Impact of hybrid surfaces on the droplet breakup dynamics in microgravity slug flow: A dynamic contact angle analysis

Microfluidic devices, which enable precise control and manipulation of fluids at the microscale, have revolutionized various fields, including chemical synthesis and space technology. A comprehensive understanding of fluid behavior under diverse conditions, particularly in microgravity, is essential for optimizing the design and performance of these devices. This paper aims to investigate the effects of discontinuous wettability on droplet breakup structures under microgravity conditions using a microchannel wall. The approach we adopt is underpinned by the volume-of-fluid methodology, an efficient technique renowned for its accurate resolution of the fluid interface in a two-phase flow. Furthermore, a modified dynamic contact angle model is employed to precisely predict the shape of the droplet interface at and near the wall. Our comprehensive model considers influential parameters such as slug length and droplet generation frequency, thereby providing crucial insights into their impact on the two-phase interface velocity. Validated against existing literature data, our model explores the impact of various configurations of discontinuous wettability on breakup morphology. Our findings highlight the significance of employing a dynamic contact angle methodology for making accurate predictions of droplet shape, which is influenced by the wall contact angle. Emphasis is placed particularly on the effects of slug length and droplet generation frequency. Notably, we demonstrate that the use of a hybrid surface at the junction section allows for precise control over the shape and size of the daughter droplets, contrasting with the symmetrical division observed on uniformly hydrophilic or superhydrophobic surfaces. This study contributes valuable insights into the complex dynamics of the droplet breakup process, which has profound implications for the design and optimization of microfluidic devices operating under microgravity conditions. Such insights are further poised to augment applications in space exploration, microreactors, and more

Bubble generation mechanisms in microchannel under microgravity and heterogeneous wettability

Advances in hybrid surfaces have revealed interesting opportunities for multiphase flow control under microgravity, as the surface tension force is dominant in this condition. However, a comprehensive investigation of bubble generation rates and slug flow parameters remains challenging. This research integrates hybrid wettability and modified dynamic contact angle models to address this important knowledge gap. Using the computational capabilities of the IsoAdvector multiphase method, we performed detailed simulations of complex multiphase flow scenarios with the OpenFOAM package. We then validated these simulation results through rigorous comparison with available experimental data, thereby strengthening the accuracy and reliability of our numerical simulations. Our comprehensive research demonstrates the profound effect of altering contact angle distribution patterns on several critical parameters. These results highlight the precise control that can be achieved through the strategic manipulation of these patterns, offering the possibility of adjusting factors such as bubble production rate, slug length, bubble diameter, the relationship of flow residence to bubble movement, bubble movement speed in the channel, and pressure drop. Interestingly, altering these patterns can also induce asymmetric behavior in bubbles under microgravity conditions, a phenomenon that has significant implications for various applications. Such insights are crucial for fields such as heat transfer in energy systems, reaction mechanisms in chemical processes, multiphase flow control in petrochemical industries, fluid dynamics in aerospace engineering, and cooling mechanisms in electronic devices. With the ability to modulate these fundamental parameters, we gain valuable insights into the design and optimization of microchannel systems. Consequently, this research presents a more efficient and innovative approach to multiphase flow control, promising improved operational performance, and efficiency in various engineering applications

Experimental investigation of regular fluids and nanofluids during flow boiling in a single microchannel at different heat fluxes and mass fluxes

