Your right. The filler will stay dry in the center on a fresh carto and the rebuild ones, too. It has to do with how the filler is packed at manufacture and how it is installed during rebuild. There was just a thread in the e-cig general session where a poster named Niclic took a picture of a dual coil clear cartomizer probably on page 2or 3 by now). It took the recommended mls (3 I think) and then started to drip out the bottom but he could see that the middle was still dry(there is a picture he took showing this). 1 hr later, still dry. 3 hrs later, still dry, 5 hrs later, still dry but just barely. So it takes time for the carto filler to absorb the juice evenly throughout the carto and for some reason it's always the middle that ends up dry. When I use one of my rebuilds or a brand new one, I always fill it last thing at night and let it sit on end overnight now. I was filling the carto up and not getting a good hit out of it. I left it and went to bed and the next morning tried it again and it hit perfect. I have a theory that the filler is compressed when dry and when filled with juice it expands and returns to it's natural density and wicks properly over time. Once it's thouroughly wet it's a snap to top off and keep it wicked but the initial fill takes a whole lot longer to wick than thought. I've been doing some experiments of filling the cartos with 3 ml and then pulling the filler immediately to see whats going on in there. I want to get some clear stiff plastic and pull a gocart filler and insert it in the clear plastic or glass of the same size and fill it with juice and see what it does. As soon as I have some difinitive conclusions, I'll post them here.
Another GO-GO user TexasTumbleWeed (Not TxTumbleWeed) had mentioned a week or so back that thier use of blanks versus pre-filled showed the prefilled seem to hold refills better and so on. I couldn't resist so I pulled the filler on a brand new prefilled and the juice was pretty evenly distributed. On a new blank I had filled and left sitting for almost 2 hours was also pulled and the center was dry.
I am not into physics but it would seem to me based on what I just decribed as well as using both blanks I filled and pre-fills I refilled that the pre-fill seems to perform better and retain a better vape. That said, I have zero idea whether it is because the pre-fills are filled at the factory, vacuum sealed and have a long time to soak the juice into all the filler before they wind up on a users battery or not.
I am purely guessing here but it seems that unless the factory is loading these with some diferent juice (unlikely I think) that the wild card is the length of time it takes for the filler to get wet througout it's entirety. Obviously I have nothing to base that on but observation but if it turns out to be slow absorption rates for blanks, it will be interesting to see Mohawkz clear tube test.
I don't mind filling the blanks I have and let them soak for a few days before use but for the price differential, I think I'll stick with the prefills with a weak flavor that my juice can over ride until we can form some good idea on how long it takes to get the filler wet throughout.