As both liquids have different density (that is why you can separate them with centrifugation) you can put the blood in the fridge and wait - the two liquids should separate. If they have a different freezing point, you can also cool them down.

This is an excellent question, and

the cost of a good answer to this very simple question could be at least ten million dollars.

This is a great idea. I have thought about the similar issues before and tried a lot of literature research, I found that gravity-driven and microfluidic technology can achieve good serum separation from plasma. These works have been reported frequently in Analytical Chemistry and Lab on a Chip.

Such as Article High-Yielding Separation and Collection of Plasma from Whole...

Article H.E.R.M.E.S: Rapid blood-plasma separation at the point-of-need

Junhang Dong Many thanks! This is extremely helpful.

Of course, the practicality and efficiency of the separation process could be decisive factors in competition with centrifugation.

If anyone, e.g. the author of this question, knows a way to attract serious investors to solve this interesting important problem and thereby eliminate the need to use centrifuges, I am ready to provide a great solution.

I apologize for the down-to-earth approach.