"Coffee-ring" effect in coating

[Ben Altman]

Interesting item on NPR the other day on some scientific research that seems relevant to Alt process coating.

The "coffee-ring" effect is that when you spill coffee on your pants you end up with a dark ring at the edge of the stain and a lighter center. In other words the particles in the fluid tend to migrate to the edges of the wet area. However, not all fluids do this. The researcher discovered that fluids containing spherical particles show the effect, while those containing elliptical particles do not - they coat evenly.

This Spring and Summer I've been doing some printing with Gold-toned Vandyke, and had noticed something like the coffee ring - a border of higher dMax around the edge of the coated area. Don't remember seeing this with POP Palladium. So maybe it has to do with the particle shape?

Not sure how this can be useful, but it's interesting to have an possible understanding of what's happening. Also would be nice to persuade the VDB not to migrate to the border of the coating - which I think tends to leave an emulsion-starved area around the edges of a print. Thoughts anyone?

Best, Ben

 

[Shawn Dougherty]

I heard this same report and thought it would surely have coating implications. Of course I don't know what they are!? =)

 

[jp498]

I would suggest not putting quite as much liquid on the paper in one coat. Two coats would create more uniformity. (sort of like painting)

 

[Gerald C Koch]

It may not have any effect. As the article pointed out, whether the effect occurs depends on the shape of the molecules in the solution.

 

[Ben Altman]

I would suggest not putting quite as much liquid on the paper in one coat. Two coats would create more uniformity. (sort of like painting)

I'm already doing two coats for VDB, first coat diluted. Van Dyke likes to be coated fairly generously, though, so can't cut down on the volume easily.

 

[Photo Engineer]

This falls into the discipline of rheology. Almost all liquids will form a high edge and a low center when coated due to the border with the dry area along the edges of a coating. This forms what is called the selvedge edge which is present in all coatings made in production. It is slit off before sale of course. Even hand coatings have this effect. It varies with viscosity and particle content.

PE