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ramé-hart Newsletter

                    

August 2024
 
The Spreading Coefficient and Contact Angle
The spreading coefficient is a quantitative measure that describes the tendency of a liquid to spread across a solid or even another liquid surface. It is particularly useful in understanding the wetting behavior of liquids on various surfaces. The spreading coefficient, S, is defined as the difference between the surface tension of the substrate and the sum of the surface tensions of the liquid and the liquid-substrate interface.

Mathematically, the spreading coefficient S is given by:

S = γSV − (γSL + γLV)

where:

  • γSV​ is the surface tension of the solid-vapor interface.
  • γSL​ is the surface tension of the solid-liquid interface.
  • γLV​ is the surface tension of the liquid-vapor interface.

The value of the spreading coefficient determines the wetting behavior as follows:

  • Positive Spreading Coefficient (S > 0): Indicates that the liquid will spontaneously spread over the surface of the solid, leading to complete wetting.
  • Negative Spreading Coefficient (S < 0): Indicates that the liquid will not spread completely and will form droplets, leading to partial wetting. In this case, the contact angle is measureable.

The relationship between the spreading coefficient S and contact angle θ is explained by Young's equation and the Young-Depré equation. The Young equation is:

γSV = γSL + γLV(COSθ)

where θ is the contact angle. If we substitute γSV from Young's equation into the spreading coefficient equation, we get:

S = γLV(COSθ - 1)

Knowing both the contact angle and the spreading coefficient provides a more comprehensive understanding of the wetting behavior and surface interactions of a liquid on a solid. While contact angle provides a snapshot of the equilibrium state of a droplet on a surface, it doesn't directly quantify the energy changes involved. The spreading coefficient not only quantifies the tendency of a liquid to spread, but can also predict dynamic wetting behavior, such as how quickly a particular liquid will wet or dewet a particular surface.

In short, while the equilibrium contact angle provides valuable feedback relative to basic wetting behavior, the spreading coefficient is capable of providing a more expansive view of wetting dynamics by taking into account not only the contact angle but also the surface tension of the liquid.

In practical terms, the spreading coefficient is crucial in applications where the control of wetting properties is important, such as in coatings, adhesives, lubrication, and the development of superhydrophobic and superhydrophilic surfaces.
 
Product of the Month - Precision Combo Calibration Device
While ramé-hart offers a variety of calibration products, the Precision Combo Calibration Device (p/n 100-27-31-U) is the only one that's required and also included with each instrument we sell. Our DROPimage software has a calibration routine1 that will take a picture of the sphere which results when the calibration device is positioned on the stage and in focus and then analyzes the resulting profile. Knowing the diameter of the ball, a value (in microns) of the pixel size in both the x and y axes is calculated. These dimensional values are then stored in DROPimage and used to determine both linear and angular distances. Since all measurements reference back to the calibration device of a known dimensional value, the resulting data are both correct and valid.


ramé-hart Precision Combo Calibration Device p/n 100-27-31-U

For customers working within certain quality standards, we also offer a certified version of the Precision Combo Calibration Device (p/n 100-27-31-C). This version of the calibration tool ships with a NIST-traceable certificate which optionally can be renewed annually.

1 Watch this video to see how easy it is to calibrate a ramé-hart instrument using a Precision Combo Calibration Device and DROPimage Advanced.

 
 
Regards,

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