Symbols used
A	apparent breakage rate constant, min–1
a 1	breakage distribution exponent
A c	cross-sectional area of the mill chamber perpendicular to suspension flow, m2
b	breakage distribution parameter
B	cumulative breakage distribution parameter
c	volume fraction of the beads in the suspension–bead mixture
CFD	computational fluid dynamics
D	diameter of the rotor, m
$\acute{D}$	dispersion coefficient, m2/s
D m	mill chamber diameter, m
DEM	discrete element method
K	total number of size classes in the laser diffraction measurement
L	length of the mill, m
mi	mass fraction of particles in size class i
$\overline{m}$ i	mass fraction density of particles in size class i
Mi	mass concentration in size class i, kg/m3
MHD	microhydrodynamic
n	number of cells in cell-based PBM
N	number of size classes used in the PBM
N to	number of theoretical passes or turnovers
N T	number of trials
ODE	ordinary differential equation
P	total number of time samples
PBM	population balance model
Pec	Peclet number of a continuous mill in recirculation operation
Pes	Peclet number of a single-pass continuous mill
PSD	particle size distribution
Q	volumetric flow rate of the recirculating suspension, m3/s
R	back-mixing ratio
$\dot{R}$	internal recirculation rate, m3/s
RTD	residence time distribution
S	specific breakage rate parameter, min–1
s f	shape factor
SI	stress intensity, Pa
SN	stress number
SSR	sum-of-squared-residuals
t	milling time, min
u a	axial interstitial velocity of the drug suspension, m/s
u s	superficial velocity of the drug suspension, m/s
V m	free volume of the milling chamber available for bead filling, m3
V s	batch volume of the suspension, m3
V T	volume of the suspension in the holding tank, m3
WSMM	wet stirred media milling
x	particle size, µm
x*	transition particle size, µm
$x_0$	normalizing reference particle size, µm
YSZ	yttrium-stabilized zirconia
Greek Letters
τ s	average residence time in a single pass, min
τ c	average residence time in the mill in recirculation (circuit) mode, min
τ cell	average residence time in a cell, min
τ T	average residence time in the holding tank, min
ω	rotational speed of the stirrer (rotor), rpm
Indices
10, 50, 90	10%, 50%, and 90% passing sizes of the cumulative volume PSD
EXP	experimental
i, j	size-class index
m	mill chamber
MOD	model predicted
n	total number of cells in the cell-based PBM
p	time index
T	holding tank