The dissipation of high heat flux from integrated circuit chips and the maintenance of acceptable junction temperatures in high powered electronics require advanced cooling technologies. One such technology is two-phase cooling in microchannels under confined flow boiling conditions. In macroscale flow boiling bubbles will nucleate on the channel walls, grow, and depart from the surface. In microscale flow boiling bubbles can fill the channel diameter before the liquid drag force has a chance to sweep them off the channel wall. As a confined bubble elongates in a microchannel, it traps thin liquid films between the heated wall and the vapor core that are subject to large temperature gradients. The thin films evaporate rapidly, sometimes faster than the incoming mass flux can replenish bulk fluid in the microchannel. When the local vapor pressure spike exceeds the inlet pressure, it forces the upstream interface to travel back into the inlet plenum and create flow boiling instabilities. Flow boiling instabilities reduce the temperature at which critical heat flux occurs and create channel dryout. Dryout causes high surface temperatures that can destroy the electronic circuits that use two-phase micro heat exchangers for cooling. Flow boiling instability is characterized by periodic oscillation of flow regimes which induce oscillations in fluid temperature, wall temperatures, pressure drop, and mass flux. When nanofluids are used in flow boiling, the nanoparticles become deposited on the heated surface and change its thermal conductivity, roughness, capillarity, wettability, and nucleation site density. It also affects heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. Flow boiling was investigated in this study using degassed, deionized water, and 0.001 vol% aluminum oxide nanofluids in a single rectangular brass microchannel with a hydraulic diameter of 229 µm for one inlet fluid temperature of 63°C and two constant flow rates of 0.41 ml/min and 0.82 ml/min. The power input was adjusted for two average surface temperatures of 103°C and 119°C at each flow rate. High speed images were taken periodically for water and nanofluid flow boiling after durations of 25, 75, and 125 minutes from the start of flow. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). Cycle duration and bubble frequencies are reported for different nanofluid flow boiling durations. The addition of nanoparticles was found to stabilize bubble nucleation and growth and limit the recession rate of the upstream and downstream interfaces, mitigating the spreading of dry spots and elongating the thin film regions to increase thin film evaporation

Microfluidic Hydrodynamic of Gas-Liquid flow in Single Microchannel and Porous Media with Microchannel Network

In this thesis, a microfluidics platform with high-speed imaging system was built to investigate gas-liquid flow in single microchannel and interfacial instability in porous media with microchannel network:The mass transfer of slug flow in the rectangular and square microchannels was experimentally studied by using water as liquid phase and CO2 as gas phase. Depending on flow rates, flow patterns including slug flow, bubbly flow, and annular flow were observed in rectangular and square microchannels. Flow pattern map was proposed and compared with the maps in the literatures. By using digital image processing, the bubble volume especially deformed bubbles in rectangular and square microchannels was calculated based on 2D projection and 3D slicing, correspondingly. Scaling laws including important parameters of bubbles were derived to provide the guidance of microreactor design. Mass transfer coefficients were calculated based on bubble volume. The empirical correlations involving dimensionless numbers were fitted to precisely predict mass transfer coefficients. Further, to be universality, a semi-theoretical model considering length ratio of liquid and gas phases was developed topredict measured mass transfer coefficients in square microchannel precisely.The gas-liquid displacement in porous media with microchannel network was experimentally investigated. By varying capillary numbers Ca and viscosity ratios M in a wide range, flow pattern involving viscous fingering (VF), capillary fingering (CF) and crossover zone (CZ) can be observed. Finger morphologies at breakthrough moment and steady state in three different flow regions was visualized. The main difference between VF and CF is that the gas stops invading in CF region after breakthrough, whereas in VF region gas can continue to expand until almost all the liquid phase is displaced. Invasion velocity, phase saturation and fingering complexity were quantified based on digital image processing. Fingering dynamical behaviors in different flow pattern before and after breakthrough was investigated. Time evolution of fingering displacement after breakthrough demonstrated an unobserved circle, consisting of new finger generation, cap invasion, breakthrough and finger disappearance. The circle repeats until steady state. Finally, local dynamical invasion behavior was studied and a stepwise way of gas invasion was exposed

Non-Newtonian Microfluidics

Microfluidics has seen a remarkable growth over recent decades, with its extensive applications in engineering, medicine, biology, chemistry, etc. Many of these real applications of microfluidics involve the handling of complex fluids, such as whole blood, protein solutions, and polymeric solutions, which exhibit non-Newtonian characteristics—specifically viscoelasticity. The elasticity of the non-Newtonian fluids induces intriguing phenomena, such as elastic instability and turbulence, even at extremely low Reynolds numbers. This is the consequence of the nonlinear nature of the rheological constitutive equations. The nonlinear characteristic of non-Newtonian fluids can dramatically change the flow dynamics, and is useful to enhance mixing at the microscale. Electrokinetics in the context of non-Newtonian fluids are also of significant importance, with their potential applications in micromixing enhancement and bio-particles manipulation and separation. In this Special Issue, we welcomed research papers, and review articles related to the applications, fundamentals, design, and the underlying mechanisms of non-Newtonian microfluidics, including discussions, analytical papers, and numerical and/or experimental analyses

