Thermal transport at a nanoparticle-water interface: A molecular dynamics and continuum modeling study

Heat transfer between a silver nanoparticle and surrounding water has been studied using molecular dynamics (MD) simulations. The thermal conductance (Kapitza conductance) at the interface between a nanoparticle and surrounding water has been calculated using four different approaches: transient with/without temperature gradient (internal thermal resistance) in the nanoparticle, steady-state non-equilibrium and finally equilibrium simulations. The results of steady-state non-equilibrium and equilibrium are in agreement but differ from the transient approach results. MD simulations results also reveal that in the quenching process of a hot silver nanoparticle, heat dissipates into the solvent over a length-scale of ~ 2nm and over a timescale of less than 5ps. By introducing a continuum solid-like model and considering a heat conduction mechanism in water, it is observed that the results of the temperature distribution for water shells around the nanoparticle agree well with MD results. It is also found that the local water thermal conductivity around the nanoparticle is greater by about 50 percent than that of bulk water. These results have important implications for understanding heat transfer mechanisms in nanofluids systems and also for cancer photothermal therapy, wherein an accurate local description of heat transfer in an aqueous environment is crucial.Comment: 22 pages, 7 figures

Lightweight Vehicle Structures that Absorb and Direct Destructive Energy Away from the Occupants

One of the main thrusts in current automotive industry is the development of occupant-centric vehicle structures that make the vehicle safe for the occupants. A design philosophy that improves vehicle survivability by absorbing and redirecting destructive energy in underbody blast events should be developed and demonstrated. On the other hand, the size and weight of vehicles are also paramount design factors for the purpose of providing faster transportation, great fuel conservation, higher payload, and higher mobility. Therefore, developing a light weight vehicle structure that provides a balance between survivability and mobility technologies for both vehicle and its occupants becomes a design challenge in this research. One of the new concepts of absorbing blast energy is to utilize the properties of “softer” structural materials in combination with a damping mechanism for absorbing the destructive energy through deformation. These “softer” materials are able to reduce the shock loads by absorbing energy through higher deformation than that of characteristic of normal high strength materials. A generic V-hull structure with five bulkheads developed by the TARDEC is used in the study as the baseline numerical model for investigating this concept. Another new concept is to utilize anisotropic material properties to guide and redirect the destructive energy away from the occupants along pre-designated energy paths. The dynamic performance of multilayer structures is of great interest because they act as a mechanism to absorb and spread the energy from a blast load in the lateral direction instead of permitting it to enter occupant space. A reduced-order modeling (ROM) approach is developed and applied in the preliminary design for studying the dynamic characterization of multilayer structures. The reliability of the ROM is validated by a spectral finite element analysis (SFEA) and a classic finite element analysis by using the commercial code Nastran. A design optimization framework for the multilayer plate is also developed and used to minimize the injury probability, along with a maximum structural weight reduction. Therefore, the goal of designing a lightweight vehicle structure that has high levels of protection in underbody blast events can be achieved in an efficient way.PHDNaval Architecture & Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135895/1/leaduwin_1.pd

Unsteady squeezing flow of a magnetized nano-lubricant between parallel disks with Robin boundary conditions

The aim of the present work is to examine the impact of magnetized nanoparticles (NPs) in enhancement of heat transport in a tribological system subjected to convective type heating (Robin) boundary conditions. The regime examined comprises the squeezing transition of a magnetic (smart) Newtonian nanolubricant between two analogous disks under an axial magnetism. The lower disk is permeable whereas the upper disk is solid. The mechanisms of haphazard motion of NPs and thermophoresis are simulated. The non-dimensional problem is solved numerically using a finite difference method in the MATLAB bvp4c solver based on Lobotto quadrature, to scrutinize the significance of thermophoresis parameter, squeezing number, Hartmann number, Prandtl number and Brownian motion parameter on velocity, temperature, nanoparticle concentration, Nusselt number, factor of friction and Sherwood number distributions. The obtained results for the friction factor are validated against previously published results. It is found that friction factor at the disk increases with intensity in applied magnetic field. The haphazard (Brownian) motion of nanoparticles causes an enhancement in thermal field. Suction and injection are found to induce different effects on transport characteristics depending on the specification of equal or unequal Biot numbers at the disks. The main quantitative outcome is that, unequal Biot numbers produce significant cooling of the regime for both cases of disk suction or injection, indicating that Robin boundary conditions yield substantial deviation from conventional thermal boundary conditions. Higher thermophoretic parameter also elevates temperatures in the regime. The nanoparticles concentration at the disk is boosted with higher values of Brownian motion parameter. The response of temperature is similar in both suction and injection cases; however, this tendency is quite opposite for nanoparticle concentrations. In the core zone, the resistive magnetic body force dominates and this manifests in a significant reduction in velocity i.e. damping. The heat buildup in squeeze films (which can lead to corrosion and degradation of surfaces) can be successfully removed with magnetic nanoparticles leading to prolonged serviceability of lubrication systems and the need for less maintenance

