The propagation of laser beam in a flow aligned nematic liquid crystal (NLC) and its interaction with liquid are illustrated in this letter. The effect of polarization and scattering on the transmitted power through the NLC under external perturbation flow is demonstrated here. It is found that the flow rate has a significant role in the modulation of refractive index of the medium leading to scattering and change in polarization.
In this paper, we report the dynamic behavior of a ferro fluid
droplet in time-dependent magnetic fields. The magnetic field was
generated by an array of planar coils, which were fabricated on a
double-sided printed circuit board (PCB). The permanent magnetic
moment of the ferro fluid droplet was created using the field of a
pair of permanent magnets. The motion of the ferro fluid droplet
is further aligned with the permanent magnetic field of a pair of
planar coils. Two other in serial connected planar coils on the
other side of the PCB displaces the droplet along a line. The
direction of the droplet motion can be controlled by switching the
electric current passing through them. Different paprameters
affecting the motion of the ferro fluid droplet such as the
droplet size, the viscosity of the surrounding medium, the
electric current, and the switching frequency were investigated.
This paper reports the results of theoretical and experimental
investigations of reciprocating thermocapillary motion of a liquid
plug in microchannels. A one-dimensional analytical model for the
transport of micro plugs in a capillary was established. The model
consists of a system of two transient one-dimensional equations:
one for temperature spreading in the capillary wall and one for
the dynamics of surface tension driven movement of the plug.
Surface tension depends strongly on temperature. Thus, a transient
temperature distribution leads to a gradient of surface stress
across a liquid plug. This surface stress difference leads to the
movement of the liquid plug. For the experimental investigation
two heaters were used for the periodic temperature gradient. Each
of the heaters was activated alternatively to induce the
reciprocating motion of the liquid plug. For quantitative
evaluation, the position of the plugs was captured and evaluated
with a CCD camera. This paper focuses on analysing the results of
this motion at different switching frequencies. The results show
that the motion of the plug exhibits a chaotic characteristics at
high switching frequencies. This actuation concept has potential
applications in post-processing stages for droplet-based
microfluidics. The chaotic motion can be explored for efficient
mixing in microplugs.
The chemical warfare agent Sarin is an organophosphate that is highly toxic to humans as they can act as
cholinesterase inhibitors, that disrupts neuromuscular transmission. As these nerve agents are colorless, odorless
and highly toxic, they can be introduced into drinking water as a means of terrorist sabotage. Hence, numerous
innovative devices and methods have been developed for rapid detection of these organophosphates. Microfluidic
technology allows the implementation of fast and sensitive detection of Sarin. In this paper, a micro-total analysis
systems (TAS), also known as Lab-on-a-chip, fitted with an optical detection system has been developed to
analyze the presence of the nerve agent sarin in water samples. In the present set-up, inhibition of co-introduced
cholinesterase and water samples containing trace amounts of nerve agent sarin into the microfluidic device was
used as the basis for selective detection of sarin. The device was fabricated using polymeric micromachining
with PMMA (poly (methymethacrylate)) as the substrate material. A chromophore was utilized to measure
the activity of remnant cholinesterase activity, which is inversely related to the amount of sarin present in the
water samples. Comparisons were made between two different optical detection techniques and the findings
will be presented in this paper. The presented measurement method is simple, fast and as sensitive as Gas
Active control of microdroplets in microchannels is an important task in droplet-based microfluidics. The break-
up process of droplets at an T-junction is usually controlled passively by the fluidic resistance of the branches.
We used thermal control to actively manipulate aqueous droplets in microchannels. The temperature affects both
viscosity and interfacial tension between the phases. The concept was first simulated with a two-dimensional
model. The simulation results show that increasing temperature at a branch can change the size ratio of the
two daughter droplets from 0 to 1. That means, droplet switching is possible with this concept. Control of
droplet size during the formation process and splitting process was demonstrated experimentally by varying the
temperature of the branches. At a critical temperature, droplet switching can be achieved. The used control
temperature of less than 40°C shows that this active control concept is suitable for biochemical applications.
Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.
