Masahiro Motosuke

Noncontact Bubble Manipulation in Microchannel by Using Photothermal Marangoni Effect

A noncontact bubble manipulation by an optically induced local surface tension gradient is described in this paper. In microfluidic devices, the effects of interfacial phenomena become dominant with decreasing of the length scale. An appropriate control of the local gradient of the interfacial properties can provide a powerful method for bubble handling in the microfluidic system. Here, the photothermal technique is used to induce the gradient in surface tension around a microbubble. The thermocapillary force around the bubble is induced and controlled by local laser focusing. The bubble in a microchannel filled with silicone oil with varied viscosity was examined to verify experimentally the optical manipulation method of bubbles based on the photothermal Marangoni effect. As a result, three applications of bubble manipulation by using photothermal Marangoni effect are obtained: detachment from channel wall, trapping in liquid phase, and transportation in the channel. Additionally, minimum optical power for the manipulation was evaluated. The effects of bubble size, liquid viscosity, and irradiated optical power on the manipulation characteristics can be summarized by a dimensionless number that is a ratio of the thermocapillary force to viscous drag force for the bubble.

Vortex Behavior of Vertical and Inclined Synthetic Jets in Cross Flow at Low Reynolds Number

An experimental investigation to obtain the three-dimensional flow structure of the synthetic jet in low Reynolds number cross flow has been conducted for the purpose of examining the effective method of vortex generation for flow control. A synthetic jet is a promising technique to promote the mixing and to control the flow separation because it introduces flow perturbation without a net mass injection and the device can be easily downsized. Jet orifice diameter is 0.5 mm, and Reynolds number based on channel height is 650. The phaseaveraged three-dimensional flow structures of synthetic jet with the vertical and the inclined injection patterns were measured by scanning stereoscopic PIV. Then, it becomes clear that both injection patterns generate the hairpin vortex. The inclined injection pattern cannot generate the higher vorticity compared with the vertical one, but it is suitable for the control of flow separation because the hairpin vortex exists near wall region.

A Noncontact Picolitor Droplet Handling by Photothermal Control of Interfacial Flow

We present a noncontact handling of droplets in a microfluidic platform by the Marangoni convection, interfacial tension driven flow, generated by a light-induced local temperature gradient in the surrounding liquid of the droplet. Droplets flowing in a microchannel experience a force due to the interfacial tension gradient when approaching the heated area. This method provides noncontact, selective and flexible manipulation for droplets flowing in microchannel network. In this study, an O/W emulsion system with oleic acid for the dispersed phase and a buffer solution for the continuous one was used. Trajectory control and trapping for droplets with 5 - 65 pL in volume was achieved by patterned laser irradiation. Also, we quantitatively evaluated the driving force exerted on droplets by measuring the fluidic temperature distribution around the droplet. From the balance of the drag force and the photo-induced Marangoni force, the driving force was determined using the measured temperature gradient of the droplet. From the results, the applicability of noncontact droplet manipulation using the photothermal Marangoni effect by continuous-phase heating has been demonstrated.

Flow Behavior behind a Circular Cylinder by DBD Plasma Actuators in Low Reynolds Number

The aim of this study is to investigate on frequency shift of vortex shedding from a circular cylinder by introducing jet with dielectric barrier discharge (DBD) plasma. The electrode of DBD plasma was mounted on the circular cylinder. A constant AC wave with voltage of 2 kV and frequency of 10 kHz is applied to the electrode. The forward and backward jets to the main flow direction were induced by electrode arrangements. The timeaveraged velocity profiles and the vortex shedding frequency in the wake behind the circular cylinder were measured by I-type hot-wire anemometer at a Reynolds number of 1.0×10 3 . We discussed on the wake using the forward jets and backward jets by Hot-Wire anemometer and flow visualization. The vortex shedding frequencies are changed by the forward jet and backward jet of the plasma actuators. From the flow visualization, the frequency shift of vortex shedding is closely related to a length scale of the vortex induced by the forward and backward jet.

Three-dimensional flow characterization of a square array of multiple circular impinging jets using stereoscopic PIV and heat transfer relation

AbstractImpinging jets are one of the most industrially essential methods of cooling, for example, the cooling of gas turbine blades and electronic devices, drying or annealing of glasses. Usually, jets are configured for a specific purpose, but the flow tends to be very complicated as each jet interacts with the others, especially near the impingement surface. In the present study, complicated three-dimensional flow from a square array of circular impinging jets was revealed experimentally to investigate the effect on heat transfer characteristics. The flow fiel d was measured by scanning stereoscopic particle image velocimetry to confirm the detailed spatial features of the multiple circular impinging jets as nozzle-to-surface distance and jet-to-jet spacing were changed. Adjacent jets generated vortex rings and roll-up toward the nozzle plate, vortex rings and roll-up sizes changing depending on the experimental parameters. Differences in vorticity and dispersions of velocity from the jets were also observed. The temperature field of the impingement surface was measured using a thermosensitive liquid crystal technique. The spatial distribution of heat transfer coefficient was related to the flow field near the impingement surface.Graphical abstract

