Tech Talk Blog

The dynamics of wetting are described below:

Spreading = A – ( B+ C )

Where:

• A = surface energy of solid (given below)
• B = surface tension of liquid
• C = surface energy of solid-liquid interface

If Spreading is:

• Negative. Then, liquid will bead up.
• Zero. Then, liquid will spread.
• Positive. Then, liquid will spread.

If the material surface energy is relatively low, then the coating will not flow well and fisheyes, pinholes, gaps, or air bubbles will form. If the material surface energy is too high, then the paint, ink, or coating may bleed or be difficult to control. Therefore, the surface tension of the liquid and the surface energy of the material must be matched for the application.

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In surface science, the use of a goniometer (contact angle meter) is common to determine the wettability of a surface. What is clear is that every material has a specific surface energy. But, may not be clear is that the surface topography of that material also effects the contact angle.

Surface roughness can change the contact angle without plasma or other surface treatment. When a device is molded or machined, the surface finish of the device can have a functional as well as a aesthetic purpose. The functional purpose can be microscopic capillary channel as in microfluidic devices. The aesthetic purpose can be feel or finish of the part as in grips or sunglasses. The data below shows the effects of surface roughness in RMS versus natural, plasma treated hydrophilic, and plasma treated hydrophobic.

Notice the general trends in contact angle relative to surface roughness.

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