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 AC: Microfluidics: Drops, Bubbles, and Wetting I |
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Chair: Paul Neitzel, Georgia Tech Room: Hilton Chicago Grand Ballroom |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AC.00001: Frictional forces associated with isothermal non-wetting Maria-Isabel Carnasciali, G. Paul Neitzel Numerous engineering applications have been proposed to exploit the load-carrying and `non-contact' nature of noncoalescing and nonwetting systems. One proposed application is a ``lab-on-a-chip'' (LOC), in which liquid samples sliding over a film of air are delivered from point-to-point without the large driving forces required to pump liquid through a microchannel. Due to the axisymmetry of the flow fields in both the lubricating gas and droplet, a stationary, thermocapillary-nonwetting droplet has a vanishing coefficient of static friction. However, once motion is imparted, the droplet deforms, requiring a force to sustain such motion. Given the very small volumes of droplets of interest in LOC applications, very little inertia must be overcome to initiate droplet motion in this near-frictionless situation. In fact, a small amount of friction in the LOC is actually desirable to enhance maneuverability of the drops on the chip. The present work seeks to quantify the frictional force for isothermal nonwetting between a drop of silicone oil and a moving, unwetted substrate due to the presence of the lubricating gas film. Current results from laboratory experiments will be presented. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AC.00002: Failure loading on a thermocapillary nonwetting droplet G. Paul Neitzel, Peter Nagy Permanent nonwetting is a phenomenon characterized by a liquid droplet not wetting a surface with which it appears to be in contact. It is realized by generating relative tangential motion between the solid and droplet surfaces that drags a thin film of lubricating air (or any surrounding fluid) into the ``contact'' region, akin to the classical slider bearing. In the case of thermocapillary-induced nonwetting driven by a temperature difference between the solid and liquid, air films have been shown to be capable of sustaining substantial applied loads between droplet and surface. By exploiting the load-carrying capability of such nonwetting systems, several applications have been envisioned. For these devices to operate successfully, however, the lubricating film must be supplied continuously within the contact region. As expected, there are thresholds of load and/or vibration under which these films cannot be sustained. Experiments will be described that quantify failure-threshold loadings for both static and dynamic environments. These results will aid in designing reliable nonwetting systems exhibiting robust operation. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AC.00003: Shape Oscillations in Sessile Liquid Drops Using Electrowetting on Dielectric R. Miraghaie, J.D. Sterling, A. Nadim Experimental results aimed at characterization of oscillating droplet microfluidics using electrowetting-on-dielectric are presented. The study involves shape oscillations of a microliter sessile drop positioned on an electrowetting chip with electrical grounding provided by thin gold lines patterned on the chip. This type of grounding avoids the use of the commonly used penetrating wire, resulting in more symmetric oscillations. Excitation of the droplet at different frequencies is achieved by applying electric potential to the electrowetting chip. Oscillation shape modes of the droplet are estimated using edge detection schemes and series decomposition in Legendre polynomials in spherical coordinates. For axisymmetric oscillations, one can show using potential flow theory that only even-mode Legendre polynomials arise, with the solid boundary being a plane of mirror symmetry. Power spectral densities of the time-dependent Legendre polynomial coefficients as well as droplet diameter and height signals are calculated for different frequencies of the applied voltage. Super- and sub-harmonics are evident in some of the response signals. As an application, acceleration of DNA hybridization as a result of oscillatory excitation of a 10 microliter mixture of molecular beacon and complementary DNA is presented and compared with the non- oscillatory case. [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AC.00004: A computational study of drop formation in microfluidic devices Chunfeng Zhou, Pengtao Yue, James J. Feng Capillarity has a prominent role in flow in microfluidic devices because of the typically small linear dimensions, large curvature and large surface area in these geometries. Applications of micro-drops and micro-bubbles in small channels range from micro-pumps to ultrasound contrast agents [1]. In this study, we concentrate on the process of drop formation in microdevices, which has been used for generating uniform emulsions of Newtonian and non-Newtonian fluids, with an emphasis on the effects of fluid rheology. The bulk rheology and interfacial motion are described in a phase-field framework [2], and the numerical solution uses a finite-element algorithm with adpative meshing to ensure proper resolution of the interfaces. We will present simulations of drop formation at the tip of a jet either within a quiescent medium or inside a flow-focusing device as demonstrated in recent experiments [1,3]. The rheology of the components may be Newtonian, viscoelastic or liquid-crystalline, with the Oldroyd-B and Leslie-Ericksen models being used for the latter. Results show that component rheology is a major determinant in the morphology of the jet, the details of the breakup process, and the size distribution of drops. The solutions compare favorably with experiments.