This page contains a Flash digital edition of a book.
BioMEMS | FEATURE


A Spotlight on BioMEMS


BOGDAN FIRTAT, MEMSOP BioMEMS are a special area of micro systems,


combining fluidic devices with micro nano systems. Modelling such devices, in the development phase, must account for all the phenomena from both worlds. Studying the fluid dynamics within the


biosensor is key to improving the functionality and optimising such devices.


CFD tools have been developed for answering questions about how fluids behave in different conditions. Problems from simple fluid flow to mixing, electrophoresis and fluid-structure interactions were solved using such software tools. However, during the miniaturisation to nano scale devices, it was found that the continuous modelling of fluids (as the CFD models) tend to break down and should no longer be taken for granted as the essential assumption for fluid dynamics. Several phenomena that take place at these small scales cannot be accurately solved, or even taken into consideration using the regular CFD models. Fluid flow and heat transfer at the nano scale is very different from the same phenomena at larger scale. This is due to several phenomena that also exist, but it can be easily neglected in the macro world.


Such phenomena include velocity slip, liquid viscosity variation, viscous dissipation, electrokinetic effects. Also, surface roughness (very important in nano channels, sometimes being comparable with the length scale of the channel itself ) cannot be neglected, as in the millimetre or micrometre sized channels.


At the nano scale, the characteristic dimension of the flow conduits are comparable to the mean-free-path of the fluid they operate in, leading to the breakdown of the ‘continuum hypothesis’.


Constitutive laws that determine stress tensor and heat flux vector for continuum flows have to be modified in order to incorporate the rarefaction effects. The usual ‘no-slip’ boundary conditions for velocity and temperature of the fluid in the walls vicinity have to be, therefore, modified. The deviation from continuum hypothesis is identified with Knudsen number, the ration of the mean-free- path of the molecules to a representative physical length scale (e.g. the radius of the body in a fluid). This type of movement is


<< Figure 2: Velocity magnitude simulation of a fluid through a shape-changing channel. >>


<< Figure 1: CFD analysis of an fluid's inertial movement within a container. >>


46 | commercial micro manufacturing international Vol 7 No.3


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52