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June
2016 |
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| The Receding Contact Angle | |
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Bonnie and Clyde, Jack and Jill, Bert and
Ernie, Tom and Jerry, Advancing and Receding. Some duos seem
inseparable. Today, however, we're going to leave advancing behind and
discuss the receding contact angle.
Traditionally, the static contact angle measured at the three-phase line of a sessile drop of liquid on a given surface is used to describe the wetting properties of the solid/liquid pair. This single measurement is somewhat one-dimensional. A more comprehensive characterization can be achieved by measuring the maximum and minimum contact angles. The receding contact angle represents the smallest possible contact angle for a particular solid/liquid combination. There are a number of ways to measure the receding contact angle. One of the most common ways is using the Tilting Plate Method. The left and right contact angles are repeatedly measured while the contact angle tool is being tilted from from 0° to 180°. The receding contact angle is measured from the side which is diminishing in value as the tilt angle is increased. This type of measurement requires a tilting base. A manual tilting base (such as ramé-hart p/n 100-25-M) can be used but a more automated measurement can be taken using a motor-controlled and software-driven tilting base (such as the ramé-hart Automated Tilting Base p/n 100-25-A). If the drop rolls off while tilting, then the last prior measurement prior to the drop releasing is used. If the drop stays on to a tilt angle of 90°, then the smallest contact angle measurement (which is also usually the final measurement on the downhill side) is used to determine the receding contact angle. This method is volume dependant as a larger drop is more inclined to roll off at a given tilt angle than a drop with smaller volume.
The Add/Remove Volume Method is another popular method. A tip or needle is embedded in a sessile drop and the contact angle is measured while volume is retracted from the drop. The receding contact angle is reached when the minimum contact angle can be measured and prior to dewetting which results in the contraction of the three-phase line.
While this measurement can be taken using a manual microsyringe, it's more automated and repeatable when using an Automated Dispensing System. The main disadvantage to this method is the disturbance of drop geometry by the needle itself. Some researchers have determined that for polymer solids, the discrepancy can be as much as 30%.1
The Evaporation Method1 requires no special accessory but only works with water and aqueous liquids that actually evaporate over time. It can also be a very time consuming measurement. The main advantage is that the drop volume can be reduced without disturbing the drop geometry. And unlike the other methods, the evaporation method can only capture the receding contact angle and not the advancing. On some surfaces, especially where surface roughness is pronounced, the drop will never dewet; it stays pinned all the way to the disappearance of the drop.2 In those cases, the evaporation method is not so useful. Likewise, this method is not very well suited for hydrophobic and superhydrophobic surfaces. The method selected is often a function of the equipment that is available. However, for superhydrophobic surfaces, the add/remove volume method can be done reliably using large drops with a small gauge needle.3 Researchers in Italy have determined that receding contact angle can predict rebound behavior of dynamic drops on hydrophobic surfaces.4 Their studies indicate that rebound only occurs when the receding contact angle is greater than 100°. Bullets
1 Determination of the
Receding Contact Angle of Sessile Drops on Polymer Surfaces by
Evaporation H. Yildirim Erbil,*,†, G. McHale,‡, S. M. Rowan,‡ and,
and M. I. Newton‡ Langmuir 1999 15 (21), 7378-7385 DOI:
10.1021/la9900831 |
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| Super June Coupon - 35% Off | |
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Regards,
Carl Clegg |
