Current Search:   Yakhshi Tafti, Ehsan (x)

View All Items

  • CSV Spreadsheet
(1 - 2 of 2)
FLOW VISUALIZATION IN MICROFLUIDIC EXPANSION AND MIXING
Title
FLOW VISUALIZATION IN MICROFLUIDIC EXPANSION AND MIXING.
Creator
Yakhshi Tafti, Ehsan, Kumar, Ranganathan, University of Central Florida
Abstract / Description
Micro particle image velocimetry (microPIV) is a non-intrusive tool for visualizing flow in micron-scale conduits. Using this investigative instrument, two experimental studies were performed to understand flow behaviors in microfluidic channels - a sudden expansion step flow and laminar velocity profile variation in diffusion driven mixing. First, flow in a backward facing step feature (1:5 expansion ratio) in a microchannel was taken as the subject of microPIV flow visualization. The onset...
Show moreMicro particle image velocimetry (microPIV) is a non-intrusive tool for visualizing flow in micron-scale conduits. Using this investigative instrument, two experimental studies were performed to understand flow behaviors in microfluidic channels - a sudden expansion step flow and laminar velocity profile variation in diffusion driven mixing. First, flow in a backward facing step feature (1:5 expansion ratio) in a microchannel was taken as the subject of microPIV flow visualization. The onset and development of a recirculation flow was studied as a function of flow rate. This flow pattern was further used to investigate two major parameters affecting microPIV measurements; the depth-of-focus and recording time-intervals between images in a microPIV image pair. The onset of recirculation was initiated at flow rates that correspond to Reynolds numbers, Re>95, which is well beyond the typical working range of microfluidic devices (Re=0.01-10). The recirculation flow has a 3D structure due to the dimensions of the microchannel and the effect of no slip condition on the walls. Ensemble cross-correlation was found not to be sensitive to variations of depth-of-focus and the output flow fields were similar as long as the overall optical focus remained within the upper and lower bounds of the microchannel. However, variations of time intervals between images in a microPIV pair, resulted in quantitatively and qualitatively different flow patterns for a given constant flow rate and depth-of-focus. In the second experiment, the effect of the laminar velocity profile and its variation on mixing phenomena at the reduced scale is studied. It is shown that the diffusive mass flux between two miscible streams, flowing in a laminar regime in a microchannel, is enhanced if the velocity at their diffusion interface is increased. Based on this idea, an in-plane passive micromixing concept is proposed and implemented in a working device (sigma micromixer). This mixer shows considerable mixing performance by periodically varying the flow velocity profile, such that the maximum of the profile coincides with the transversely progressing diffusion fronts repeatedly throughout the mixing channel. microPIV has been used to visualize the behavior of laminar flow inside the micromixer device and to confirm the periodic variation of the velocity profile through the mixing channel.
Show less
Date Issued
2009
Identifier
CFE0002826, ucf:48079
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0002826
THERMALLY-INDUCED MOTION OF DROPLETS ON A THIN LIQUID LAYER AND ITS APPLICATION TO DROPLET MANIPULATION PLATFORMS
Title
THERMALLY-INDUCED MOTION OF DROPLETS ON A THIN LIQUID LAYER AND ITS APPLICATION TO DROPLET MANIPULATION PLATFORMS.
Creator
Yakhshi-Tafti, Ehsan, Kumar, Ranganathan, University of Central Florida
Abstract / Description
In the recent years, there has been a growing interest in droplet-based (digital) microfluidic systems due to their ability to handle multiple discrete samples in a self-contained configuration compared to continuous flow systems. Various methods for droplet manipulation are currently available based on hydrodynamic, electrostatic, chemical, photonic and thermal interactions. High speed, controlled response and minimal thermal loading with least contamination are required in practical...
Show moreIn the recent years, there has been a growing interest in droplet-based (digital) microfluidic systems due to their ability to handle multiple discrete samples in a self-contained configuration compared to continuous flow systems. Various methods for droplet manipulation are currently available based on hydrodynamic, electrostatic, chemical, photonic and thermal interactions. High speed, controlled response and minimal thermal loading with least contamination are required in practical applications, especially in chemistry and biology. Although, thermal actuation of droplets has been recognized as an attractive choice due to a wide range of thermomechanical properties that can be exploited, the previous studies yielded limited success in addressing issues such as droplet evaporation, contamination, pinning, hysteresis and irreversibility that are associated with using solid substrate platforms In order to overcome shortcomings of traditional approaches, a novel thermally-actuated droplet manipulation platform based on using an inert liquid film was proposed and its working mechanisms were studied. Droplets at the air-liquid interface of immiscible liquids usually form partially-submerged lens shapes (e.g. water on oil). In the thermally-induced motion of droplets on the free surface of immiscible liquid films, lens-shaped droplets move from warm toward cooler regions. In addition to this structure, we showed that droplets released from critical heights above the target liquid can sustain the impact and at the end maintain a spherical ball-shape configuration above the surface, despite undergoing large deformation. It was discovered in this study that such spherical droplets migrate in the opposite direction to lens droplets when subject to a thermal gradient; i.e. direction of increasing temperatures. The existence of this metastable spherical state above the free surface and its transition into more stable lens configuration was investigated using optical diagnostic tools and theoretical analysis. Opposite direction of motion observed for droplets at the free surface of immiscible liquids was explained based on droplet shape at the interface and the dynamics of thin liquid films subject to lateral thermal gradients: mainly 1) deformation of the free surface and 2) development of an outward moving flow (hot to cold) at the free surface due to surface tension gradients caused by thermal gradients. A lens droplet moves due to the free surface flow caused by Marangoni convection which is from hot to cold. On the other hand, the spherical droplet moves towards the maximum depression on the free surface, occurring at the hottest region as a result of the balance between gravity and drag forces from the opposing free surface flow. The proposed theoretical models predict experimental observations of droplet motion due to thermal gradients satisfactorily. Opposite responses of thermally-induced motion of lens and spherical droplets on a thin liquid layer, were characterized experimentally and compared to theory by studying droplet motion in an exponentially-decaying temperature field maintained across the length of a shallow liquid layer. The effect of droplet size and magnitude of thermal gradient (slope) on drop velocity were investigated. The down-scaling effect is prominent, which shows that the proposed concept of droplet manipulation could be used favorably in miniaturized platforms. Based on the theoretical development and measurements obtained from meso-scale experiments, a silicon-based droplet transportation platform with embedded metal film micro heaters was developed. A thin layer of a chemically-inert and thermally stable liquid was chosen as the carrier liquid. Heaters were interfaced with control electronics and driven through a computer graphical user interface. By creating appropriate spatio-temporal thermal gradient maps, transport of droplets on predetermined pathways was demonstrated with a high level of controllability and speed.
Show less
Date Issued
2010
Identifier
CFE0003547, ucf:48908
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0003547