Chih-Ming Ho

Subharmonics and vortex merging in mixing layers

In the present study, it is shown that the spreading rate of a mixing layer can be greatly manipulated at very low forcing level if the mixing layer is perturbed near a subharmonic of the most-amplified frequency. The subharmonic forcing technique is able to make several vortices merge simultaneously and hence increases the spreading rate dramatically. A new mechanism, ‘collective interaction’, was found which can bypass the sequential stages of vortex merging and make a large number of vortices (ten or more) coalesce. A deeper physical insight into the evolution of the coherent structures is revealed through the investigation of a forced mixing layer. The stability and the forcing function play important roles in determining the initial formation of the vortices. The subharmonic starts to amplify at the location where the phase speed of the subharmonic matches that of the fundamental. The position where vortices are seen to align vertically coincides with the position where the measured subharmonic reaches its peak. This location is defined as the merging location, and it can be determined from the feedback equation (Ho & Nosseir 1981). The spreading rate and the velocity profiles of the forced mixing layer are distinctly different from the unforced case. The data show that the initial condition has a longlasting effect on the development of the mixing layer.

Dynamics of an impinging jet. Part 1. The feedback phenomenon

In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle ( x 0 / d

Chaotic mixing in electrokinetically and pressure driven micro flows

We present two micro-devices, fabricated by using MEMS technology, in which mixing of fluid and particles takes place. The systems are designed to induce folding and stretching of material lines, leading to chaotic-like mixing. In a first case, we use unsteady pressure perturbations superimposed to a mean stream, and in the second case, time-dependent dielectrophoretic forces to induce folding and stretching. The first device shows chaotic-like mixing is achieved in an efficient way, leading to rapidly homogenizing concentration fields. Folding and stretching effects inducing mixing are shown for the second system. The systems are simple in their conception and may favorably be integrated within complex bio-MEMS.

IC-integrated flexible shear-stress sensor skin

This paper reports the successful development of the first IC-integrated flexible MEMS shear-stress sensor skin. The sensor skin is 1 cm wide, 2 cm long, and 70 /spl mu/m thick. It contains 16 shear-stress sensors, which are arranged in a 1-D array, with on-skin sensor bias, signal-conditioning, and multiplexing circuitry. We further demonstrated the application of the sensor skin by packaging it on a semicylindrical aluminum block and testing it in a subsonic wind tunnel. In our experiment, the sensor skin has successfully identified both the leading-edge flow separation and stagnation points with the on-skin circuitry. The integration of IC with MEMS sensor skin has significantly simplified implementation procedures and improved system reliability.

Deformation of DNA molecules by hydrodynamic focusing

(Received 23 July 2002 and in revised form 1 July 2003) The motion of a DNA molecule in a solvent flow reflects the deformation of a nano/microscale flexible mass–spring structure by the forces exerted by the fluid molecules. The dynamics of individual molecules can reveal both fundamental properties of the DNA and basic understanding of the complex rheological properties of long-chain molecules. In this study, we report the dynamics of isolated DNA molecules under homogeneous extensional flow. Hydrodynamic focusing generates homogeneous extensional flow with uniform velocity in the transverse direction. The deformation of individual DNA molecules in the flow was visualized with video fluorescence microscopy. A coil–stretch transition was observed when the Deborah number (De) is larger than 0.8. With a sudden stopping of the flow, the DNA molecule relaxes and recoils. The longest relaxation time of T2 DNA was determined to be 0.63 s when scaling viscosity to 0.9 cP.

A flexible micromachine-based shear-stress sensor array and its application to separation-point detection

Abstract A new microfabrication technique has evolved and has been applied to the development of a flexible shear-stress sensor array. The flexible sensor array is composed of many silicon islands that are interconnected by two layers of polyimide film. The silicon islands are formed by RIE etching and contain individual thermal shear-stress sensors. Each sensor on the flexible sensor array has been proven to behave the same as those on a rigid substrate. There are more than 100 sensors distributed inside a 1 cm×3 cm area. The flexible sensor array has a thickness of 80 μm and can be easily attached on a highly curved surface to detect shear-stress distribution. Applications of the flexible sensor array to flow separation-point detection have been demonstrated in this task, including flow over a circular cylinder and instantaneous separation line detection on the rounded leading edges of a delta wing.

Micromachined membrane particle filters

Abstract Particle membrane filters (8×8 mm2) with circular, hexagonal and rectangular through holes are designed, fabricated and tested. By varying hole dimensions from 6 to 12 μm, opening factors from 4 to 45% are achieved. In order to improve the filter robustness, a composite silicon nitride/Parylene membrane technology is developed, and the burst pressure of the filters is increased more than 4 times. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical calculation using the Navier–Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified.

A Micromachined Permalloy Magnetic Actuator Array for Micro Robotics Assembly Systems

Arrays of permalloy magnetic actuators have been studied for the use as precision micro robotics assembly systems. The actuator arrays have been tested for lifting and moving silicon and glass chips. The actuator unit consists of a permalloy plate 1 mm x 1 mm X 5/spl mu/m in size together with polysilicon bending supports. Experimentally, it can lift a 87 /spl mu/N (or 8.88 mg) force under a magnetic field of approximately 2 x 10/sup 4/ A/m. A proposed synchronous driving mode has been observed, and both translation and rotation of a silicon chip has been demonstrated.

Micro heat exchanger by using MEMS impinging jets

A micro impinging-jet heat exchanger is presented. Heat transfer is studied for single jet, slot arrays and jet arrays. In order to facilitate micro heat transfer measurements with these devices, a MEMS sensor chip, which has an 8/spl times/8 temperature-sensor array on one side, and an integrated heater on the other side has been designed and fabricated. This sensor chip allows 2-D surface temperature measurement with various jets impinging on it. It is found that micro impinging jets can be highly efficient when compared to existing macro impinging-jet microelectronics packages such as IBM 4381. For example, using a single nozzle jet (500-/spl mu/m diameter driven by 5 psig pressure), the sensor chip (2/spl times/2 cm/sup 2/) temperature can be cooled down from 70 to 33/spl deg/C. The cooling becomes more efficient when nozzle arrays (4/spl times/5 over 1 cm/sup 2/ area) are used under the same driving pressure. Interestingly, although higher driving pressure gives better cooling (lower surface temperature), the cooling efficiency, defined as h/0.5 /spl rho//spl nu//sup 2/, is actually higher for lower driving pressure.

A MEMS thermopneumatic silicone rubber membrane valve

Abstract In this paper, a technology for fabricating silicone rubber membranes and integrating them with other processes on a silicon wafer has been developed. Silicone rubber has been found to have exceptional mechanical properties, including low Youngs modulus, high elongation, and good sealing. An integrated normally open valve using a silicone rubber membrane and 3M PF5060 liquid for thermopneumatic actuation has been fabricated and tested. For a 1.34 LPM (1 min −1 ) air flow, 280 mW power input is required to close the valve at 20 psi inlet pressure. Due to the high permeability of silicone rubber, most liquids used in thermopneumatic systems will be lost, necessitating more work to find a suitable barrier material compatible with silicone rubber.