Ferrofluid is a type of smart material consisting of stable colloidal suspensions of magnetic nanoscale particles dispersed in a nonmagnetic carrier liquid. It is known that ferromagnetic ...nanoparticles may improve the magnetic and dielectric properties of host materials, which exhibit novel functionality to provide better microwave absorption properties. A ferrofluid layer subjected to an external magnetic field resulting in cone-shaped structures may further enhance the reflection loss of the incident electromagnetic wave. This study investigates the effect of the ferrofluid crests on the enhancement of the electromagnetic wave shielding effectiveness in the range of 12-18 GHz. It is noted that such a ferrofluid has significant shield effectiveness at the frequency of 16 GHz. When the magnetic field is applied to create the ferrofluid crests, the value of shielding effectiveness initially increases, then decreases as the field strength is further increased. The variation of the EMI shield effectiveness is dominated by the arrangement of the ferrofluid crests array, which is closely related to the thickness, composition of the ferrofluid layer, and the field strength uniformity.
We experimentally investigate the motion of a ferrodrop array in a rotating magnetic field. Magnetized and driven by the external field, the ferrodrops are stretched and self-aligned to form a drop ...array along the field orientation. An interesting planet-like dual rotation, including local self-spins of individual drops and a global revolution of the drop array, is newly identified. While the drops spin nearly synchronized with the external field, the revolution always lags behind the field and appears a forth and back movement. Prominence of the net revolutionary movement depends on the strength and uniformity of the overall field as well as the number of drops containing in the array. In general, more uniform and stronger rotating field leads to a more prominent global revolution. Phenomenon of such planetary motion can be applied to mix two fluids more effectively than self-spin drops.
Superparamagneticmicro-bead chains and microswimmers under the influence of an oscillating magnetic field are studiedexperimentally and numerically. The numerical scheme composed of the lattice ...Boltzmann method,immersed boundary method, and discrete particle method based on the simplified Stokesiandynamics isapplied to thoroughly understand the interaction between the micro-bead chain (orswimmer), the oscillating magneticfield, and the hydrodynamics drag. The systematic experiments andsimulations demonstrated the behaviors of the microchains and microswimmers as well as thepropulsive efficiencies of the swimmers. The effects of key parameters, such as fieldstrengths, frequency, and the lengths of swimmer, are thoroughly analyzed. The numericalresults are compared with the experiments and show good qualitative agreements. Ourresults proposed an efficient method to predict the motions of the reversible magneticmicrodevices which may have extremely valuable applications in biotechnology.
博士
國立交通大學
機械工程系所
102
We investigate experimentally the dynamics of microchains containing superparamagnetic particles and the further application in an oscillating field. The chains are first formed ...by a static directional field, and then manipulated by an additional dynamical perpendicular field. The present methodology represents a simple reversible chaining process, whose particles can be re-dispersed after removal of the field. The motion of superparamagnetic chains is dominated by magnetic torque and induced hydrodynamic drag. The effects of key parameters, such as field strengths and the lengths of particle chains, are thoroughly analyzed. Distinct behaviors, from rigid body oscillations and bending distortions to rupture failures, are observed by increasing the amplitudes of oscillating fields or chains’ lengths. Because of lower induced drag, a shorter chain follows the field trajectory closely and oscillates more synchronically with the external field. On the other hand, the influences of field strengths are not consistent. Even the overall oscillating phase trajectory in a stronger external field deviates less significantly from the corresponding field trajectory, a stronger dynamical component of the external field results in larger phase angle lags at certain points. The experimental results confirm the criterion of ruptures can be effectively determined by the value of (N*Mn1/2), where Mn is the Mason number defined as the ratio of induced drag to dipolar attraction, and N represents the number of particles contained in a chain.
In addition, we report an interesting phenomenon of “trajectory shift” of magnetic chains in an oscillating field as well. When the phase angle lags of chains to the external field exceeding 90 degrees, the phenomenon shifts the chain’s oscillating trajectory along a new axis, which is perpendicular to its original axis. Applicability of the phenomenon to a stable chain in various conditions is experimented systematically. The trajectory shift provides an effective manipulating mechanism in a MEMS system, such as steering of micro-swimmers. By utilizing the mechanism of the stable oscillating chain and trajectory shift, we successfully develop a micro-swimmer by connecting particles of different sizes to break the symmetry of the chains to generate thrust to move forward and reverse. Further research is required to refine the manipulating methodology and expand the applicability of the micro-swimmers.