Intrusion and extrusion of liquids in highly confining media: bridging fundamental research to applications

Wetting and drying of pores or cavities, made by walls that attract or repel the liquid, is a ubiquitous process in nature and has many technological applications including, for example, liquid separation, chromatography, energy damping, conversion, and storage. Understanding under which conditions intrusion/extrusion takes place and how to control/tune them by chemical or physical means are currently among the main questions in the field. Historically, the theory to model intrusion/extrusion was based on the mechanics of fluids. However, the discovery of the existence of metastable states, where systems are kinetically trapped in the intruded or extruded configuration, fostered the research based on modern statistical mechanics concepts and more accurate models of the liquid, vapor, and gas phases beyond the simplest sharp interface representation. In parallel, inspired by the growing number of technological applications of intrusion/extrusion, experimental research blossomed considering systems with complex chemistry and pore topology, possessing flexible frameworks, and presenting unusual properties, such as negative volumetric compressibility. In this article, we review recent theoretical and experimental progresses, presenting it in the context of unifying framework. We illustrate also emerging technological applications of intrusion/extrusion and discuss challenges ahead

Nano metal additive enhanced magnetorheological fluid based on rheological and thermophysical properties

Controllable rheological properties through external magnetic field makes magnetorheological materials useful in many applications such as brakes and clutches. However, the performance of magnetorheological fluid is affected by heat generation due to operating condition and particle friction. The heat must be dissipated by producing magnetorheological fluid with enhanced thermal conductivity. This study aims to develop stabilized nano metal added magnetorheological fluid for enhanced thermal conductivity and to evaluate the effects of nano metal added magnetorheological fluid on magnetorheological response. The materials for this study were the nano metal added magnetorheological fluid with fumed silica additive. The materials were prepared with specific concentration of suggested components by using two-step method. 10 samples were developed under three different categories namely MRF, MRF-Cu, and MRF-Al. Experiments were designed based on combined D-optimal model for mixture design with the target to optimize the configuration of magnetorheological fluid for high sedimentation ratio and high thermal conductivity. The study started with investigation of sedimentation ratio through visual observation method. Sedimentation of synthesized samples was observed by inspecting the sediment for 28 days. Then, the study followed with investigation of thermal conductivity in the absence and the presence of magnetic field. The investigation was conducted by thermal properties analyzer and by a module that was based on guarded hot-plate method. The module was developed due to the limitations of thermal properties analyzer in measuring thermal conductivity under magnetic field. Next, the study was finalized with determination of magnetorheological response in the presence of magnetic field. The rheological response was conducted by rheometer and the magnetization response was conducted by vibrating sample magnetometer. The results from every investigation were compared with the commercial MRF-132DG. From the sedimentation ratio investigation, the sample with 5% aluminum recorded the highest enhancement at 14% due to the low density of aluminum particles and the addition of fumed silica. From the thermal conductivity investigation, the highest thermal conductivity without magnetic field was recorded at 0.902 W/m·K from the sample with 5% copper due to the high thermal conductivity value of copper material and its particle size. The enhancement from the sample was 153%. By using the developed module, the sample showed an increase from 0.925 W/m·K without magnetic field to 1.102 W/m·K with magnetic field. The result was from the effect of the chain-like structures formed by the particles when experiencing magnetization. The sample has the highest enhancement at 137% due to the maximum concentration of copper. Finally, from the magnetorheological response determination, the sample with 5% copper demonstrated the highest shear stress (90.3 kPa) in the presence of magnetic field with 276% enhancement due to the higher number of particle chains. The sample recorded the highest magnetization (30.98 emu) and magnetic saturation with 71% enhancement. The addition of copper nanoparticles has avoided the formation of aggregates from magnetic particles by forming clouds around each magnetic particle and resulted to stronger bonds between the particles. The findings in this work provided encouraging results of enhanced magnetorheological fluid properties. With optimized configuration of magnetorheological fluid components, the addition of nano metal additive has enhanced the sedimentation ratio, thermal conductivity, and magnetorheological responses. Hence, nano metal added magnetorheological fluid is suitable to be used for improved performance in elevated temperature applications. It is recommended that redispersibility of the nano metal added magnetorheological fluid is investigated in the future