This paper reports a new mixing concept in microscale using hydrodynamic focusing and sequential segmentation. Both focusing and segmentation were used in the present study to reduce mixing path, to shorten mixing time, and to enhance mixing quality. Transversal mixing path is reduced by hydrodynamic focusing, while sequential segmentation shortens the axial mixing path. Assuming the same viscosity in the different streams, the focused width can be adjusted by the flow rate ratio. The axial mixing path can be controlled by the switching frequency of the inlet valves and the mean velocity of the flow. Both flow rate ratio and pulse width modulation of the switching signal can adjust the desired mixing ratio. This paper first presents a time-dependent two-dimensional analytical model for the mixing concept. This model considers an arbitrary mixing ratio between solute and solvent as well as the axial Taylor-Aris dispersion. A polymeric micromixer was designed and fabricated by CO2 laser micromachining and hot lamination. Sequential segmentation was realized by two piezoelectric valves. The sheath streams for hydrodynamic focusing are introduced through other two inlets. We also designed a measurement system that can synchronize of the mixer's switching signal with the camera's trigger signal. The system allows our relatively slow and low-resolution CCD camera to freeze and to capture a large transient concentration field. The concentration profile along the mixing channel agrees qualitatively well with the analytical model.
This paper reports on a hybrid polymeric microfluidic device with optical detection for droplet-based systems. The optical part of the device is integrated by a hybrid concept. The microfluidic structures were fabricated using CO2 laser on PMMA (poly methylmethacrylate) substrate. The microfluidic network consists of two microchannels for forming droplets of an aqueous liquid in an immiscible carrier liquid. The optical component consists of two optical fibers for guiding laser light from the source, through the detection point, to a photo diode. The formed droplets pass the detection point and diffract the incoming laser light. The detected signal at the photo diode can be used for evaluating droplet size, droplet shape, and droplet formation frequency. The device can detect very high formation frequencies, which are not detectable using a conventional CCD camera/microscope setup.
Microfluidics has become one of the most intense research fields in MEMS technology. In this paper, the use of a 1064nm Q-switched solid state laser to fabricate micro-pump cavities in copper is presented. The focusing technique is employed fore directly structuring the micro-pumps and mixers. In this case, a laser beam with a focal spot of 50 micrometers is canned over the surface and the substrate material is ablated track by track and layer by layer. Machining results such as surface finishing and dimensional resolution are discussed. The dependence of the ablation depth, ablation rate and surface roughness on the process parameters and on the scan overlap are investigated. The laser micromachined structures are free of cracks and without any deposition of debris on the surface. The assembly and first characterization results of the pumps are reviewed. The capability as well as the potential of laser micromachining are also discussed.
This paper presents a high performance micropump based on the printed circuit board technology. This pump was our first effort to lower the packaging cost by combining functional elements (diffuser/nozzle) with packaging materials (PCB, inlet/outlet tubes). The paper presents in details the numerical simulation, the fabrication and the experimental characterization of the pump. Experimental results have shown a high pump performance. A flow rate of 3 ml/min can be achieved with a drive voltage of 120V. Pumps with different distances between inlet and outlet were tested. Characterization results agreed well with simulation results. Pumps with a short distance between inlet and outlet have a better performance.
Based on an article in print this paper presents a hybrid assembled bi-directional micro dosing system for a water flow range of -40 (mu) l/min to 80 (mu) l/min. The system consists of a silicon micropump/valve chip (9 mm X 9 mm) and a silicon flow sensor (6 mm X 12 mm). The valve/pump can be driven by either a piezoelectric disk or an electrostatic actuator. Both, piezoelectric and electrostatic actuation for the pump/valve, the technology of each component and the hybrid assembling of the whole system are described. Results of transient numerical simulation of the pump and the mass flow sensor are presented and compared with experimental results. Descriptions of two different operational modes of the micro dosing system are given. The new pulse-width modulated control method for the actuator makes controlling the system easier. It allows an open-loop control of the pump rates without changing the driving frequency. All reported micropumps were driven by a square-wave signal which causes a relatively high noise level. In contrast to this, sawtooth and sinusoidal signals generate a smooth and quiet operation because of the small drag force on the fluid ports. Results of different driving methods are presented and compared.