Toward Sophisticated Controls of Two-Phase Transport at Micro/Nano-Scale

Through the use of latent heat evaporating, flow boiling in microchannels offers new opportunities to enable high efficient heat and mass transport for a wide range of emerging applications such as high power electric/electronic/optical cooling, compact heat exchangers and reactors. However, flow boiling in microchannels is hampered by several severe constraints such as bubble confinement (e.g., slug flow), viscosity and surface tension force-dominated flows, which result in unpredictable flow pattern transitions and tend to induce severe flow boiling instabilities (i.e. low-frequency and large magnitude flows) and suppress evaporation and convection. In this dissertation, three novel micro/nanoscale thermo-fluidic control methodologies were developed to address these aforementioned constraints faced in flow boiling in microchannels. These include a) unifying two-phase flow patterns to radically avoid pattern transitions, b) nano-tips induced boundary layers to promote evaporation and advections by reconstructing boundary layers, and c) high frequency self-sustained two-phase oscillations to generate strong mixing in the laminar flow. Using superhydrophilic silicon nanowires, the first methodology successfully formulated a new, single and periodic annular flow during the entire flow boiling process, i.e., from the onset of nucleate boiling to the critical heat flux (CHF) conditions by reducing the characteristic bubble size and transforming the direction of the dominant capillary forces from the cross-sectional plane to inner-wall plane. In the second methodology, boundary layers were induced along vertical walls by hydrophilic nanotips using surface tension forces, which is the first time to achieve the design of boundary layers that ultimately govern heat and mass transfer. In the last approach, novel microfluidic transistors were devised to passively introduce and sustain high frequency bubble growth/collapse processes and hence to create strong mixing in microchannels. Compared with the state-of-the- art techniques, by directly targeting on manipulating bubble dynamics and governing forces, consequently, the fluid structures, these three novel principles can enable substantially higher flow boiling performance in terms of heat transfer coefficient, CHF, and flow boiling stabilities. Equally important, pressure drop was also well managed or even greatly reduced. Theoretical study was also conducted to understand the mechanisms and provide insight to new flow boiling phenomena

Two-Phase Flows With Dynamic Contact Angle Effects For Fuel Cell Applications

Liquid water management is still a very critical challenge in the commercialization of proton exchange membrane fuel cell (PEMFC). Fundamental understanding of two-phase flow behaviors is of crucial importance to the investigation of water management issues. Recently, it has been noted that the dynamic contact angle (DCA) plays a critical role in the two-phase flow simulations and the conventional static contact angle (SCA) model has obvious limitations in the prediction of droplet behaviors. This thesis mainly focuses on the numerical modeling and simulation of two-phase flow problems with dynamic contact angle (DCA) and is presented by four papers. The first paper proposes and validates an advancing-and-receding DCA (AR-DCA) model that is able to predict both advancing and receding dynamic contact angles using Hoffman function (Chapter 2). In the second paper, the AR-DCA model is further applied to simulate droplet behaviors on inclined surfaces with different impact velocities, impact angles and droplet viscosities (Chapter 3). The third paper introduces a methodology to improve the evaluation method of contact line velocity in the AR-DCA model and an improved-AR-DCA (i-AR-DCA) model is developed (Chapter 4). The last paper presents different flow regimes in a single straight microchannel under various air and water inlet flow rates (Chapter 5)

Fabrications and Applications of Micro/nanofluidics in Oil and Gas Recovery: A Comprehensive Review

Understanding fluid flow characteristics in porous medium, which determines the development of oil and gas oilfields, has been a significant research subject for decades. Although using core samples is still essential, micro/nanofluidics have been attracting increasing attention in oil recovery fields since it offers direct visualization and quantification of fluid flow at the pore level. This work provides the latest techniques and development history of micro/nanofluidics in oil and gas recovery by summarizing and discussing the fabrication methods, materials and corresponding applications. Compared with other reviews of micro/nanofluidics, this comprehensive review is in the perspective of solving specific issues in oil and gas industry, including fluid characterization, multiphase fluid flow, enhanced oil recovery mechanisms, and fluid flow in nano-scale porous media of unconventional reservoirs, by covering most of the representative visible studies using micro/nanomodels. Finally, we present the challenges of applying micro/nanomodels and future research directions based on the work