\\ 1. H.A. Stone etc, Ann. Rev. Fluid Mech. 36, p.381(2004).\\ 2. P. Yue etc, J. Fluid Mech. 515, p.293 (2004).\\ 3. B. Steinhaus etc, Bulletin APS, 49, No. 9, p. 67 (2004). [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AC.00005: Length scale effect on thin film drainage during droplet deposition inside microchannels Amy Shen, Ben Steinhaus When a droplet in a liquid-liquid system approaches a solid surface a thin liquid film forms between the droplet and the surface. This thin film drains until an instability forms, resulting in the coalescence of the droplet and the surface. A microfluidic device is utilized to investigate the effects of length scale on the drainage of these thin films on both hydrophobic and hydrophilic surfaces. Droplets ranging in size from microns to millimeters are examined. The rate of film drainage, as well as the time to coalescence, are compared between millimeter and micron scale droplets. Results show that van der Waal's and electrostatic forces become dominate over viscous forces at small length scales. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AC.00006: Imbibition in microchannels Abraham Medina, Erick Luna, Marina Medina In this work we studied the imbibition in straight microchannels with circular and noncircular cross-sections. The formulation of a motion equation allowed us to obtain closed-form analytical solutions for the imbibition front when there is an initial head pressure P, the gravity is acting, and the cylinder may be inclined any angle respect to the vertical. It is shown that the limit case, Bo=0, where Bo is the Bond number, give solutions that cover the main aspects related to imbibition in microchannels, as the competition between the strength of the capillary phenomena and the hydraulic resistance depending on the shape. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AC.00007: Arnold tongues in a microfluidic drop emitter. Herve Willaime, Valessa Barbier, Patrick Tabeling In this paper, we present experimental study of microfluidic droplets produced in T junctions and subjected to local periodic forcing. Droplets of water are generated periodically in the main channel. The emission frequency depends mainly on both oil and water flow rates. External forcing allows the control of the emission of droplet. Synchronized and quasiperiodic regimes -- structured into Arnold tongues and devil staircases -- are reported. The nature of the dynamical regime controls the droplets characteristic. Depending on the flow conditions, one may either favor synchronized regimes giving rise to mono-disperse emulsions of controlled size or promote quasi-periodic regimes giving rise to polydisperse emulsion. The width of each regime is affected by the characteristics of the unforced regime and also by the strength of the external forcing. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AC.00008: Liquid Droplet Detachment and Entrainment in Microscale Flows Carlos Hidrovo, Fu-Min Wang, Julie Steinbrenner, David Fogg, Eon Soo Lee, Jae-Mo Koo, Ching-Hsiang Cheng, John Eaton, Kenneth Goodson In this talk we will present a first order study of liquid water detachment and entrainment into air flows in hydrophobic microchannels. Silicon based microstructures consisting of 23 mm long U-shaped channels of different geometry were used for this purpose. The structures are treated with a Molecular Vapor Deposition (MVD) process that renders them hydrophobic. Liquid water is injected through a side slot located 2/3 of the way downstream from the air channel inlet. The water entering the air channel beads up into slugs or droplets that grow in size at this injection location until they fill and flood the channel or are carried away by the air flow. The slugs/droplets dimensions at detachment are correlated against superficial gas velocity and proper dimensionless parameters are postulated and examined to compare hydrodynamic forces against surface tension. It is found that slug/droplet detachment is dominated by two main forces: pressure gradient drag, arising from confinement of a viscous flow in the channel, and inertial drag, arising from the stagnation of the air due to obstruction by the slugs/droplets. A detachment regime map is postulated based on the relative importance of these forces under different flow conditions. