a	Radius of nanobubble
a 1	Radius of nanobubble 1
a 2	Radius of nanobubble 2
A	Hamaker constant for the gas in water
C	Concentration
dh	Hydrodynamic diameter
DT	Diffusion coefficient
e	Electron charge
g	Gravitational acceleration
h	surface-to-surface distance between two nanobubbles
k	Boltzmann constant
Ka1, Ka2	Equilibrium constant
K	hydrophobic force constant for the gas in water
K′	constant for Lemlich’s theory
n	Concentration of anion or cation in solution
NA	Avogadro number
ΔP	Laplace pressure difference
p	Instantaneous bubble mean radius
R	Distance between bubbles 1 and 2
t	Time
T	Absolute temperature
u	Terminal velocity
VA	Van der Waals interaction potential energy
Vh	Hydrophobic force interaction potential energy
VR	Electrostatic interaction potential energy
VT	Total potential energy of interaction between two nanobubbles
x	Distance
z	Ionic valence
γ	Surface tension
ε	Electron charge
ε 0	Permittivity of vacuum
εr	Relative dielectric constant
Ψ1, Ψ2	Surface potential of the nanobubbles of radius a1 and a2
η	Viscosity
κ	Debye–Hückel parameter
ρ	Density of water
μ	Electrophoretic mobility
ζ	Zeta potential