Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: What buoyancy really is. A Generalized Archimedes Principle for
  sedimentation and ultracentrifugation   Particle settling is a pervasive process in nature, and centrifugation is a
much versatile separation technique. Yet, the results of settling and
ultracentrifugation experiments often appear to contradict the very law on
which they are based: Archimedes Principle - arguably, the oldest Physical Law.
The purpose of this paper is delving at the very roots of the concept of
buoyancy by means of a combined experimental-theoretical study on sedimentation
profiles in colloidal mixtures. Our analysis shows that the standard
Archimedes' principle is only a limiting approximation, valid for mesoscopic
particles settling in a molecular fluid, and we provide a general expression
for the actual buoyancy force. This "Generalized Archimedes Principle" accounts
for unexpected effects, such as denser particles floating on top of a lighter
fluid, which in fact we observe in our experiments.

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The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
24
The categories of this paper are: dg-ga
13
The categories of this paper are: quant-ph