International Journal of Micro-Nano Scale Transport

Editor-in-Chief: Dr. Sarit Kumar Das
published quarterly • ISSN 1759-3093 • 2014 journal prices/format options
2014 is volume 5

 

The applications of transport processes posed tremendous new challenges in the last part of 20th century in emerging areas like electronic cooling and MEMS (and NEMS) devices where heat, species and fluid flows are involved within very small dimensions, challenging the established paradigms of transport processes. This has given a boost to the area of micro-scale transport. In addition to this, extending micro/nano scale technologies to health sciences through Bio-MEMS and micro-nanofluidic devices, bringing the concept of lab-on-chip in biological sciences have emerged as the latest important initiatives in this field. This has given rise to inter-disciplinary research where engineering concepts are used to address the problems at smaller scales required for the problems related to fundamental sciences and their applications. The International Journal of Micro-Nano Scale Transport aims to bring all this research work together.

The International Journal of Micro-Nano Scale Transport will focus on transport processes of all kinds applicable to smaller dimensions. The processes may include the transport of momentum, mass, chemical species, thermal, biological and electro-kinetic/ electrochemical quantities at micro and/or nanoscale in natural as well as engineered systems. The observations of the characteristic features of these transport processes, analysis of these observations and theorization as well as modeling and simulation of these processes will be the broad scope of this journal. The journal will specifically emphasize both the fundamentals of microscale transports of heat, mass, momentum and species as well as the application of these to specific areas like electronic cooling, micromachines for miniaturized mechanical devices including MEMS (and NEMS), synthesis and characterization of nanoparticles and nanostructured surfaces like super hydrophobic surfaces and their applications in industrial and health care systems, micro and nano scale measurements, microfluidic devices for surgery and medicine, bio-MEMS for novel biological assays and similar other applications. The journal will also publish articles on newer concepts in the above areas with proof of concepts as rapid communications. Reviewing the above and related areas through well structured review articles will be a special feature of the journal.

 

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abstracts of papers to be published in early issues of the journal

Electrowetting-induced droplet transport on smooth and superhydrophobic surfaces : fundamentals, applications and future directions
Vaibhav Bahadur and Suresh V. Garimella
School of Mechanical Engineering and Birck Nanotechnology Center Purdue University, West Lafayette, Indiana 47907-2088 USA

Abstract: Microfluidic electrical control of liquid droplet motion and morphology has many applications in the field of MEMS and lab-on-chip devices resulting from enhanced flow control opportunities, minimal power consumption, and the absence of mechanical moving parts. The present article summarizes recent progress towards understanding of the fundamentals governing the electrical actuation of droplets on smooth and superhydrophobic surfaces. The first section of the article analyzes the physics underlying actuation of liquid droplets with a wide range of electrical properties on smooth surfaces. Electromechanical considerations are employed to study the actuation force on a generic liquid droplet across the entire spectrum of electrical actuation regimes. The challenges and recent progress in understanding the fluid flow and dissipation mechanisms associated with a discrete moving droplet are discussed. The second section of the article maps out the role of electrical voltages, surface chemistry
and surface morphology in determining droplet states (nonwetting Cassie state and wetting Wenzel state) and triggering state transitions on superhydrophobic surfaces. Recent progress in understanding critical phenomena associated with droplet transitions on superhydrophobic surfaces (energy barrier for the Cassie-Wenzel transition, absence of spontaneous reversibility of the Cassie-Wenzel transition, robustness of Cassie state, nanostructured versus microstructured roughness elements) is reviewed and analyzed. The third section of the article highlights avenues for future research in the fields of dropletbased microfluidics and superhydrophobic surfaces. Specific applications are outlined and discussed to highlight the potential impact of this research in the areas of energy systems, lab-on-chip devices, optics and microscale heat transfer.

Development of Multi-scale Models for Transport Processes Involving Catalytic Reactions in SOFCs
Bengt Sunden and Jinliang Yuan
Department of Energy Sciences, Lund University, Box 118, 22100 Lund, Sweden

