Dispensing Glycerol and other High Viscosity Liquids
Glycerol is the typical example of a high-viscosity liquid that users might want to dispense for contact angle measurements. It has a viscosity of 934 mPa-s or cp at 25C.
Liquids develop a significant pressure from viscosity when flowing through narrow tubes. This pressure alters the effective pumping rate. For this reason, one has to limit the dispense rates for high-viscosity liquids. FTA recommends using a needle no smaller than #20, which has an inside diameter of 0.635mm. With a #20 needle, flow rates of 2ul/s are possible with coarse control. Flow rates of 0.5 or 1ul/s will yield fine control. For example, one might take 10s to dispense 10ul.
The larger question with high viscosity liquids is that they take significant time to reach their equilibrium contact angle. Before this time, the drop shape is meaningless because it is affected by both surface tension and viscosity. For gylcerol, one should wait about 5s after placing the drop on the solid surface before the drop shape and contact angle are meaningful. The exact time depends on the contact angle. Lower angles take longer to stabilize.