Symbols used
a	Average frequency of drug particle compressions, Hz
c	Bead loading or fractional volumetric concentration of the beads, –
d	Particle diameter (size), m
e	Restitution coefficient, –
F b n	Average maximum normal force during collision of two identical elastic beads, N
g 0	Radial distribution function, –
i	Observation (run) index, –
I	Maximum # of observations, –
j	Predictor index, –
J	Maximum # of allowed predictors, –
k	Breakage rate constant in Equations.(2) and (3), min–1 and µm1–n∙min–1, respectively
k(t)	Time-dependent breakage rate parameter in Equation (4), min–1
k 0	Breakage rate constant in Equation (6), min–n
K	Coefficient obtained from an empirical correlation, –
n	Exponent in the kinetic models, –
p	Probability for a single drug particle to be caught between the beads, –
PSD	Particle size distribution
P w	Average stirrer power per unit volume, W/m3
Q	Volumetric flow rate, m3/s
R	Radius, m
R diss	Dissipation coefficient of the bead, –
R diss0	Dissipation coefficient when relative motion of the bead/liquid is absent, –
t	Milling time, s
T	Available # of predictors for a given MLRM approach, –
V	Volume, m3
u b	Average bead oscillation velocity, m/s
V m	Volume of the milling chamber, m3
Y	Young’s modulus, Pa
Y*	Reduced elastic modulus for the bead–drug contact, Pa
Greek letters
α b	Radius of the contact circle formed at the contact of two beads, m
ε coll	Energy dissipation rate due to partially inelastic bead–bead collisions, W/m3
ε ht	Power spent on shear of milled suspension of the slurry at the same shear rate, but calculated (measured) when no beads were present in the flow, W/m3
ε m	Non-dimensional bead–bead gap thickness at which the lubrication force stops increasing and becomes a constant, –
ε tot	Total energy dissipation rate, W/m3
ε visc	Energy dissipation rate due to both the liquid–beads viscous friction and lubrication, W/m3
Φ	Warped time, minn
η	Poisson’s ratio, –
θ	Granular temperature, m2/s2
µ L	Apparent shear viscosity, Pa·s
ν	Frequency of single-bead oscillations, Hz
Π	Energy dissipation rate attributed to the deformation of drug particles per unit volume, W/m3
Πσ y	Pseudo energy dissipation rate, J2/m6s
ρ	Density, kg/m3
σ b max	Maximum bead contact pressure at the center of the contact circle, Pa
σ y	Contact pressure in drug particle when the fully plastic condition is obtained, Pa
Mean residence time, s
ω	Stirrer (rotational) speed, rpm
Ψ	Volumetric fraction of drug particles in the drug suspension, –
Indices
b	Bead
cm	Chamber of the mill
L	Equivalent liquid (milled drug suspension)
m	Mill
p	Drug particle
sm	Suspension in the mill
T	Holding tank
50	Median (50% passing) particle size
90	90% passing particle size