Alexandru Herescu

A Theoretical Discussion of a Menisci Micropump Driven by an Electric Field

The potential for miniaturization of analytical devices made possible by advances in micro-fabrication technology is driving demand for reliable micropumps. A wide variety of micropumps exist with many types of actuating mechanisms. One such mechanism is electrohydrodynamic (EHD) forces which rely upon Coulomb forces on free charges and/or polarization forces on induced dipoles within the liquid to induce fluid motion. EHD has been used to pump liquid phases and to displace gas-liquid interfaces for enhanced boiling heat transfer as well as to displace gas/vapor bubbles. A novel concept for using EHD polarization forces to deflect a stationary meniscus in order to compress and pump a gaseous phase is described. The pumping mechanism consists in alternative compression of two gas volumes by continuous deflection of the two pinned menisci of an entrapped liquid slug in an electric field. Using the Maxwell stress relations, the electric field strength necessary to operate the pump is determined. The operational limits are determined by analyzing the stability limits of the two menisci from inertial and viscous standpoints, corroborated with the natural frequencies of the gas-liquid interfaces.

Film Deposition in Non-Wetting Tubes: An Experimental Film Thickness Law

Deposition of a liquid film on non-wetting tubular surfaces gives rise to the unexpected behavior of simultaneously coexisting thick and thin films. Experiments show that a discontinuity in the film thickness, a jump between the thick and thin films being laid from the meniscus towards the bounding moving contact line, occurs after a considerably thicker than the expected Bretherton film is deposited. Bretherton assumed the film to be uniform and, unlike the case of a non-wetting surface, the visco-capillary deposition process was not affected by the presence of a contact line. In reality this phenomenon lies at the confluence between a dewetting process and the deposition itself, being the result of the influence claimed by the dynamics of the zone adjacent to the moving contact line. The film thickness is calculated directly from the experimental data and a correlation is obtained by matching the measured and the theoretical shock velocities associated with the hydraulic jump. The non-wetting film is significantly thicker than Bretherton’s prediction and follows in turn a different law which is determined experimentally. The non-wetting film thickness is found to vary as Display FormulahR4/3, Display FormulahR being the Ca-dependent non-dimensional Bretherton film thickness.Copyright

Implications of Contact Line Dynamics on Taylor Bubble Flow Morphology

Film deposition experiments are performed in circular glass capillaries of 500 μm diameter. Two surface wettabilities are considered, contact angle of 30° for water on glass and of 105° when a hydrophobic coating is applied. It was observed that the liquid film deposited as the meniscus translates with a velocity U presents a ridge that also moves in the direction of the flow. The ridge is bounded by a contact line moving at a velocity UCL as well as a front of velocity UF , and it translates over the deposited stagnant film. The behavior of the ridge presents striking dissimilarities when the wettability is changed. Both UCL and UF are approximately twice as large for the non-wetting case at the same capillary number Ca. The Taylor bubbles forming due to the growth of the ridge are also differentiated by wettability, being much shorter in the non-wetting case. The dynamics of the contact line is studied experimentally and a criterion is proposed to explain the occurrence of a shock at the advancing front of the ridge. The hydraulic jump cannot be explained by the Froude condition of shock formation in shallow waters, or by an inertial dewetting of the deposited film. For a dynamic contact angle of θd = 6° and according to the proposed criterion, a hydraulic jump forms at the front of the ridge when a critical velocity is reached.© 2010 ASME

