How Do I Get a Constant Drop Size?
Constant drop volume is not strictly necessary for contact angle measurements as the angle is not a function of volume. However, when making comparative measurements, it is always worthwhile to keep all parameters as similar as possible. When the sample absorbs the test liquid, it is important to keep the drop volume the same so there is the same relative amount of saturation.
Even if exactly the same pendant drop volume is hanging on the tip, the actual amount deposited into a sessile drop will vary slightly, maybe +/-10%, because the hanging drop breaks in two below the dispense tip, not right at the tip. Therefore some liquid remains attached to the tip while the majority falls off. The location of the break line varies from drop to drop. More particularly, it is a chaotic process.There are three basic techniques for achieving the same net sessile drop volume, deposition after deposition:
Let the tip diameter set the deposited volume. Dispense until the pendant drop falls off the tip. Smaller tips will support smaller drops. Lower surface tension liquids will have smaller drops for any given needle size. The disadvantages of this approach are that only certain volumes can be supported, because only certain needle diameters are available, and the liquid will fall an unnecessary distance unless the needle tip is placed carefully above the solid surface. If the drop falls too far, the momentum it gains will cause it to spread out on the sample and the measured contact angle will be low. Nevertheless, this is a widely used technique.
Let the tip height set the deposited volume. Place the tip closer to the surface so the drop is still hanging on the tip when the bottom of the drop touches the solid surface. If the tip is placed high enough, the drop will detach when its bottom touches. This is called "touching off" because the adhesion of the liquid to the solid pulls the drop away from the tip. This approach lets you adjust sessile drop volume over some range by adjusting height. The primary disadvantage is that the drop will stay attached to the tip if the tip is too low, so there is a range of acceptable heights and volumes for any liquid and surface.
Measure pendant drop volume. Real time analysis can measure the volume hanging on the tip. With the desired amount on the tip, the drop can be touched off.
High end FTA systems, such as the FTA2000 and FTA4000, provide an additional means of controlling dispensed volume. They contain autosamplers that can pickup a precise volume in a tip with an air gap above the liquid. Then the entire liquid volume can be dispensed and will be precisely known. The volume is precise because the air gap sets a definite end to the liquid.
Real time volume measurement is a feature of 1.98 and was added to 2.0 with Build 149.