of the silicon film (tSi) on the performance of lateral thin-film SOI PIN photodiodes when illuminated by low wavelengths, in the blue and ultraviolet (UV) range. The experimental measurements performed with the wavelengths of 396 nm, 413 nm, and 460 nm in a temperature range of 100 K to 400 K showed that the optical responsivity of the SOI PIN photodetectors has larger dependence on the incident wavelength than on temperatures variation. Two-dimensional numerical simulations showed the same trends as the experimental results as a function of temperature and as a function of wavelength. Numerical simulations were used to investigate the responsivity and total quantum efficiency of PIN SOI photodetectors with intrinsic length region ranging from 5 µm to 30 µm and silicon film thickness ranging between 40 nm to 500 nm. From the results can be concluded that by properly choosing Li and tSi it is possible to optimize PIN SOI photodiodes performance for detecting specific wavelengths.

Three-dimensional numerical simulation of soft-tissue wound healing using constrained-mixture anisotropic hyperelasticity and gradient-enhanced damage mechanics

Healing of soft biological tissues is the process of self-recovery or self-repair after injury or damage to the extracellular matrix (ECM). In this work, we assume that healing is a stress-driven process, which works at recovering a homeostatic stress metric in the tissue by replacing the damaged ECM with a new undamaged one. For that, a gradient-enhanced continuum healing model is developed for three-dimensional anisotropic tissues using the modified anisotropic Holzapfel–Gasser–Ogden constitutive model. An adaptive stress-driven approach is proposed for the deposition of new collagen fibres during healing with orientations assigned depending on the principal stress direction. The intrinsic length scales of soft tissues are considered through the gradient-enhanced term, and growth and remodelling are simulated by a constrained-mixture model with temporal homogenization. The proposed model is implemented in the finite-element package Abaqus by means of a user subroutine UEL. Three numerical examples have been achieved to illustrate the performance of the proposed model in simulating the healing process with various damage situations, converging towards stress homeostasis. The orientations of newly deposited collagen fibres and the sensitivity to intrinsic length scales are studied through these examples, showing that both have a significant impact on temporal evolutions of the stress distribution and on the size of the damage region. Applications of the approach to carry out in silico experiments of wound healing are promising and show good agreement with existing experiment results.

Simulação e caracterização elétrica de dispositivos fotossensores implementados em tecnologia SOI

This work presents an analysis of the main performance characteristics of lateral PIN photodiodes implemented in thin layer SOI technology, when illuminated by wavelengths, in the range between blue and ultraviolet (UV), and subjected to temperature variations. Twodimensional numerical simulations were performed to analyze characteristics such as photocurrent, absorption, quantum efficiency, and responsivity. In this analysis, the influence of the variation between 40 nm and 500 nm of the silicon film thickness (tSi) and the intrinsic length region (Li) between 5 and 30 ?m was considered to evaluate the performance of the photodiode at different wavelengths, in the range blue and ultraviolet (UV). Different sets of physical models were studied in the simulations, to reproduce trends reported in the literature. Through experimental measurements of the intensities of incident powers as a function of distance, light sources were characterized using light-emitting diodes at wavelengths, UV (390 nm), violet (410 nm), and Blue (460 nm), adapted for providing light energy in the photosensitive region of experimental photodiodes also characterized for temperatures between 100 K and 400 K. The simulations show that there is a dependency relationship between the silicon film thickness and the intrinsic length region (Li), that when evaluated and scaled simultaneously it is possible to optimize the quantum efficiency and responsivity of the PIN SOI photodiodes in the definition technology for specific wavelength applications. The results show that the quantum efficiency around 28 % and responsiveness around 85 mA / W for a given technology showed the same trend as the experimental results, taking into account the wavelength and temperature range. The results also show an almost linear trend in the relationship between silicon film thickness (tSi) and absorption (light penetration depth), so that, in thinner silicon film thickness, the device will be more selective for low wavelengths, that is, closer to UV

Thermal and Quantum Fluctuation Effects in Quasiperiodic Systems in External Potentials

We analyze the many-body phases of an ensemble of particles interacting via a Lifshitz–Petrich–Gaussian pair potential in a harmonic confinement. We focus on specific parameter regimes where we expect decagonal quasiperiodic cluster arrangements. Performing classical Monte Carlo as well as path integral quantum Monte Carlo methods, we numerically simulate systems of a few thousand particles including thermal and quantum fluctuations. Our findings indicate that the competition between the intrinsic length scale of the harmonic oscillator and the wavelengths associated to the minima of the pair potential generically lead to a destruction of the quasicrystalline pattern. Extensions of this work are also discussed.

Studies on thermo-electro-mechanical coupling bandgaps of a piezoelectric phononic crystal nanoplate with surface effects

Applying surface piezoelectricity theory and plane wave expansion (PWE) method to the model of Kirchhoff plate, the calculation method of band structure of a piezoelectric phononic crystal (PC) nanoplate with surface effects is proposed and formalized. In order to investigate the bandgap properties of first order in the nanoplate in detail, the corresponding influence rules of thermo-electro-mechanical coupling fields, surface effects and geometric parameters on bandgaps are studied. During the researches, temperature variation, electrical voltage and external axial force are picked as the influencing parameters corresponding to thermo-electro-mechanical coupling fields. Residual surface stress and material intrinsic length are chosen as the influencing parameter related to surface effects. Lattice constant, radius of PZT-4 hole and thickness of nanoplate are picked as the influencing parameters of geometric parameters. All the results are expected to be helpful for the design of micro and nanodevices based on piezoelectric periodic nanoplates.

Programmable and robust static topological solitons in mechanical metamaterials

Solitary, persistent wave packets called solitons hold potential to transfer information and energy across a wide range of spatial and temporal scales in physical, chemical, and biological systems. Mechanical solitons characteristically emerge either as a single wave packet or uncorrelated propagating topological entities through space and/or time, but these are notoriously difficult to control. Here, we report a theoretical framework for programming static periodic topological solitons into a metamaterial, and demonstrate its implementation in real metamaterials computationally and experimentally. The solitons are excited by deformation localizations under quasi-static compression, and arise from buckling-induced kink-antikink bands that provide domain separation barriers. The soliton number and wavelength demonstrate a previously unreported size-dependence, due to intrinsic length scales. We identify that these unanticipated solitons stem from displacive phase transitions with periodic topological excitations captured by the well-known $${\varphi }^{4}$$φ4 theory. Results reveal pathways for robust regularizations of stochastic responses of metamaterials.