PDMS density

the density to be less than water. I've tried with above product and after it's cured, it floats on water due surface tension, but will sink if submerged. The data sheet from Dow Corning also said that Sylgard 184 has specific gravity of around 1.03.

make Sylgard 184 less dense than water?

Possibly using an additive with a lower specific gravity that you can tolerate. I google that there are some hollow glass beads with low spg although most are above 1.

I've used glass microspheres in suspension in plastisol and I was never happy with the resulting buoyancy. I always felt that to many of the sphere burst at the working temperature of the plastisol. 300-350F. When I had the patience to slowly kick the plastisol, and then add microspsheres at below the kick temperature I got only slightly better results. They are also very hazardous to work with. They are very light and will waft across the room at the lightest breeze. Even the air from my respirator discharge was enough to cause them to waft around a little bit.

Where glass microspheres are used routinely is in fiberglass resin as a filler to make a glue paste to fillet joins. They are much lighter than chopped glass, and the result is less prone to hold moisture like wood flour.