July 2010

In recent years surface scientists have become less satisfied with a simple static contact angle measurement. Increasingly they are becoming more interested in the range of metastable contact angles that are observable between two bounds. This range is defined by the maximum possible contact angle (the advancing contact angle) and the minimum (the receding contact angle). We last discussed this topic at length in our February 2008 Newsletter and at that time we discussed some of the methods that are typically employed to measure advancing and receding contact angles.

The tilting base method is becoming the preferred method for capturing the advancing and receding contact angles. This method involves the formation of a sessile drop on the sample solid - just as is required for a static contact angle measurement. But the contact angle is continuously measured while the sample (and entire instrument) is tilted between 0° and 90°. If the drop slips off during the measurement (it sometimes hangs on all the way to 90° such as the drop in the video below), the thermodynamic equilibrium is broken and the last valid pair (L and R) of contact angle measurements prior to the slip represent the advancing and receding contact angles.

In the video below we are conducting a simple tilting base experiment in which we are taking a measurement every second while simultaneously adding 1° of tilt until the tilt reaches 90°. Since the drop does not fall off, we are actually taking a total of 91 readings. We are moving at a rate of 1° per second which is a speed that works well for most applications. If you're worried about the effect of evaporation, you can step the speed up - up to 5° per second. In the experiment in the video, you will notice a slight loss in volume due to evaporation since the drop phase is pure deionized water. If evaporation is not a concern but you would like higher resolution results, you can slow the tilt speed down or increase the number of measurements - up to 60 per second. Since we are tilting in the normal angular direction (CCW), Theta (Left) is the advancing contact angle while Theta (Right) is the receding contact angle. In the end, the contact angle hysteresis is calculated by subtracting the receding from the advancing contact angle.

The tilting base method has some distinct advantages over alternate methods. Consider the following:

• Unlike the add and remove volume method, the tilt method allows both the advancing and receding to be measured simultaneously.
   

• While the add and remove volume method produces fairly repeatable advancing angles, the receding angles are difficult to accurately capture. The primary source of the problem is the disturbance to drop by the needle or tip which must be embedded in the drop. This disturbance can be reduced by increasing the needle gauge size which results in less surface area. The alternative of removing volume through a small hole in the solid is difficult or impossible to implement.

• An evaporation method can be used (see http://tinyurl.com/23mh664) but this can be very time consuming, only results in the receding contact angle, and only works on drops that will actually evaporate.
   

• Unlike other methods, the tilting base method also captures the roll-off angle which offers another perspective on the wetting and dewetting forces.

If you are using the tilting base method, you should be aware of these considerations:

• You will want to keep the drop volume constant from one run to the next to ensure repeatability. Generally a larger drop, say 5-10µl, works better than a small drop, say 1µl. There are four reasons for this: first, a larger drop is less affected by evaporation; second, a larger drop has more volume and thus more mass and will be more responsive to the gravitational force which is compelling the drop to roll off; and third, the competing force which is keeping the drop from releasing is described by Tate's law which is a function of the length of the three-phase line; and fourth, as surface roughness increases, larger drops are necessary to maintain repeatability.
   

• Measurements need to be taken in a controlled vibration-free environment. While our ramé-hart Model 100-25-A Automated Tilting Base and Model 100-25-M Manual Tilting Base provide an excellent vibration-free tilt movement at high or low speeds, the surrounding air flow and stray movements from the lab bench can inadvertently cause premature drop slip if they are not controlled or eliminated.
   

• In many cases the sample will slip off the specimen stage before the drop rolls off the sample. For films and thin samples we have clamps (p/n 100-15) and a vacuum chuck (p/n 100-21-VCx). For most other samples, we recommend double-sided tape.
   

• The left and right contact angles should be the same or close to the same when the drop is first dispensed at a tilt angle of 0°. If this is not the case, then any difference will bias the contact angle hysteresis by that the difference after tilt is added. 

For more information on our Automated and Manual Tilting Base options, including a downloadable PDF brochure, visit: http://www.ramehart.com/tilting_base.htm