ramé-hart Tech Note on Needle Riding Issue
In some cases, especially with low viscosity and low surface tension liquids, the drop will ride up the needle or tip when attempting to produce a pendant drop.
If this occurs, the first line of action is to thoroughly clean the tip or needle prior to use using acetone (on stainless steel needle) or IPA (on plastic tips).
When using the Automated Dispensing System, you will want to speed up the dispensing. If you dispense too slowly, it gives too much time for the drop to ride up the needle or tip. You can control the dispensing speed my adjusting the full stroke time. By changing the setting to a lower value, dispensing will be faster. If the drop ejects, you are going too fast and will need to slow it down. Start with a 4 second full stroke time and adjust accordingly.
If the issue continues, we recommend using a Manual Syringe (or Microsyringe Assembly) and a higher gauge needle - e.g., 30 gauge. You can order new needles here. ramé-hart keeps all needles sizes in stock all the way to 33g.
ramé-hart also sells needles made from solid PTFE which are omniphobic. These can also greatly reduce the propensity for pendant drops to ride up the needle. They are available here. Using a sharp razor you can easily trim the tip if needed.
Lastly, ramé-hart also offers PTFE-lined needles. They can be ordered here. These needles feature an outer stainless steel shell with a PTFE inside.
No matter which tip or needle you use, be sure to clean it thoroughly before starting your experiment. And be sure to dispense promptly to reduce the riding issue. The faster you can make a larger drop, the sooner the force of gravity can out strengthen the capillary forces that cause it to want to ride up the side of the needle.
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