Video: How to Prevent Chloroform From Dripping Out of Pipette Tips
This video demonstrates the dripping effects from halogenated solvents (in this case chloroform) and how to prevent this from happening.
The first thing to do to prevent the ink dropping out of the pipette before it is supposed to is to use the smallest size pipette tip that is available as it will have a smaller diameter aperture at the end and reduce dripping. It also helps to pipette the minimum amount of ink required for coating as that will reduce the effect of the weight compared to the surface tension (10 µl of solution will weigh half as much as 20 µl but the surface tension remains the same).
If the solvent is halogenated (such as chloroform) then it can also help to leave the pipette tip in the ink for a few seconds before removing it. The reason for this is that most pipette tips are made of polypropylene which will not dissolve in halogenated solvents but will swell slightly which will reduce the effective diameter of the aperture.
Note that this technique does not work for non-halogenated solvents such as acetone as they will not swell the polypropylene. As such, an alternative is to tilt the pipettor at an angle to reduce the weight to surface tension ratio, returning the pipettor to vertical only just before dispensing the solvent. Care must be taken not to tilt the pipettor to the extent that solvent will enter the pipettor and damage it.