ramé-hart Newsletter

                  

March 2017
 

If the drop is too large - say over 10 to 20µl or larger - the effects of gravity pull the drop down and distort the contact angle. If the drop is too small - say under 1µl - not only is it more difficult to dispense and view, but smaller drops will also evaporate much faster, lack axisymmetry, and often exhibit a high standard deviation among a series of measurements on the same surface for reasons explained below.

An ideal solid surface is one that is smooth (i.e., no roughness), rigid, chemically homogeneous, insoluble, and nonreactive.¹ Likewise, an ideal contact angle is the static, or equilibrium contact angle on an ideal surface. In the case of an ideal surface, it doesn't matter what the drop volume is. Whether it's 1µl or 5µl, the resulting static contact angle should be the same.

In the real world, it turns out that most surfaces are not ideal. This means that drop volume can matter. Generally, as the contact angle hysteresis² increases, the less ideal the surface is. In other words, an increase in contact angle hysteresis indicates an increase in roughness, a decrease in chemical homogeneity, or some combination of the two.

Contact angle hysteresis is the difference between the maximum possible contact angle, referred to as the advancing contact angle, and the minimum, or receding contact angle. One method for measuring advancing and receding contact angle is the add/remove volume method illustrated in the graphic below.³

Another method, illustrated below, is the tilting base method. If the drop rolls off at a low tilt (t) angle, the contact angle hysteresis is generally low. It's important when using the tilting plate method that uniform drop volumes are used. On a given surface, as the drop volume is increased, the tilt angle at roll off decreases due to the effects of gravity (except in cases when Wenzel state is present).

Since most surfaces exhibit some contact angle hysteresis and thus are less than ideal, it's recommended that the same drop volume be used when testing multiple locations on the same sample or multiple variants of the material. Additionally, hydrophobic surfaces will need larger drop volumes than hydrophilic surfaces.

Bullets

• On an ideal surface, drop volume does not affect static contact angle.
• Since most surfaces are not ideal, it's best to use the same drop volume from location to location.
• Generally as the contact angle hysteresis increases, the less ideal a surface is.
• The add/remove volume method and the tilting plate method are two common methods used to measure the advancing and receding contact angles as well as the contact angle hysteresis.
• The tilting plate method is drop volume sensitive. As the volume is increased, the required tilt angle to produce roll off decreases.
• Smaller drop volumes (e.g., 1µl to 4µl) are needed for hydrophilic surfaces.
• Larger drop volumes should be used on hydrophobic surfaces (e.g., 2µl to 8µl).
• As the contact angle hysteresis increases, so too will the standard deviation of multiple contact angle measurements. Thus, the less ideal the surface is, the more measurements should be collected to find an accurate average static contact angle.
• On a less than ideal surface, as drop volume decreases, the standard deviation increases. Thus, use drops sufficiently large to correlate with the scale of surface topology and chemical heterogeneity as well as provide good axisymmetry.

Notes

¹ Abraham Marmur, "Solid-Surface Characterization by Wetting", Annu. Rev. Mater. Res. 2009. 39:475.
² For more detail on contact angle hysteresis, see our October 2013 Newsletter.
² For more discussion on advancing and receding contact angle, see our July 2010 Newsletter.

Keywords

Drop volume, ideal surface, ideal contact angle, contact angle, contact angle hysteresis, advancing contact angle, receding contact angle, surface roughness, surface heterogeneity, tilting plate method, add/remove volume method, standard deviation, drop axisymmetry 
 

Eight years ago we introduced Model 290.¹ This incredibly versatile model has become one of our top-selling products and is well-suited for all types of contact angle measurements, static and dynamic, advancing and receding, as well as surface tension, interfacial tension, and surface energy studies, and much more. Model 290 includes our powerful DROPimage Advanced software, Automated Dispensing System, and Automated Tilting Base.

Model 290 Automated Goniometer / Tensiometer

If your lab could benefit from a powerful and multipurpose surface science tool, please contact us today for more information and a quotation for our Model 290.

¹ See our July 2009 Newsletter.

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
Contact us