Fluid Dynamics in a Syringe I am currently working on a project where we use syringes to extrude some viscous materials. I will explain what I am trying to do and I just want to know what size of tubing would be better for me. I have a 10 cc syringe connected to 3/32" tubing with luer lock and at the very end, it is attached to either a 27 gauge or 30 gauge needle. The tubing is 24 inches. When I do this, it takes a lot of pressure to push the stuff out but when the stuff comes out, its not a continuous flow. What happens is it takes a lot of force to push but at some point, i will have pushed enough with my hand to make it come out but it comes out very fast and not in a continuous flow. If I make the tubing diameter larger, would the pressure inside be smaller even if I have the same 27 or 30 gauge needle at the very end.
Would I be able to have a more smooth/continuous flow? The reason I use such small needles is because I want to be able to draw a design with precision.
 A: I believe you want a smaller tube. The pressure has to reach a point where it is great enough to push through the needle. I larger tube has more space to fill, so you have to push the plunger farther, to build enough pressure, to push the fluid through the small needle. Pressure will build faster in a small tube, which means you do not have to push the plunger as far into the syringe to build the necessary pressure. It will likely come out faster and sooner, but should be more regular and predictable.
A: please check the syringe plunger tip (the black rubber thing on the end of the plunger shaft). In contact with certain fluids, the plunger tip tends to adhere to the syringe cylinder walls and break loose suddenly, then get stuck again, break loose, etc. as you push on it. the fluid then spurts out suddenly, stops, spurts, stops and so on. you can eliminate this temporarily by lubricating the rubber tip with petroleum jelly or sewing machine oil. I say temporarily because these lubricants can soften the rubber itself, making the problem worse with time. 
I recommend you try a ground-glass syringe; these have a precision-ground plunger made of glass which gets a good seal inside of the (glass) syringe body without a rubber tip. They do not exhibit the stick-slip-stick-slip behavior you describe, but are more expensive than disposable syringes.  
