Why is Kapton film used to hold liquid in waveguide?
Here a Kapton film is used to hold liquids, so that they don't spill.  Why is this so?


Liquids flow to take the shape of the container you put them in. They need to use something to hold the liquid in a specific area of the waveguide structure. It happens they chose a kapton film.


  Does this film act as perfect conductor at microwave frequencies ?


It had better not. If it were a perfect conductor it would block all rf signals from passing from the feed waveguide to the sample chamber. 

Ideally, the film should act as a perfect dielectric. 


  Won't it affect the measured S-parameters ?


Unfortunately there is no perfect dielectric. However, the thickness of the film is likely quite small, compared to the sample chamber dimensions and also  so that even if it has some loss it won't significantly affect the measurement results.

It appears they did their measurements at around 12 GHz, corresponding to a wavelength of 25 mm. Kapton films are readily available with thickness less than 0.5 mm, so the effect of the film on the system should be minimal.