1.  Field of the Invention
The invention relates to a micromixing chamber.  The invention also relates to a micromixer comprising a plurality of such micromixing chambers connected fluidically in series.  The invention further relates to a method for manufacturing such a micromixing chamber, and a method for manufacturing such a micromixer.  The invention also relates to a method for mixing by means of such a micromixing chamber, and a method for mixing by means of such a micromixer.  In the context of the invention ‘micromixing chamber’ and ‘micromixer’ are understood to mean: ‘microstructural mixing chamber’ and ‘microstructural mixer’, wherein ‘microstructural’ is defined within the context of the present invention as: comprising at least one essential element or essential formation characterized by the very small size thereof, in particular within the range of 10−3 to 10−7 meter.  The invention can advantageously be applied particularly in the field of microfluidics, in which the flows are generally of laminar nature. 
2.  Description of Related Art
Microfluidics is concerned with microstructural devices and systems with fluidic functions.  This may relate to the manipulation of very small quantities of liquid or gas in the order of microliters, nanoliters or even picoliters.  Important applications lie in the field of biotechnology, chemical analysis, medical testing, process monitoring and environmental measurements.  A more or less complete miniature analysis system or synthesis system can herein be realized on a microchip, a so-called ‘lab-on-a-chip’, or in specific applications a so-called ‘biochip’.  The device or the system can comprise microchannels, mixers, reservoirs, diffusion chambers, integrated electrodes, pumps, valves and so forth.  The microchip is usually constructed from one or more layers of glass, silicon or a plastic such as a polymer.  Glass in particular is highly suitable for many applications due to a number of properties.  Glass has thus been known for many centuries and many types and compositions are readily available at low cost.  In addition, glass is hydrophilic, chemically inert, stable, optically transparent, non-porous and suitable for prototyping; properties which in many cases are advantageous or required. 
In many fluidic devices one or more volumes or flows have to be mixed.  The Reynolds number, which indicates the ratio between the occurring inertia forces and viscous forces, will generally be so low in microfluidic devices, usually a maximum of about 500, that we are dealing with laminar flow and turbulence cannot be achieved, so that in principle mixing of flowing volumes does not occur.  In order to nevertheless bring about mixing it is possible to make use of active or passive mixing.  Active or dynamic micromixers comprise moving parts which set the relevant media into motion, although this is also possible by applying for instance pressure differences or with ultrasound.  Such mixers are however complex and often difficult to make, and therefore expensive.  In passive or static micromixers flows are ‘folded and deformed’ by opting for a determined geometry and specific dimensions of the channels, tunnels, passages and so on such that the interfaces between volumes are enlarged.  The diffusion areas will thus be enlarged and the diffusion distances will decrease, whereby mixing by diffusion is more likely.  The flows can here for instance be split, rotated and subsequently recombined, see for instance WO 2005/063368.  Diffusion can also be enhanced by bringing about a transverse flow component, i.e. perpendicularly of the main direction of a flow, by means of grooves or protrusions arranged for this purpose in the wall of a microfluidic channel, see WO 03/011443.  Many more other embodiments of passive or static micromixers are thus known, to be found for instance in patent documents classified in B01F13/00M (European classification). 
Design variables in passive or static micromixers are the geometry and the dimensions of channels, tunnels, passages and so on.  Together with the properties of the media and components involved (viscosity, density and diffusivity) and the flow rate, these determine the pressure drop over the mixer, the values of the Reynolds number, the flow regime, the values of the Peclet number, the mixing regime, the efficiency (mixing achieved), the speed (time required), the number of mixing elements required and the necessary volume or area (‘footprint’).  It is possible to attempt to achieve a better mixing by operating at higher Reynolds numbers greater than 500, but it will usually then be no longer possible to meet stricter specifications in respect of pressure drop, speed, volume and footprint.  A micromixer is thus described in WO 2004/054696 which comprises a first mixing chamber and a second mixing chamber which are mutually connected by means of a connecting channel which is relatively narrow and long in relation to the chambers.  The liquid is caused to flow tangentially via a feed channel into the first mixing chamber and to flow tangentially via a discharge channel out of the second mixing chamber such that a circulating, more or less planar flow is created in each chamber, wherein the flow directions are opposed in the two mixing chambers.  This can result in a good mixing but, due to the relatively wide and low mixing chambers and due to the relatively narrow and long connecting channel, the pressure drop over such a micromixer is great, as are the required footprint and the total volume of the micromixer.  US 2006/079003 specifies a conical mixing chamber which tapers in the flow direction and in which a flow is created in the form of a narrowing helix.  The thus achieved mixing is however found to be too limited for many applications. 
There therefore exists a need for an improved passive or static micromixer with a higher efficiency, a higher speed, a small number of required mixing elements, a smaller volume and footprint, and a lower pressure drop than the usual micromixers.  This is preferably compatible here with known microfluidic devices and can be manufactured from materials usual for the purpose, such as glass, preferably by means of techniques usual in the relevant field, such as powder blasting, etching and bonding.  The object of the invention is to fulfil this need.