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AC.00009: WITHDRAWN: Fragmentation of charged aqueous nanodroplets Kengo Ichiki, Styliani Consta The whole evaporating process of charged aqueous nanodroplets is studied by systematic molecular dynamics simulations until most of the solvent molecules are evaporated. % The solvent evaporation makes the droplet smaller and smaller, and at a certain point the repulsive force among ions causes an instability, where typically single ion and 10 to 20 water molecules are disintegrated from the main droplet. % This ion fragmentation occurs around 70 to 80\% of the charge predicted by the Rayleigh theory [Lord Rayleigh, Phil. Mag. {\bf 14}, 184 (1882)]. % The numerical results are summarized in the function $R(z)$ which is the fragmentation radius at the charge $z$. From the fitting by the power law $R\propto z^\beta$, we find that at lower temperature $T=350$ and $370$ K the result is close to the Rayleigh theory $\beta = 2/3$, while at higher temperature $T=400$ and $450$ K it is like $\beta = 1/2$. % Another fitting on $R(z)$ by the extended ion evaporation mechanism [M. {Gamero-Casta\~{n}o} and J. {Fern\'{a}ndez de la Mora}, Anal. Chim. Acta {\bf 406}, 67 (2000)] works well for both cases. % The final state of the evaporation process is typically a single ion with several water molecules. If we put an alanine dipeptide in zwitterionic form at the beginning, two charges remain in some cases. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AC.00010: Dripping and Jetting in a Co-flowing Fluid Stream Andrew Utada, Alberto Fernandez-Nieves, David Weitz We present preliminary data on drop formation from a nozzle in a co-flowing fluid. This problem has been addressed for cases where the outer fluid is a gas or a stationary viscous fluid. We observe that drop formation is affected by the co-flowing fluid. We focus on the dripping-jetting transition, jet size and shape, and drop sizes. Finally, we relate the physics of drop formation to a recently reported microcapillary device (1,2) that generates monodisperse double emulsions. \newline \newline (1) A. S. Utada, E. Lorenceau, D. R. Link, P. Kaplan, H. A. Stone, D. A. Weitz, Science 308, 537 (2005) \newline (2) E. Lorenceau, A. S. Utada, D. R. Link, G. Cristobal, M. Joanicot, D. A. Weitz, Langmuir in press, (2005). [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:23AM |
AC.00011: Discrete Droplet Microfluidics for Biological Assays A. Kalra, R. Miraghaie, A. Nadim, J.D. Sterling The use of droplet microfluidics to miniaturize biological assays has the potential to achieve high throughputs with small volumes of reagents. We control the discrete movement of microliter volume droplets containing DNA molecules on a surface by exploiting electrowetting, which refers to the reduction in contact angle of a liquid drop when the surface is electrically charged. A droplet containing molecular beacon is actuated and coalesced with a droplet containing complementary DNA. The hybridization is implemented on an electrowetting chip and detected using fluorescence microscopy. Electrowetting is also applied to conduct an isothermal DNA amplification reaction used for rapid detection of biological agents. A droplet containing the DNA sequence to be amplified is actuated and merged with a droplet containing enzymes, template DNA and an intercalating fluorescent dye on a heated electrowetting chip. Our chips can be activated by relatively low electrical potentials and consume little power. Time histories of the detected fluorescence intensities of the mixed reagents are presented for several hybridization and amplification reactions. [Preview Abstract] |
Sunday, November 20, 2005 10:23AM - 10:36AM |
AC.00012: Polygonal Drops formed by AC field induced resonance. Ping Wang, Siddharth Maheshwari, Hsueh-Chia Chang Liquid phase polarization of a drop in a DC field gives rise to the formation of the classical Taylor cones whose singular Maxwell pressure cancels the singular azimuthal capillary pressure. We report the observation of a novel interfacial singularity that produces polygonal drops. This new singularity also arises due to Maxwell stress and liquid polarization, but when both are selectively and resonantly amplified by an AC field. A particular frequency is chosen such that the period is larger than the ion diffusion time across the interfacial double layer. The drop size is controlled with the Maxwell force such that the drop dimension is smaller than the capillary length and the resonance frequency of the undeformed spherical drop is commensurate with the AC frequency. The resonant interaction selects different interfacial modes with different patterns and wavelengths. The liquid polarization focuses the ridges of the interfacial patterns into conical protrusions that deform the drop into polygonal shapes. The capillary stress at the cusp is now compensated by a Maxwell stress. Due to the symmetry of the drop, only certain polygonal shapes are selected at specific resonant frequencies that are consistent with the inviscid resonant dispersion relationship for a spherical drop. [Preview Abstract] |
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