Micro-channels mixing time

The mixing process was experimentally studied in mini-channels manufactured by traditional mechanical machining as well as micromachined channels. In the characterisation it was shown that mixing is possible by the introduction of secondary flow or local turbulence in miniaturised channels at high flow rates. In both minichannels and micro-channels, mixing by diffusion was observed at low flow rates when the transit time through the channel exceeded 20 seconds. 

P 33] The catalyst bed was manually positioned in the micro channel (300 pm wide 115 pm deep) at room temperature using a 10% (v/v) solution offormamide and potassium silicate [6, 7]. Micro reactor bottom and top plates were thermally bonded thereafter. Then, 75% THF (aqueous) solutions of 4-bromobenzonitrile and phenylboronic acid having equimolar concentrations were placed in the two reservoirs of a micro-mixing tee chip. In the collection reservoir, 30 pi of the THF solution was placed. Voltages ranging from 100 to 400 V were used, but kept constant only for one reservoir. The other one was switched on and off at 200 V for given time periods. 

GL 22] [R 3] [R 9] [R 10] [P 23] The mass transfer efficiency of different gas/liquid contactors as a function of residence time was compared qualitatively (Figure 5.29), including an interdigital micro mixer, a caterpillar mini mixer, a mixing tee and three micro bubble columns using micro channels of varying diameter [5]. 

Table 1.3 Liquid mixing times for T-channel micro mixers of varying hydraulic diameter [68].

Channel width, depth (fan) Hydraulic diameter of the micro mixer (fan) Reynolds number Mixing time (ms)... 

The bended micro channels had a width of 180 pm and a depth of 25 pm and a reduced length (25 mm) compared with the mini channels [151]. The flow in such channels was characterized at two very low Re (1.0 and 0.1) and compared with the flow in straight channels under some hydrodynamic conditions. In all four cases, undisturbed laminar flow was found. Mixing was only detectable at Re = 0.1 owing to diffusion mixing at a much prolonged residence time. At Re = 1, no mixing could be detected. 

By fluorescence monitoring of the concentration profile along the channel width in a special microfluidic chip, it could be shown that mixing times may be of the order of several tens of seconds for low molecular weight molecules and of several hundred seconds for high molecular weight molecules in aqueous solutions [161], This fits expectations from simple calculations on the rate of diffusion. Therefore, a micro mixer is required. 

Mixing by inertial forces has been demonstrated using a 80 zigzag micro-channel (300 pm x 600 pm x 1 mm). The mixing time achieved in this case is 1 second at the Reynolds number of 33 [17]. 

A micro-reactor is generally defined as a device consisting of a number of interconnecting micro-channels in which small quantities of reagents are manipulated, mixed and allowed to react for a specified period of time (Ehrfeld et al., 2000 Wirth, 2008). The movement of fluids within such a device can be achieved in a number of ways with the most common being mechanical micro-pumping and electro-osmotic flow, which may include electrophoresis separations. The typical cross-sectional dimensions of such micro-channels are in fhe range of 10-500 pm and are normally fabricated on the planer surface of substrates... 

It is well known that the flow within micro-channels is restricted to diffusive mixing under laminar flow conditions. Based on Pick s law, the relationship between the travel distance (L) of a molecule by diffusion and time (t) can be simplified as L = (2D tf where D is the diffusion coefficient. 

If the thickness of the lamella corresponds initially to the half diameter of the micro-channel i.e. do=dtl2, we obtain the following relationship for the characteristic mixing time ... 

Interdigital micro mixers comprise feed channel arrays which lead to an alternating arrangement of feed streams generating multi-lamellae flows [39 2]. If processes have to be carried out with extended residence times (> 1 s) and/or at a temperature level different from the mixing step, tubes have to be attached to the interdigital micro mixers. Their internals comprise millimeter dimensions or below, if necessary. 

M 3] [P 3] Fluorescence images at various times were taken in the main channel, i.e. along the direction of the electric field, of the first-generation micro mixer [25, 93]. After a period of 2 s, the flow becomes unstable and transverse velocities stretch and fold material lines in the flow. The initial seeded/unseeded interface becomes rapidly deformed. Finally after about 13 s, a random distribution of the tracer transverse to the applied AC field is achieved. EKI action is visible throughout the whole channel length of 7 mm. Thus, feasibility of EKI action for micro mixing has been demonstrated. 

At a certain point they merge and mixing takes place by diffusion at constantly recreated and renewed interfaces, (b) A similar flow arrangement with square bed objects however, this time a micromachined packed bed is shown, (c) Artificial new structure of a packed bed where some objects of the packed bed have been removed. Now, flow conduits exist which are intended to promote lateral mixing, (d) Bimodal intersecting channel micro mixer, which is a micromachined design resembling the idea depicted in (C) [160] (by courtesy of ACS). 

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