I'm trying to build a device to deliver a N-drop (N * .05ML) of liquid via a tube into a container. The liquid might change from one application to the next, so it might be water in one, and oil in another.
I've been using an electric solenoid valve that uses a gravity fed vessel into the solenoid input, and controlling the solenoid with an arduino to release a drop from the output end of the solenoid. Depending on the viscosity of the liquid, I've been able to determine how long to open the valve for. 12.5 milliseconds for water gives me .05ML about 95% of the time, which is more than acceptable for my purposes.
The vessel which is to receive the liquid is below the solenoid,so I've been trying to use a gravity approach using plastic tubing. The problem I'm running into is that the path is not a straight line down. While there is always a downward slope, sometimes it is close to flat, so sometimes the drop just gets stuck in the tube.
I've tried tubing made of vinyl, silicone, of various diameters. When the N-Drop leaves the solenoid, it is released into a funnel connected to the tubing. I use the funnel because this prevents a vacuum forming which will hold the drop with a vice like grip. With smaller diameter tubing the N-Drop forms a "plug" and makes its merry way down the tube. When it gets stuck, another drop can push this one along. I can prime the tube with drops, but this isn't ideal as evaporation may come into play. With larger diameter tubing, instead of a plug, it's more like a blob inching it's way to the bottom. Once again, on the less steep portions of the tube, the blob sometimes just gets stuck.
There's not much I can do with making the slope steeper.
Can you think of a way of ensuring that the drop makes its way through the tube 90% of the time?