Microstrip patch design

I am using water as patch which is enclosed by plexiglass. what type of feed should I use to excite it?
If I use coax feed, can i make contact of the coax pin with water?

Depending on the conductivity, water can be expected to act more as a dielectric than a conductor. But it's probably very lossy in both roles. What do you want to achieve?

I want to design a transparent antenna for mobiles. so I am using water patch. i referred the following paper. http://ieeexplore.ieee.org/document/7987716/
can i use coaxial feed instead?

I had a quick look at the article and I think it is not a typical patch antenna. The water "patch" serves as a reflecting layer (due to high dielectric contrast between air and water) for the waves emanating from the feeding probe. So the probe is not generating conductive currents on the patch (as for the case of a standard patch). Therefore, there is no reason to make physical contact between the probe and the water. It might have impact on antenna parameters, but not on operating principle.

Why don't you use the same feeding as described in the paper?

Yes I can use the same feed. But since I am designing it for mobiles, I am not sure what dimension is to be applied to such a feed and mainly the disc load mentioned in that paper.