Liquid Climbs Needle
When a pendant drop for IFT measurements climbs the dispense needle, you have several choices:
The easiest and best is to use a larger diameter needle (smaller gauge number), rather than switching to a different needle material. For a listing of commercial stainless steel needles of different diameters, see the Contact East webpage for Kahnetics.
There are few suppliers of Teflon coated stainless steel needles. The ones we know make needles in limited diameters and generally do not have Luer hubs. A commercial product called Nyebar can be used to dip coat stainless steel needles.
Pure Teflon needles are too flexible to be of use.
If you have microbore tubing available, it is possible to slide the tubing over your existing needle if the diameters fit well. If you have to order tubing, you might as well order needles of a larger size.
A final possibility is to use the pump to work the drop down to the bottom of the needle by first increasing the volume enough that the drop will slide down and then decreasing the volume a little so it will hang on the bottom.
A surface tension measurement can be correctly made with the drop up the tip as long as the bottom of the tip does not come through to the surface of the liquid and change the profile. The operator must make a judgement here and that is the risk. A needle simply hidden inside the drop does not affect the measurement.
See also IFT tip size.