Biophysical Tools - Expert in Microfluidic Flow Control and Fluid Dynamics

We can flexibly adapt the software of the system and implement a number of individual adaptations of our microfluidic devices.

Fast reaction of the pressure control system results in a more stable and constant flow, as small perturbations are compensated more swiftly.
For example FACS, if you want to sweep cells through the channel, stop them in the focus of the observing microscope, take measurements and remove them and inject the next cell.
You strongly accelerate the screening with high and low pressures.

If you wish to change the flow speed you want to know how long this may take before the new value becomes „stable“. What does „stable“ mean?
Maybe, once the measured value equals the desired value for the first or second time? This can easily be realized by tuning the system to be too „nervous“, i.e. with tendency to overshoot.

What is settling time?
Click here to find a good explanation on this very instructive wikipedia page

This may be good for the sales manager but overshoots may perturb your experiment significantly.
Furthermore, the settling time depends strongly on the tuning of the control loop and it is therefore quite arbitrary.
The engineer may tune the settling time to any value when disregarding the overshoot dynamics.

The ideal case would be that the observed quantity rises swiftly but smoothly until it reaches the new value and settles there. We could realize that with the pressure control system P2CS. Therefore we claim that the dynamics of P2CS relies uniquely on quantities as the compressibility of materials and air and the dead volumes of the valves and manifolds. Otherwise the system is optimal limited only by physical laws (causality).

Now the problem is how to characterize the transition time in a well defined manner: theoretically speaking the settling time is infinite in this case and of no use.
The solution is to characterize the pressure control system with the rise and fall time (see the next FAQ).

First: What is rise and fall time?
Click here to find a good explanation on this very instructive wikipedia page

The P2CS lacks any overshoot and the flow increases/decreases until it reaches the desired value. Therefore we cannot define an error band.

The rise/fall time, however, gives the delay between 10% and 90% of the pressure and flow speed difference. These values are fixed by convention.

In physics 1/e is often used to characterize the time scale for the exponential approach to the final value.

This time period can be obtained from the 10%-90% time perriod, or shortly 10%-value by t_1/e = t_10% / ln(10) (The numerical value of ln(10) ~2.3).

In conventional designs being on the market you observe a damped oscillation before the object stabilizes its position in the microscopy image.

This is a consequence of the classical control method called „PID“ (For more information about PID check this very instructive wikipedia page) and non-linear response function of the system.
If you choose the PID-parameters too tight, overshoots appear. This is similar to a harmonic oscillator with a too small damping constant.
If choosing the parameters so that the overshoot disappears, the system’s settling time becomes much more slowly – no choice to get around this limitation – its the law of physics!
Therefore we developed a new control method, which removes such artifacts and improves strongly the response time, for rising as well as falling edges.

As physicist I can say that this method presents the best possible allowed by hard physical laws (there is a hard limit because of a kind of uncertainty principle very similar to quantum mechanics).

You will be surprised of the result because it gives a feeling as if you pushed the cells with your own fingers directly.

You need it

• if you want to have a very fine control an stop objects (e.g. cells or molecules) and at the next moment remove them fast and get the next object, or,
• if you want to study the effect of shear flow over a flow range of 6 orders of magnitude
• if you are working with small channels of very different lengths or sizes
• if you mix large and very small channels (meso- and microfluidics)
• if you need to remove debris of dead cells or dust from your channel and you do not want to remove the chip from the set-up

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