Abstract: Depending on specific configurations and designs, several physical phenomena are present in anode-supported solid oxide fuel cells (SOFCs), such as multi-component gas/surface species flow, thermal energy and mass transfer. The physical composition of the porous Ni-YSZ anode structure may be uniform throughout the thickness (in terms of materials, porosity and microstructure). The major domain of the supporting anode serves as the purpose transporting thermal energy and gas/surface chemical species, and Ni in which servers as a catalyst for the steam reforming reactions of hydrocarbon fuels. Due to the reactions, generation and consumption of gas- and surface species together with electric current production are involved at the active sites. Therefore, various reactions in SOFCs are strongly coupled with the transport processes to make the physical phenomena more complicated. In this study, a fully three-dimensional numerical calculation procedure (CFD approach) is further developed. The gas mass/heat generation and consumption related to the microscopic catalytic chemical reactions have been modeled and integrated with the CFD code. The variable thermal-physical properties and transport parameters of the fuel gas mixture have also been taken into account. Furthermore, the heat transfer due to the fuel gas diffusion is implemented into the energy balance based on multi-component diffusion models. A multi-step heterogeneous steam reforming reaction scheme based on the micro and detailed reaction mechanisms of Ni catalyst is employed. The catalytic reactions include 42 irreversible elementary ones, evaluated for temperatures between 220 and 1700oC, accounting for the steam reforming, the water-gas shift reforming and Boudouard reactions. This microscopic model describers the adsorption and desorption reactions of 6 gas-phase species (H2, CO, CH4, CO2, H2O and O2) and the catalytic reactions of 12 surface adsorbed species (Nis, Hs, Os, OHs, HCOs, CHs, CH2s, CH3s, CH4s, COs, CO2s, H2Os). Simulation results are presented and discussed in terms of, among others, gas-phase species and temperature distribution, the chemical reaction rates of the gas- and surface chemical species, the catalyst surface coverage, and the effects on the transport processes.
Keywords: CFD Approach; Multi-scale; Catalytic Reactions; Transport Phenomena; Anode-support; Solid Oxide Fuel Cells (SOFCs).

Visualization and High Resolution Temperature Measurements of Condensation on Nanostructured Surfaces
Carter R. Dietz
G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States
Yunhyeok Im
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-Dong, Yuseong-Gu, Daejeon, 305-701, Republic of Korea
Yogendra K. Joshi
G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States
S. S. Lee
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-Dong, Yuseong-Gu, Daejeon, 305-701, Republic of Korea

Superhydrophobic surfaces exhibit a high contact angle and a low contact angle hysteresis. These surfaces are formed by increasing the surface roughness and modifying the surface chemistry. By condensing vapor on a superhydrophobic surface, permanent drop-wise condensation may be achieved. Drop-wise condensation offers a heat transfer coefficient that is an order of magnitude greater than film-wise condensation. In turn, a larger heat transfer coefficient can lead to the design of a more compact condenser for thermal management of portable electronic devices. In this study, superhydrophobic surfaces were fabricated by electroplating copper through a porous anodized aluminum (AAO) template. The resulting copper nanowires were 200 nanometers in diameter, had an average pitch of 400 nm, and had a parameterized height varying from 1 to 8 micrometers. The copper nanowires were coated with n-octyltriethoxysilane using an aqueous alcohol solution. The resulting contact angle of the superhydrophobic surface was between 159° and 168°. Finally, the silane coated copper nanowires were attached to a silicon thermal test die using silver filled epoxy. The silicon thermal test die had a footprint of 10 mm by 10 mm and had 9 patterned resistive temperature detectors (RTDs) to measure the surface temperature at various locations. The RTDs were circular with a diameter of 600 micrometers. The superhydrophobic samples were compared to a silicon thermal test die without copper nanowires at inclination angles of 0°, 45°, and 90° with respect to gravity. Visualization was done using a high-speed CMOS camera with a field of view of 500 micrometers by 500 micrometers. Images were captured at 60 fps for 90 seconds from the onset of drop-wise condensation. The number of nucleation sites, average surface temperature, and local heat transfer coefficient are measured and compared among the samples.

Concentration gradients in microfluidic cell culture systems
Roger Kamm, Seok Chung, Ioannis Zervantonakis
Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge MA

Abstract: Microfluidic technology enables the creation of well-defined cell culture environments, which integrate the control of multiple biophysical and biochemical cues for designing novel in vitro assays. Growth-factor concentration gradients play a critical role in a wide range of biological processes ranging from development to cancer, guiding cell migration and influencing cell signaling. We present a microfluidic device that is capable of generating stable concentration gradients in a 3D matrix, while allowing for direct imaging of cellular behavior. The design consists of polydimethylsiloxane microchannels interconnected through 3D matrices where cells can be seeded on 2D or 3D for monitoring of cell invasion. An important characteristic of the microfluidic platform is the capability of generating reproducible, stable and quantifiable concentration gradients that are essential for systematic studies of soluble factor signaling in chemotaxis assays. To characterize the concentration gradients in the device we combine intensity measurements using fluorescent markers and a finite element model, while we also provide estimates of the chemoattractant diffusion coefficient across the 3D matrix. The numerical model solves the coupled convection-diffusion-Brinkmann equations using a commercial finite element solver. Comparison of measured and computed concentration profiles demonstrate good agreement, while the simulation can be used as a tool for optimizing the microfluidic design. To demonstrate the device capabilities and the effects of concentration gradients on cell migration, we seeded a glioblastoma cell line on the microfluidic channels and monitored cell invasion in 3D collagen type I matrices under control and epidermal growth factor (EGF) gradient conditions. We found that in the presence of a 20ng/ml/mm concentration gradient tumor cells were guided towards higher EGF level, while under control conditions no preferential direction in invasion was observed, suggesting that EGF is a critical regulator of cell invasion.

