Micro-evolution

The Evolution of Methyl Iodide. The flask A (Fig. 89) is now heated with the non-luminous flame of the micro-burner. The immediate result of the heating will be an increase in the rate of bubbles passing up the absorption spiral no endeavour should be made to decrease this flow, however, as it will return to the original rate as soon as the hydro-... 

Place 0 -5 g. of 3 4 5 triiodobenzoyl chloride in a small test-tube, add 0 -25 ml. of the alcohol - ether and heat the mixture gently over a micro burner until the evolution of hydrogen chloride ceases (3-5 minutes). Pour the molten mass into 10 ml. of 20 per cent, alcohol to which crushed ice has been added. Some derivatives solidify instantly those which separate as oils change to solids in a few minutes without further manipulation. Recrystallise from rectified spirit (use 50 per cent, alcohol for esters of methyl and butyl carbitol ). 

The classical polarizing light microscope as developed 150 years ago is still the most versatile, least expensive analytical instrument in the hands of an experienced microscopist. Its limitations in terms of resolving power, depth of field, and contrast have been reduced in the last decade, in which we have witnessed a revolution in its evolution. Video microscopy has increased contrast electronically, and thereby revealed structures never before seen. With computer enhancement, unheard of resolutions are possible. There are daily developments in the X-ray, holographic, acoustic, confocal laser scanning, and scanning tunneling micro-... 

Kandlikar SG, Grande W (2002) Evolution of micro-channel flow passages - thermo-hydraudc performance and fabrication technology. In Proceedings of IMECE ASME International Mechanical Engineering Congress and Exposition, New Orleans, 17-22 November 2002, IMECE 2002-32043, pp 1-13... 

The bubble dynamics in a confined space, in particular in micro-channels, is quite different from that in infinity still fluid. In micro-channels the bubble evolution depends on a number of different factors such as existence of solid walls restricting bubble expansion in the transversal direction, a large gradient of the velocity and temperature field, etc. Some of these problems were discussed by Kandlikar (2002), Dhir (1998), and Peng et al. (1997). A detailed experimental study of bubble dynamics in a single and two parallel micro-channels was performed by Lee et al. (2004) and Li et al. (2004). 

Some Historical Information on Micro-reactor Evolution... 

Figure 1.12 Timetable for the micro-reactor evolution, marking important steps in the two last decades.

In the following, the impact of the micro-channel diameter on the temperature rise due an exothermic gas-phase reaction is investigated. For simplicity, a homogeneous reaction A —> B of order n with kinetic constant k is considered. Inside the micro channel, the time evolution of the radially averaged species concentration c and temperature T is governed by the equations... 

A multitude of information is supplied on the spatio-temporal temperature evolution at the outside of the reactor housing and via conduits even within the micro reactor (bottom) [15]. 

Figure 2.43 Model geometry for the CFD calculations on flow in curved micro channels (above) and time evolution of two initially vertical fluid lamellae over a cross-section of the channel (below), taken from [139].The secondary flow is visualized by streamlines projected on to the cross-sectional area of the channel. The upper row shows results for fC = 150 and the lower row for K = 300.

Figure 2.46 Section of a micro channel with electrodes embedded in the channel walls (left). When an electric field is applied along the channel, different flow patterns may be created depending on the potential of the individual electrodes. The right side shows the time evolution of an ensemble of tracer particles initially positioned in the center of the channel for a flow field alternating between the single- and the four-vortex pattern shown on the left [144].

Figure 2.48 The top shows the schematic design of a magneto-hydrodynamic mixer with equally spaced electrodes arranged In a micro channel and an external magnetic field oriented along the z-axis. On the bottom theoretical results for the evolution of two parallel liquid lamellae as a function of dimensionless time are shown [146].

In most cases the only appropriate approach to model multi-phase flows in micro reactors is to compute explicitly the time evolution of the gas/liquid or liquid/ liquid interface. For the motion of, e.g., a gas bubble in a surrounding liquid, this means that the position of the interface has to be determined as a function of time, including such effects as oscillations of the bubble. The corresponding transport phenomena are known as free surface flow and various numerical techniques for the computation of such flows have been developed in the past decades. Free surface flow simulations are computationally challenging and require special solution techniques which go beyond the standard CFD approaches discussed in Section 2.3. For this reason, the most common of these techniques will be briefly introduced in... 

Figure 3.42 Evolution of a pulse at the entrance of a micro channel for different diffusion coefficients. Calculated concentration profile (left) and cumulative residence time distribution curve (channel 300 pm x 300 pm x 20 mm flow velocity 1 m s f = 10 s) [27],...

Micro reactors allow the sequential injection of the reaction partners, rather than introducing them in one step. By this means, the course of the reaction, in particular concerning the dynamic evolution of the concentration of reactants, may be changed. It is hoped that this may impact on selectivity. 

Zhou et al. obtained nitrogen-doped titanium dioxide replicas via a two-step infiltration process with natural leaves as templates [220]. The replicas inherited the hierarchical structures of the natural leaf at the macro-, micro-, and nanoscales. These materials showed enhanced light-harvesting and photocatalytic hydrogen evolution activities. The photocatalytic water splitting activity of the artificial leaf structures was eight times higher than that of titanium dioxide synthesized without templates. 

---