Molecules Aggregating on a Surface that includes a Scratch

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Discussion Overview

The discussion revolves around the aggregation of molecules on a fused silica surface that has been altered by laser irradiation, specifically examining the differences in aggregation between burned and unburned regions. Participants explore the underlying physics of surface energy and morphology, as well as the potential effects of surface geometry on wetting properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the aggregation of molecules is more favorable on the burned region due to changes in surface energy caused by the removal of contaminants, resulting in a higher surface energy compared to unburned silica.
  • Others question how the geometry of the surface, such as whether a groove or a bump is present, might influence the aggregation and wetting properties, suggesting that the mechanism may remain consistent unless the geometry is nanostructured.
  • One participant references the "lotus effect" to illustrate how surface morphology can affect wetting properties, indicating that roughness may play a significant role in this context.

Areas of Agreement / Disagreement

Participants express differing views on the impact of surface geometry on molecular aggregation and wetting, indicating that the discussion remains unresolved regarding the specific effects of bumps versus grooves.

Contextual Notes

Limitations include assumptions about the cleanliness of the silica surface prior to laser treatment and the potential influence of ambient conditions on surface properties. The discussion also does not resolve the specific effects of surface roughness on wetting behavior.

Sciencestd
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fused silica.jpg

View from above of fused silica chip.

In the image is fused silica and the black line is burned fused silica caused by laser, when I cast droplet that contains molecules, never mind which kind of aggregation I tried different kinds of molecules, the aggregation is more preferably on the burned fused silica rather than unburned fused silica, the burning it seems that it caused to a groove in the sample.. so I have two questions:
1- Why the molecules prefer to aggregate more on the burned region, what is the physics behind that??
2- Could the result be changed if the burned region is like a bump and not a groove?

Thank you,
 
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Surface morphology can change wetting properties, especially if there is a regular structure at the nanoscale (google "lotus effect" for example). But if your laser was powerful enough to melt silica (mp 1710 C) and you're seeing the irradiated areas gather liquid, I imagine what happened was that you started with a silica chip that's had no special precautions taken to protect it from ambient, so that it's got a nice thick layer of low-surface-energy crud on it that's come from the air. This happens: bare clean silica has a very high surface energy, and if there's non-volatile schmutz floating around in the air (aerosolized pump oil, dust and dirt from HVAC units, or whatever), it will land on the surface, stick permanently, and lower the surface energy. Irradiating with a laser in air will burn that crud off, leaving behind an area of clean silica, which has a much higher surface energy. When a liquid meets a surface, it will preferentially cover the areas with the highest surface energy. That's my guess as to what's happening here.
 
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Thank you so much for your answer! :) It gives me the way to think about it, really thank you!
I just have couple of questions more, do you think if the groove was like a bump so it will be different? and how the roughness of the surface can change the wetting?
 
Sciencestd said:
do you think if the groove was like a bump so it will be different?
The mechanism I hypothesized would be the same regardless of the geometry, as long as that geometry wasn’t nanostructured.
Sciencestd said:
how the roughness of the surface can change the wetting?
Please google “lotus effect.”

Edit: also, the Wiki page on ultrahydrophobicity has some enlightening pictures.
 
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