31 research outputs found
Designing nonlinear thermal devices and metamaterials under the Fourier's law: A route to nonlinear thermotics
Nonlinear heat transfer can be exploited to reveal novel transport phenomena
and thus enhance peo-ple's ability to manipulate heat flux at will. However,
there hasn't been a mature discipline called nonlinear thermotics like its
counterpart in optics or acoustics to make a systematic summary of rele-vant
researches. In the current review, we focus on recent progress in an important
part of nonlinear heat transfer, i.e., tailoring nonlinear thermal devices and
metamaterials under the Fourier's law, especially with temperature-dependent
thermal conductivities. We will present the basic designing techniques
including solving the equation directly and the transformation theory. Tuning
nonlinearity coming from multi-physical effects, and how to calculate effective
properties of nonlinear conductive composites using the effective medium theory
are also included. Based on these theories, researchers have successfully
designed various functional materials and devices such as the thermal diodes,
thermal transistors, thermal memory elements, energy-free thermostats, and
intelligent thermal materials, and some of them have also been realized in
experiments. Further, these phenomenological works can provide a feasible route
for the development of nonlinear thermotics
Qi and Garden Wall
By using Feng Shui’s principles of Qi in relation to specific residential properties – site organization, surrounding environment and existing structures – this thesis will demonstrate new spatial, formal, and material potentials of the garden wall, as the medium for occupation. The prototypes that achieving therapeutic qualities of Qi and phenomenological effects of the garden wall will be developed for diverse residential landscapes
PENTALENE-EMBEDDED ACENES AND THIENOACENES: SYNTHESIS, PROPERTIES AND APPLICATIONS
Ph.DDOCTOR OF PHILOSOPH
Convective Heat Transfer in Porous Materials
Thermal convection stands out as an exceptionally efficient thermal transport
mechanism, distinctly separate from conduction and radiation. Yet, the
inherently elusive nature of fluid motion poses challenges in accurately
controlling convective heat flow. While recent innovations have harnessed
thermal convection to achieve effective thermal conductivity, fusing thermal
convection in liquids and thermal conduction in solids together to form hybrid
thermal metamaterials is still challenging. In this review, we introduce the
latest progress in convective heat transfer. Leveraging the right porous
materials as a medium allows for a harmonious balance and synergy between
convection and conduction, establishing stable heat and fluid flows. This paves
the way for the innovative advancements in transformation thermotics. These
findings demonstrate the remarkable tunability of convective heat transport in
complex multicomponent thermal metamaterials
Diffusive Pseudo-Conformal Mapping: Anisotropy-Free Transformation Thermal Media with Perfect Interface Matching
Transformation media provide a fundamental paradigm for field regulation, but
their tricky anisotropy challenges fabrication. Though optical conformal
mapping has been utilized to eliminate anisotropy, two key factors still hinder
its development in thermotics, i.e., the distinct diffusion nature and
inevitable interface mismatching. Here, we put forth the concept of diffusive
pseudo-conformal mapping, overcoming the inherent difference between diffusion
and waves and achieving perfect interface matching. The proposed mapping
directly leads to heat guiding and expanding functions with anisotropy-free
transformation thermal media, whose feasibility is confirmed by experiments or
simulations. Besides diverse applications, we provide a unified perspective for
two distinct types of prevailing bilayer cloaks by uncovering their profound
ties with pseudo-conformal mapping. These results greatly simplify the
preparation of transformation thermotics and have implications for regulating
other diffusion and wave phenomena
On Transformation Form-Invariance in Thermal Convection
Over the past two decades, effective control of physical fields, such as light fields or acoustics fields, has greatly benefited from transforming media. One of these rapidly growing research areas is transformation thermotics, especially embodied in the thermal conductive and radiative modes. On the other hand, transformation media in thermal convection has seldom been studied due to the complicated governing equations involving both fluid motion and heat transfer terms. The difficulty lies in the robustness of form invariance in the Navier–Stokes equations or their simplified forms under coordinate transformations, which determines whether the transformation operations can be executed on thermal convection to simultaneously regulate the flow and thermal fields. In this work, we show that thermal convection in two-dimensional Hele–Shaw cells keeps form-invariance, while its counterpart in general creeping flows or general laminar flows does not. This conclusion is numerically verified by checking the performances of invisible devices made of transformation media in convective environments. We further exploit multilayered structures constituted of isotropic homogeneous natural materials to realize the anisotropic inhomogeneous properties required for transformation media. Our results clarify the long-term confusion about the validation of the transformation method in thermal convection and provide a rigorous foundation and classical paradigm on inspiring various fascinating metadevices in both thermal and flow fields
Paper-Based Electrodeposition Chip for 3D Alginate Hydrogel Formation
Hydrogel has been regarded as one significant biomaterial in biomedical and tissue engineering due to its high biocompatibility. This paper proposes a novel method to pattern calcium alginate hydrogel in a 3D way via electrodeposition process based on a piece of paper. Firstly, one insulating paper with patterned holes is placed on one indium tin oxide (ITO) glass surface, which is put below another ITO glass. Then, 1% sodium alginate solution with 0.25% CaCO3 nano particles is filled between these two glasses. In the bottom glass, patterns of electrodes followed patterns of holes on the insulating layer. Hydrogel forms on patterned electrodes when electrochemical potential is applied due to electrodeposition. The experiments demonstrate that the pattern of alginate hydrogels follows the pattern of electrodes exactly. In addition, the hydrogel’s height is controllable by applied potential and reaction time. An equivalent circuit model and a hydrogel growth model have been built to predict the electrodeposition current and hydrogel’s growth. This method for gel formation is easy and cheap since the main material is one piece of insulated paper, which provides an easy and controllable method for 3D hydrogel patterning
Multi-Image Encryption Algorithm Based on Cascaded Modulation Chaotic System and Block-Scrambling-Diffusion
To address the problem of a poor security image encryption algorithm based on a single chaotic map, this paper proposes a cascade modulation chaotic system (CMCS) that can generate multiple chaotic maps. On this basis, a multi-image encryption algorithm with block-scrambling-diffusion is proposed using CMCS. The algorithm makes full use of the features of CMCS to achieve the effect of one encryption at a time for images. Firstly, the key-value associated with the plaintexts is generated using a secure hash algorithm-512 (SHA-512) operation and random sequence, and the three images are fully confused by the double scrambling mechanism. Secondly, the scrambled image is converted into a bit-level matrix, and the pixel values are evenly distributed using the bit-group diffusion. Finally, the non-sequence diffusion of hexadecimal addition and subtraction rules is used to improve the security of the encryption algorithm. Experimental results demonstrate that the encryption algorithm proposed in this paper has a good encryption effect and can resist various attacks
Nonlinear thermotics: nonlinearity enhancement and harmonic generation in thermal metasurfaces
We propose and investigate a class of structural surfaces (metasurfaces). We develop the perturbation theory and the effective medium theory to study the thermal properties of the metasurface. We report that the coefficient of temperature-dependent (nonlinear) item in thermal conductivity can be enhanced under certain conditions. Furthermore, the existence of nonlinear item helps to generate high-order harmonic frequencies of heat flux in the presence of a heat source with periodic temperature. This work paves a different way to control and manipulate the transfer of heat, and it also makes it possible to develop nonlinear thermotics in the light of nonlinear optics