Recipe for Advancing and Receding Contact Angle Measurements
An advancing contact angle is measured when the sessile drop has the maximum volume allowable for the liquid-solid interfacial area: any addition will make the drop expand and increase the liquid-solid interfacial area. This can be thought of as the "wetting angle" because the drop is ready to wet additional area.
The receding angle is the opposite: if any liquid is removed from the drop, the liquid-solid interfacial area will decrease. This is the "de-wetting angle."
The advancing angle is the largest possible angle and the receding is the smallest possible contact angle. Both are presumed to be measured at thermodynamic equilibrium.
A more formal technical discussion, complete with images and data, is available in the Papers section of the website. See Contact Angle Measurements Using the Drop Shape Method.
Embedded Needle Measurements
A contact angle may be measured with the dispense needle embedded in the sessile drop. There is one requirement for this measurement to be accurate: the overall drop shape must not be distorted by the needle. This is true for both advancing and receding angles. However, on a practical basis, this requirement is easy for advancing and difficult for receding angles. The reason is that the needle itself has its own contact angles which potentially distort the drop shape. When you study the details, you see that the needle's receding angle tends to pull the drop up around the needle. This distortion results in significant errors. The only remedy is to make the needle body very, very small when receding angles are required.
The alternative to the embedded needle technique is the tilt table. This simplifies the needle requirement, because the needle is no longer touching the drop, but requires additional hardware to tilt the specimen table and, perhaps, the camera itself.
The embedded needle works well when fine gauge needle are used. Drawn glass capillary or fused silican needles in the #35 gauge range are satisfactory.
The Drop Expansion and Contraction Process
It is important to have a picture of what the drop does during the experiment. You will have to empirically determine how much liquid volume is involved in each of the following phases:
Pendant drop phase: Starting with a bare dispense needle, a certain volume must be dispensed before the hanging pendant drop touches the specimen surface. The needle tip must be close enough to the surface that the drop does not detach upon touching. Normally the tip will be placed within a millimeter of the surface. After the drop touches, it must be further expanded until its overall shape is not significantly affected by the needle. For example, the drop might touch after 5ul and reach an acceptable shape after 20ul.
Advancing angle phase: Continued dispense makes the sessile drop larger and larger. This must be done slowly to maintain "quasi-equilibrium" conditions. A typical incremental volume might be 30ul, giving a total of 50ul dispensed. There is nothing special about 50ul. You might achieve satisfactory drop shapes with 25ul. It all depends on the magnification used, the needle size, and drop spreading caused by the specimen's contact angle.
No angle phase: After you have advanced the drop sufficiently on the specimen, you will want to reverse the process and start removing liquid from the drop by pumping in reverse. However, the drop will not immediately start receding. Instead, there is a significant time in which the liquid-solid interfacial area does not change as liquid is pumped out, i.e., the contact line of the drop remains fixed. The liquid removal changes the shape of the drop but not the contact line position. This is the period in which the contact angle decreases from its advancing angle (the largest possible angle) to its receding angle (the smallest possible angle). You will not use any data taken during this phase.
Receding angle phase: Once the sessile drop reaches its receding angle, any further removal of liquid will cause the contact line to move, the interfacial area to decrease, and the contact angle to be the receding angle. You take data in this phase until the liquid volume becomes so small that the needle distorts it or the remaining liquid detaches from the drop.
Clean-up phase: Once the drop shape is no longer satisfactory or the sessile drop has detached, no further data is available. However, you may wish to remove the remaining liquid from the specimen.
Capture Times and Pump Program
The overall time for the various phases must be determined empirically. The problem is that you do not know the volumes required to achieve good drop shapes until you experiment some.
You determine the times by calculating pumping speeds once you know the required volumes. In general, you must pump very slowly to not introduce vibration and viscosity forces into the sessile drop. Low rates are necessary. Again, values depend on needle size chosen, but rates of 0.1 and 0.2ul/s might be expected. To achieve the lower rates, you may need to select a syringe with 500 or 250ul volume. The 10ml syringes will be too large. (The syringe plunger has a range of possible rates in mm/s that translate into ul/s according to the syringe diameter.)
You can operate the pump in two ways. One way is to manually set the pump to Pump Out during the pendant drop and advancing angle phases, then click to Pump In during the remaining phases. Set the rate by entering the rate on the Pump tab under Manual Rate.
The other way is to use the pump program. Enter the total time and volume for each phase in the program grid. The rate is then computed internally. For example, if you choose 0.2ul/s and 25ul volume total for the pendant drop and advancing phases, the total time would be 25/0.2 = 125s. The pump program is more effort to setup but has the advantage that the rates for each phase can be set individually. Positive volumes dispense and negative volumes aspirate or pick up. Note you will check Start on Run but not Stop on Trigger for the pump because you want the pump to run throughout the experiment. You will not check Reverse Direction.
After you have chosen the pump rates, you can set Capture timing. Sometimes it is easiest to draw a time-line of the whole experiment with cartoons showing what the drop should look like during each phase. This helps visualize the process. At any rate, with this kind of experiment it is best to run at a constant rate and to use Images after Trigger rather than Images before Trigger for data acquisition.
One way is to trigger by using the Video Trigger that generates a trigger when the drop passes the cursor cross-hairs location. Another way is to manually click Trigger after starting Run, say when the pendant drop touches the surface. In either case, you will ignore the images that occur before the trigger. Set Images before Trigger = 1 and the Image Period before Trigger = .033s. This will keep the Live Video window updated prior to the actual trigger.
Now you must compute the Image Period after Trigger and the Images after Trigger to capture. Consider the total time for the experiment, including all the phases. For example, this might be 500s. A reasonable number of samples (images) during this time might be 250 or 500. While this might seem like a lot, many will occur during the no-angle phase while the drop is shifting from an advancing (higher contact angle shape) to a receding (lower contact angle shape). Certainly you will want at least 100 images over the experiment. Say you choose 250 images for the 500s duration. In this case, you would enter Images after Trigger = 500 and Image Period after Trigger = 2 (500s/250 = 2s).
As a final note, you have to run the experiment a time or two on any new sample to get the parameters matched to the sample.
Tilting Plate Technique
As mentioned above, the alternative is the tilting plate technique, where the sample is mechanically tilted so the drop wants to run downhill. This requires more hardware but is a well-respected technique. See the Tilting Plate Example paper for a more complete description and actual data.