Computational Analysis and Optimization of Wire – Sandwiched Micro Heat Pipes
R. L. Rag and C. B. Sobhan
Nanotechnology Research Laboratory, School of Nano Science and Technology, National Institute Technology Calicut, India 673 601

Abstract: Micro heat pipes are a promising option for the thermal management of microelectronic systems with high heat flux dissipation rates. A computational analysis is performed on wire-sandwiched micro heat pipes, a relatively new design of micro heat pipes, which utilizes an array of wires sandwiched between metallic plates to produce the flow channels. A transient one-dimensional model incorporating the longitudinal variations in the flow cross sections of the liquid and vapor media has been utilized, while developing the governing equations for the analysis. A fully implicit finite difference scheme is utilized to obtain solutions for the velocity, pressure and temperature distributions in the vapor and the liquid phases. The performance of the heat pipe has been obtained, with the effective thermal conductivity as the indicator, and extensive optimization studies have been performed with respect to the geometric parameters namely the length of the heat pipe sections, the diameter and pitch in the sandwiched structure, and operational parameters such as the heat input and the condenser heat transfer coefficient. The analysis provides guidelines for the geometric design of the wire-sandwiched micro heat pipes for heat dissipation from micro electronic chips, based on the results corresponding to the thermal management conditions encountered in such applications.

Transport of Flexible Molecules in Narrow Confinements
Suman Chakraborty and Siddhartha Das
Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, Kharagpur-721302, India

Abstract: Response of flexible polymer molecules to strong nanoscopic confinements is primarily dictated by the relative value of the confinement length scales with respect to the polymer persistence length. Depending on whether the channel height is larger (de Gennes regime) or smaller (Odijk regime) than the polymer persistence length, altogether different polymer dynamics is ensued as illustrated in the pioneering theoretical studies by de Gennes (1979) and Odijk (1983). Rapid advances of nanofabrication and polymer handling, over the last few years, have been able to provide experimental validation to these studies and at the same time been able to unravel different intriguing physical issues unique to nanoconfinement induced dynamics of polymer molecules. These studies have led to a plethora of new applications ranging from the estimation of structural and mechanical properties of polymer to fabrication of novel, portable diagnostic tool kits. In this review article we shall revisit the different intriguing physical and technological issues involved in polymer dynamics in nanoconfinements. First we shall identify the effect of varying degrees of confinement (de Gennes and Odijk regime) on the stretching dynamics and the overall representation of the polymer molecule. Next, we discuss the possible physical interaction forces on the polymer molecule introduced by the presence of the confining walls and the resulting effects like formation of wall adjacent depletion layers, asymmetric distribution of polymer mass density from the wall to the channel center line etc. Thirdly, possible effects of a background field (flow field or electric field or a combination of both and they may bear signatures of the involved nanoscopic length scales) on the overall nanoscale polymer dynamics is highlighted. We also briefly discuss the technological intricacies involved in the relevant nanofabrication and polymer handling schemes and also the issues governing the polymer modeling in such scales. Finally we conclude by indicating the possible directions and the anticipated outcomes and significances of future research in polymer transport and dynamics in nanoscopic confinements.

 

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Editor-in-Chief

Dr. Sarit Kumar Das
Department of Mechanical Engineering
Indian Institute of Technology Madras
Chennai – 600 036, India

 

Editorial Board

Dr. Gang Chen, Massachusetts Institute of Technology, USA

Dr. Bengt Sunden, Lund University, Sweden

Dr. Stephan Kabelac, Helmut-Schmidt University, Germany

Dr. Peter A Kew, Heriot-Watt University, UK

Dr. Yimin Xuan, Nanjing University of Science and Technology, China

Dr. Stephen U. S. Choi, University of Illinois at Chicago, USA

Dr. Suman Chakraborty, Indian Institute of Technology Kharagpur, India

Dr. Roger Kamm, Massachusetts Institute of Technology, USA

Dr. Suresh Garimella, Purdue University, USA

Dr. Yogendra Joshi, Georgia Institute of Technology, USA

Dr. Avijit Bhunia, Teledyne Scientific Company, USA

Call for Papers

Manuscripts should be sent to the editor: skdas@iitm.ac.in