Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BC: Microfluidics: Drops, Bubbles, and Wetting II |
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Chair: Neelish Patankar, Northwestern University Room: Hilton Chicago Grand Ballroom |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BC.00001: Capillary pinching in a pinched micro-channel Olivier Amyot, Franck Plourabou\'e We study experimentally the capillary pinching of a gas bubble by a wetting liquid inside a pinched channel. We show how this capillary pinching is geometrically controlled by the channel shape. The capillary pinching induces a very reproducible bubbling, at a very well-defined frequency. The dynamic exhibits two distinct regimes : a long-time elongation of the air bubble and a rapid relaxation of the interface after the interface break-up. The slow regime depends on the imposed flux and on the channel geometry. The rapid deformation dynamical regime very weakly depends on the boundary conditions. Scaling arguments are proposed in the context of the lubrication approximation to describe both regimes. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BC.00002: Localized laser forcing for breaking, sorting or blocking droplets in a microchannel Charles Baroud, Jean-Pierre Delville Heat from a laser beam focused at the interface between two immiscible fluids is used to produce thermocapillary stresses along this interface. When combined with the device geometry, these stresses allow us to act on individual droplets as they are transported by a carrier fluid. We demonstrate the application of this technique to perform a wide range of basic operations such as determining the size of a droplet upon breakup, sorting droplets or breaking them into calibrated sizes. These fundamental building blocks may then be combined to develop a contactless, scalable, method for droplet microfluidics within the microchannel environment. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BC.00003: Fourier Transform Analysis of Pressure Fluctuations Due to Microscale Phase Change Brenda Haendler, Albert Pisano, Dorian Liepmann Microscale boiling in channels smaller than the critical bubble radius results in an abrupt change from liquid to vapor. This abrupt change creates a diameter spanning meniscus when boiling pure fluids or mixtures of like fluids. Pressure changes in constant cross section microchannels due to the periodic movement back and forth of the phase change meniscus are measured for a variety of flow conditions. While there has been extensive research using water as the working fluid for electronics cooling applications, focus is placed on using commonly available fuels as the working fluid for portable power applications. A discrete Fourier transform is performed on the pressure data to determine the dominant frequencies in the signal and their relative strengths. Critical trends are discussed comparing both the frequency and the amplitude of the pressure spikes for a variety of temperatures, flow rates, working fluids, and channel diameters. The results of these trends give insight into how the fluid properties change the pressure signature, for a given set of flow conditions. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BC.00004: Steady streaming in bubble microfluidics David Hansen, Philippe Marmottant, Sascha Hilgenfeldt Ultrasound-driven microbubbles attached to a plane wall undergo a combination of transverse and volumetric oscillations, setting up a steady streaming flow constrained by both bubble and wall boundary conditions. Large flow speeds and large shear forces are obtained even for small bubble oscillation amplitudes. We solve the vorticity equation inside and outside the bubble boundary layer and derive analytical approximations for the far-field Eulerian and Lagrangian streaming. The results show that the flow can be represented by a finite number of singularities. Thus, we obtain an easy-to-use ``toolbox'' for analytical modeling of bubble-driven microfluidic flows in more complex situations relevant to lab- on-a-chip and bioengineering applications. We compare the theoretical flows with experimental data and find good agreement. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BC.00005: Rough hydrophobic substrates made from hydrophilic materials? Neelesh Patankar Superhydrophobic surfaces have been shown to offer reduced drag. This is useful in microfluidic applications. Surface roughness is one way to make flat hydrophobic surfaces to behave superhydrophobic. This observation is explained by either Wenzel's or Cassie-Baxter's formulas for the apparent contact angle of a drop on rough surfaces. Recent experimental results have shown that rough surfaces made of hydrophilic materials can exhibit hydrophobic behavior. In this work, this problem is considered theoretically. The idea is to make a rough surface with cavities. The rough substrate could show hydrophobic behavior if air remains trapped in the cavities after a liquid drop is deposited on it. The surface geometry should be such that the trapping of air in the cavities is energetically feasible, thus leading to rough hydrophobic surfaces made from hydrophilic materials. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BC.00006: Multiphase Microfluidics Near the Speed of Sound Axel Guenther, Klavs F. Jensen Concepts borrowed from classical low Reynolds number fluid mechanics have been translated into a variety of multiphase microfluidic applications: segmented flows, e.g. allow to the rapid mixing of species, to carry out highly exothermic gas-liquid reactions, or to grow narrowly distributed nanoparticles. In all cases, interfacial forces dominate by several orders of magnitude over gravity, viscous and inertia forces. We focus on conditions where this is \textit{not} the case. The velocities are sufficiently high so that viscous and inertia forces are on the same order or larger than interfacial ones. At gas superficial velocities on the order of one hundred meters per second, we observed Kelvin-Helmholtz type instabilities as a result of this interplay. The obtained flow patterns are characterized by large velocities/interfacial areas, small film thicknesses and can potentially be combined with very rapid/non-equilibrium transport processes. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BC.00007: A Bubble Reconstruction Method for Two-Phase Microchannel Flows Evelyn Wang, Shankar Devasenathipathy, Hao Lin, Carlos Hidrovo, Juan Santiago, Kenneth Goodson, Thomas Kenny Understanding bubble dynamics is critical to the design of two-phase microchannel heat sinks. This work presents a hybrid experimental and computational methodology that reconstructs 3D bubble geometry, and that provides other important information associated with nucleating bubbles in microchannels. The reconstruction methodology combines experimental measurements with micron-resolution particle image velocimetry (uPIV) and numerical simulations with FEMLAB. Heating power was applied to the microchannel and bubbles formed via heterogeneous nucleation. 2D images and two-component liquid velocity fields during bubble growth were obtained using uPIV. The limited information from the measurements was combined with iterative numerical simulations to determine the 3D geometry of the bubble and corresponding flow field. Various trial 3D bubble shapes were used in the simulations to solve for the flow fields. By identifying the combination that yielded the best match between the computed flow field and uPIV data, a best approximating 3D geometry of the experimentally captured bubble was determined. The methodology developed to reconstruct bubble geometry and the 3D flow field provides insight to modeling bubble dynamics in microchannels. [Preview Abstract] |
Sunday, November 20, 2005 12:27PM - 12:40PM |
BC.00008: Experimental and Computational Investigation of Microbubble Production in Microfluidic Flow-Focusing Devices Michael Weber, Robin Shandas Micron-sized bubbles have been effectively used as contrast agents in ultrasound imaging systems and have the potential for many other applications including targeted drug delivery and tumor destruction. The further development of these applications is dependent on precise control of bubble size. Recently, microfluidic flow-focusing systems have emerged as a viable means of producing microbubbles with monodisperse size distributions. These systems focus co-flowing liquid streams surrounding a gas stream through a narrow orifice, producing bubbles in very reproducible manner. In this work, a photopolymerization technique has been used to produce microfludicic flow-focusing devices which were successfully used to produce micron-sized bubbles. The flow dynamics involved in these devices has also been simulated using a volume-of-fluid approach to simultaneously solve the equations of motion for both the gas and liquid phases. Simulations were run with several variations of the flow-focuser geometry (gas inlet width, orifice length, gas-liquid approach angle, etc.) in an effort to produce smaller bubbles and increase the working range of liquid and gas flow rates. These findings are being incorporated into the production of actual devices in an effort to improve the overall effectiveness of the bubble production process. [Preview Abstract] |
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