Exercise 2 – Diffusion
Questions:
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What is the diffusion coefficient?
What does Ficks law describe?
Consider the microfluidic channel above. The temperature is 20°C (293 K). The colouring of the water in the channel
is achieved using small molecules (such as rhodamine or anthocyanin) of radius r≈1nm.
a) Calculate the time it would take to have both coloured mixed.
a. η(water at 20°C)=10-³ kg/ms
b. kB=1.38 *10-23 m² kg/s²K
b) The flow speed is 400 μm/s along the channel. Calculate the distance in the microfluidic channel at which
both colours would be mixed.
Consider the microfluidic diffusive mixer above. The temperature is 20°C. Calculate the average time it will take
different particles to diffuse across the red middle stream for:
a) a protein (r = 5 nm)
b) Trifluoroethanol (r = 2A)
c) Rhodamine (r = 1 nm )
Assuming a flow speed of 400 μm/s, calculate the distance in the microfluidic channels these times correspond to.
Question: How can the mixing be improved/accelerated in terms of channel length? What role does the Reynolds
number play here?