0050b// Triglycerides & Surface Finishing & Water Contact Angle Test
Recently, I'm self-studying how to use the water droplet foot print (droplet diameter) to calculate the water angle. (Just for fun and my curiosity!!! Ah-ha !!!)
But surprisingly, I observe something I'm didn't expected and I decided to discuss in this post.
Below shown a photographs for water droplets (Reversed Osmosis Water) on a metal surface.
The volume of each droplet is 0.6 micro liter and this is the smallest volume which I able to control. The smaller the droplet is, the lesser the gravitational effect to the droplet. Therefore, I can use mathematics to convert the droplet diameter to the contact angle (wetting) with assume the droplet is truncated spherical shape.
However, after I introduce my facial onto the surface, Eureka Moment happen!!!
Water droplets skew, have a oval shape and its major axis along its machining mark. (As photograph show below)
Side Note:
My facial oil in professional jargon called Sebum, below shown its chemical composition:
Triglycerides (~41%),
Wax esters (~26%),
Squalene (~12%)
Free fatty acids(~16%).
(Source: https://en.wikipedia.org/wiki/Sebaceous_gland)
There are some doubt for this observation. My facial oil consists of tons of Triglycerides which is a ester with three fatty acids. This suppose give the surface with hydrophobic properties which repel the water droplets and give me a huge contact angle. However, observation shown the droplet have smaller contact angle and spread along the machining mark. This imply the surface is hydrophilic after I introduce my facial oil.
Because my curiosity ! I decide to go into detail why this weird phenomenon happen. Wenzel's Model, Cassie–Baxter model, Petal Effect, Lotus Effect and many others things, none of it explain how a hydrophobic nature surface show a hydrophilic droplet.
After my self-study, I cannot find any explanation. If you know what is happening, can share your thought by leave me a message. Cheer!
http://gbwonggbwong.wix.com/dimensionalmetrology#!contact/c1et
Learning Outcome from this experiment:
Surface finishing play an important role in water contact angle test. Please refer to Wenzel's Model and Cassie–Baxter model.
https://en.wikipedia.org/wiki/Wetting
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