A Study of Periodic Flow Behavior over Backward Facing Step in Low Reynolds Number

A study on the low Reynolds number flow such as the flow in or around the downsized device is strongly required along with the development of the micro manufacturing technology. It is important to clarify the flow behavior of the separating shear layer in small channel or around small device. The low Reynolds number flow over a backward facing step is selected as an example of the recirculation vortex in the present study, because the mixing promotion is expected from an oscillatory motion of the vortex in the separating and reattaching shear layer by the step edge. The Reynolds number based on step height is set at 130 to 3700 in the micro sensing wind tunnel. The behavior of the near wall flow is measured by Micro Flow Sensor (MFS), and the flow field is visualized with a high-speed video camera. The results show that the frequencies of 36 Hz and 24Hz appear in the flow on the lower and upper wall downstream of the step. To identify the periodic motion near the wall, the dominant frequency of the separating shear layer is analyzed by the linear instability theory.

Vortex Interaction of In-line Synthetic Jets Injected at Different Phase in Low Reynolds Number Cross Flow

Synthetic jets are a promising technique to promote the flow mixing and to prevent flow separation, because they introduce periodic disturbance which is more effective for the flow separation control at low Reynolds number. Synthetic jets are also capable of decreasing a size of the actuator device. An experiment was conducted to study the three-dimensional flow structure and interaction of the two synthetic jets in low Reynolds number cross flow. Jet orifice diameter is 0.5 mm and Reynolds number of the cross flow based on channel height is 650. Stokes number based on the injection condition of synthetic jets is 4.6. The two jets were injected through the two orifices in the in-line configuration at the different injection phase angle where the downstream jet velocity is the same as the upstream one. The phase-averaged three-dimensional flow structure of the two synthetic jets was obtained by a scanning stereoscopic PIV system. Then, it is found that complicated vortex interaction introduces the transverse position reversal of the two jets due to the strong induced velocity by the longitudinal vortices

Classification of Vortex Regime and Three-dimensional Vortex Structure of Synthetic Jet in Quiescent Fluid

A synthetic jet is a promising technique to promote the mixing and to control the flow separation because of the periodic flow disturbance without a net mass injection. The synthetic jet has three types of vortex structures in quiescent fluid. An experimental investigation on the three-dimensional flow structure of the synthetic jet in quiescent fluid has been conducted in order to classify the regime of the vortex structure. Jet orifice diameter is 1.0 mm, Stokes number is 6.0a 12.7, and dimensionless stroke length is 0.25a 10. The phase-averaged three-dimensional flow structures of the synthetic jet were measured by scanning stereoscopic PIV. The vortex structure of the synthetic jet is changed according to Stokes number and stroke length. The parameter map for classification of vortex regime was obtained.

Development of Closed-loop Control System of Reattachment Flow in Low Reynolds Number - Validity of Sensor Position -

There is the separation and reattachment process generated in or around the fluid machinery, as a significant factor to affect the energy efficiency of the fluid machinery in the low Reynolds number. It is important to control the separating and reattaching flow in a backward facing step flow in low Reynolds number. In the present study, a synthetic jet is employed as actuator because the acceptability of the frequency in the separating shear layer was confirmed in the previous study. The orifice of the synthetic jet is placed discretely in the spanwise direction. The effect of the orifice pitch on the spanwise uniformity of the reattachment process is investigated. The normalized pitch by the orifice diameter ranges from 10 to 40. In the orifice pitch range of 10 to 30, it is confirmed that the flow of the reattachment point on the lower wall has the two-dimensional structure in spite of the threedimensional disturbance by the synthetic jets.

Total Temperature Measurement of Laminar Gas Flow at Microtube Outlet: Cooled From the Wall

This paper presents experimental results on heat transfer characteristics of laminar gas flow in a microtube with constant wall temperature whose wall temperature is lower than the inlet temperature (cooled case). The experiment was performed for nitrogen gas flow through microtubes with 163 and 243 μm in diameter and 50 mm in length. The gas was heated in an upstream section of the microtube to Tin ≈ 315 K, 335 K, and 355 K. The wall temperature was maintained at 305 K by circulating water around the microtube. The stagnation pressure was chosen in such a way that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure was fixed at the atmospheric condition. The total temperature at the outlet, the inlet stagnation temperature, the mass flow rate, and the inlet temperature were measured. The numerical computations based on the Arbitrary–Lagrangian–Eulerian (ALE) method were also performed for the same conditions of the experiment for validation of numerical results. The two results are in excellent agreement. The total and bulk temperatures obtained by the present study are also compared with the temperature of the incompressible flow.