Subject: Fluid Flow and the Viscosity of Wallpaper paste

I am doing an alevel investigation into fluid flow and the viscosity of 
wallpaper paste. I have made up pastes of different thicknesses and have 
measured the time taken for the fluids to move down a tube. I have found some 
results that I can't quite explain myself and I was hoping that you could help. 
Firstly a thin tube caused the fluid to move slower down it than a thicker tube 
of the same length, also I know why fluid in the middle of a pipe moves quicker 
than that of the sides however for the more viscous liquids once the majority 
of the fluid has moved out of the middle of the tube some of it is still left 
at the sides and this takes alot longer than the rest of the fluid to move down 
the pipe, is this because of the amount of the fluid left and is there a 
formula that relates the thickness of the layer of fluid left and its rate of 
flow.
Also I discovered that after adding a certain amount of wallpaper paste powder 
to the water,adding more didn't have as much effect on the viscosity of the 
fluid. I was wondering whether this was because the water had become saturated 
with the powder or if it was something to do with the way the powder bonds with 
the water in order to form a paste. I would really appreciate it if you would 
be able to answer my questions, thanks

Re: Fluid Flow and the Viscosity of Wallpaper paste