The Effect of Surface Wettability on Viscous Film Deposition

The viscous deposition of a liquid film on the inside of a capillary has been experimentally investigated with a focus on the relationship between the film thickness and surface wettability. With distilled water as a working fluid tests were run in a 622 microns diameter glass tube with contact angles of 30° and 105°, respectively. In the first set of experiments the tube was uncoated while in the second set a fluoropolymer coating was applied to increase the contact angle. A film thickness dependence with the contact angle θ (surface wettability) as well as the Capillary number in the form hR ∼ Ca2/3 /cosθ is inferred from scaling arguments. For partial wetting it may explain the existence of a thicker film for nonzero contact angle. It was further found that the non-wetting case of 105° contact angle deviates significantly from the existing theories, the film thickness presenting a weak dependence with the Capillary number. This deviation as well as the apparent non-uniqueness of the solution is thought to be caused by the film instability (rupture) observed during the tests. The thickness of the deposited film as a function of the Capillary number was estimated from the liquid mass exiting the capillary and the gas-liquid interface (meniscus) velocity, and compared to Bretherton’s data and a correlation proposed by Quere. The film thickness measurements as well as the meniscus velocity were determined with the aid of a Photron high speed camera with 10000 frames per second sampling capability coupled with a Nikon TE-2000 inverted microscope and a Precisa electronic balance.© 2009 ASME

Wetting Effects on Two-Phase Flow in a Microchannel

In the recent years there has been an increasing interest in the study of two-phase flows in low Bond number systems (where capillary forces are important relative to gravitational forces). Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. At the capillary scale, surface tension forces play an important role in two-phase flow regime transitions, pointing out the need to take into account the geometry of the cross section and the surface properties (wettability). Surface tension is generally considered in flow transitions, but the wetting properties of the fluid-surface material pairs (contact angle) are rarely given any importance. The researchers investigating two-phase flows should take extreme care when choosing the material of the test sections, as the flow morphology and the the pressure drop accordingly can vary widely with contact angle. In order to show these morphological changes high speed visualization experiments of air-water flow through 500 μm square and round microchannels were conducted. For the round channels, contact angles of less than 20° (wetting) and 105° (non-wetting) were investigated. For the square section, things are complicated by the presence of the corners. According to the Concus-Finn criterion, the liquid will wick into (wet) the corner if the contact angle is less then 45°, or will de-wet the corner if the contact angle is above 45°. A new case not previously mentioned in the literature arises for a contact angle of 45° ≤ θ ≤ 90°, for which the liquid is wetting the walls but dewetting the corners. Three contact angles of less than 20°, 80° and 105° are considered to investigate the possible morphologies in the square geometry. Images aquired with a high speed camera depicting the different flow morphologies that exist at the same air-water flow rates for each of the considered contact angle and geometry are presented.Copyright

Periodic Interface Destabilization in High-Speed Gas-Liquid Flows at the Capillary Scale

Recent research efforts have illustrated the importance of capillarity on the behavior of two-phase flow (gas-liquid) in low Bond number systems; that is, systems where capillary forces are important relative to gravitational forces. Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. High speed microscopy experiments visualizing air-water flow through a 500 micrometer square glass capillary, 10 cm long were conducted. The flow rates are significant with velocities of 6.2 m/s for the air and 0.2 m/s for the water. A unique annular flow with periodic destabilization of the gas-liquid interface has been observed. Standing waves develop on the liquid film and grow into annular lobes typical of that observed in low speed two-phase flow in capillary tubes. Atypical is the interface destabilization phenomena. The leading face of the lobe will decelerate and suddenly become normal to the wall of the square capillary while the trailing face of the lobe will remain gently sloped back into the annular liquid film. The transition between the leading and trailing faces acquires a sharp edge having a exceptionally large curvature. The entire structure then rapidly collapses and produces travelling waves which propagate upstream and downstream along the annular liquid film. The entire sequence of events takes approximately a half millisecond. This destabilization phenomenon is regular and periodic. Visualization of the destabilization from the high speed microscopy setup and preliminary analysis are presented.Copyright

Compressibility Effects in the Gas Phase for Unsteady Annular Two-Phase Flow in a Microchannel

High speed microscopy experiments investigating two-phase (gas-liquid) flow behavior in capillary-scale systems, that is, systems where capillary forces are important relative to gravitational forces, have revealed a unique unsteady annular flow with periodic destabilization of the gas-liquid interface. Standing waves develop on the liquid film and grow into annular lobes similar with those observed in low-speed two-phase flow. The leading face of the lobe will decelerate and suddenly become normal to the wall of the capillary, suggesting the possibility of a shock wave in the gas phase at a downstream location from the minimum gas flow section. Visualization of the naturally occurring convergent-divergent nozzle-like structures as well as a discussion on the possibility of shock wave formation are presented.Copyright