The cause of solution dripping can be due to several factors, including: the syringe type being used, the presence of air bubbles, the viscosity of the solution, the density of solution, and the surface tension.
The most common cause of this occurrence is when a pressure difference between the inside of the syringe and the outside remains after the solution has been dispensed. This can happen when air bubbles are trapped inside the solution, resulting in a compression of the gasses during dispensing. After the pump has stopped, the compressed air expands and pushes solution out of the syringe. Similarly, for plastic syringes and pipes, elastic deformation. Over time, this can result in dripping of the solution.
This can be avoided by removing bubbles within the syringe before loading, using tubing and syringes that show limited amounts of elastic deformation, and reducing the overall pressures within the syringe. To avoid a build-up build up in the syringe lower dispense rates should be used, the viscosity of solution can be reduced, or the velocity of the solution can be reduced by increasing the diameter of pipes and needles.
Sometimes, dripping can be due to the use of dense solutions with low surface tensions and viscosities. The adhesive forces between i) the molecules in the solution, and ii) the surface of the needle and tubing can become lower than the gravitational force acting upon the solution within the tube. This results in the continuous dripping of solution, until the gravitational force acting on the solution in the tubing becomes lower than the combined adhesive forces of the solution and the increasing pressure difference due to the displaced volume.
To reduce the effects of dripping from dense, low surface tension solutions, a lower diameter of tubing and needles should be used. Alternatively, the tubing and needle can be tilted horizontally to reduce the gravitational force being exerted in the direction of the tubing.