Drop Size for IFT Measurements
The drop volume must be large enough for gravity to distort the drop. This is what forms the force balance with Laplace's pressure rule.
Most IFT measurements are liquid-vapor measurements, meaning the liquid is surrounded by atmosphere. For these measurements, the required volume is normally at least 10 microliters.
When liquid-liquid measurements are made, the required volume depends on the relative densities between the two liquids. While it is possible to make a calculation, it is easier to simply form the liquid-liquid interface and keep increasing the volume until the distinctive "tear" shape is achieved.
IFT measurements are normally made on pendant (hanging) drops but can also be made on sessile (sitting) drops. A satisfactory volume for sessile drop measurements is much larger than that for pendant drops. A good starting point is 50 microliters.