Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Self-propulsion of a spherical electric or magnetic microbot in a polar
  viscous fluid   The self-propulsion of a sphere immersed in a polar liquid or ferrofluid is
studied on the basis of ferrohydrodynamics. In the electrical case an
oscillating charge density located inside the sphere generates an electrical
field which polarizes the fluid. The lag of polarization with respect to the
electrical field due to relaxation generates a time-independent electrical
torque density acting on the fluid causing it to move. The resulting propulsion
velocity of the sphere is calculated in perturbation theory to second order in
powers of the charge density.

7
The categories of this paper are: hep-th
6
The categories of this paper are: astro-ph
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
1
The categories of this paper are: cs
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph