Overview of quadrupole trapping of charged particle. (a) Schematics of a linear aqueous quadrupole trap. At electrode 1 and 4, the potential is U + V cos(Ωt), while on electrode 2 and 3, the potential is -U - V cos(Ωt). r0 is the trap radius, and Ω is the driving angular frequency of AC input. (b) The charged particle is confined around the trap axis.