碩士
國防大學中正理工學院
兵器系統工程研究所
92
The objective of the present study is to develop a numerical model that will enable us to investigate non-Newtonian fluids flow in microchannels with electrokinetic ...effects. The governing equations are derived from three parts : the first part is solving a nonlinear Poisson equation governing the electric potential which results from potential of solid liquid boundary and the applied electrical potential ; the second part is solving the Nernst-Planck equation governing the ionic concentration distribution ; the third part is solving the Navier-Stokes equations with electrokinetic effects. The third part can be derived from complete Navier-Stokes equations with theories of Power-Law fluids, and extend the applications to the non-Newtonian fluids. So we can solve all the governing equations of the system by the primitive variables. We can change the value of the Power Law index n, and discuss the effects of the shear thinning and shear thickening property on the fluids. When the mathe
Objective
Tuberous sclerosis complex (TSC) is a genetic neurocutaneous syndrome with variable and unpredictable neurological comorbidity that includes epilepsy, intellectual disability (ID), autism ...spectrum disorder, and neurobehavioral abnormalities. The degree of white matter involvement is believed to be associated with the severity of neurological impairment. The goal of the present study was to evaluate diffusion characteristics of tubers, white matter lesions, and brain structural network alterations in TSC patients using diffusion tensor imaging (DTI), graph theoretical analysis (GTA), and network-based statistical (NBS) analysis.
Materials and methods
Forty-two patients with a definitive diagnosis of TSC were recruited for this study. All patients underwent brain DTI examination using a 3 T magnetic resonance imaging system. Mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD) values, and fractional anisotropy (FA) mapping in 52 tubers and white matter lesions were measured and compared with those of contralateral normal regions. GTA was performed on the inter-regional connectivity matrix, and NBS analysis was used to identify the significance of any connected subnetworks evident in the set of altered connections. For neurological severity subgrouping, a neurological severity score was assigned to TSC patients including those with ID, seizure, autism, and other neuropsychiatric disorders (NPDs).
Results
Significantly higher MD, AD, and RD, and lower FA values, were found in TSC lesions compared with those measured in contralateral normal regions for tubers (
P
< 0.05). GTA and NBS analysis provided better local segregation but worse global integration of the structural network (regular-like network) in TSC patients with ID, seizure, and higher Neurological Severity Score. Disrupted subnetworks in TSC patients with severe status included connections from the frontal lobe to the parietal lobe, temporal lobe to the caudate, and temporal lobe to the insula.
Discussion
DTI has the potential to provide valuable information about cytoarchitectural changes in TSC lesions beyond morphological MRI findings alone. Using GTA and NBS, current results provide the information of disrupted white matter connectivity and organization in TSC patients with different neuropsychological impairments.
The research reported in this article clarifies how employee-organization relationships (EORs) work. Specifically, the authors tested whether social exchange and job embeddedness mediate how ...mutual-investment (whereby employers offer high inducements to employees for their high contributions) and over-investment (high inducements without corresponding high expected contributions) EOR approaches, which are based on
Tsui, Pearce, Porter, and Tripoli's (1997)
framework, affect quit propensity and organizational commitment. Two studies evaluated these intervening mechanisms. Study 1 surveyed 953 Chinese managers attending part-time master of business administration (MBA) programs in China, whereas Study 2 collected cross-sectional and longitudinal data from 526 Chinese middle managers in 41 firms. Standard and multilevel causal modeling techniques affirmed that social exchange and job embeddedness translate EOR influence. A second multilevel test using lagged outcome measures further established that job embeddedness mediates long-term EOR effects over 18 months. These findings corroborate prevailing views that social exchange explains how mutual- and over-investment EORs motivate greater workforce commitment and loyalty. This study enriches EOR perspectives by identifying job embeddedness as another mediator that is more enduring than social exchange.
