b	length of the vertical sub-channel in O-structure (m)
${\overline{C}}_p$	mean input power coefficient
c	mass fraction
Dh	hydraulic diameter of the main channel (m)
d	spacing between two O-structure (m)
F	objective function
H	total depth of the channel (m)
h1	depth of the horizontal sub-channel in O-structure (m)
h2	depth of the sub-channel in X-structure (m)
Lo	length of initial part of main channel (m)
L	total length of the mixing unit (m)
Le	length of the channel outlet (m)
Lt	total length of the micromixer (m)
LHS	Latin hypercube sampling
M	mixing index
Mo	mixing index at the exit
MOGA	multi-objective genetic algorithm
N	number of sampling points
n	number of mixing units
P	pressure (Pa)
ΔP	pressure drop (Pa)
Pi	pitch length (m)
POD	Pareto-optimal design
POF	Pareto-optimal front
p	length of the horizontal sub-channel in O-structure (m)
RBNN	radial basis neural network
Re	Reynolds number
SAR	split and recombine
V	average velocity (m/s)
W	width of the main channel (m)
w1	width of the sub-channel in O-structure (m)
w2	width of the sub-channel in X-structure (m)
x, y, z	Cartesian coordinates
Greek Symbols
α	fluid diffusivity coefficient (m2/s)
η	mixing efficiency
µ	fluid dynamic viscosity (kg·m−1·s−1)
ν	fluid Kinematic viscosity (m2/s)
ρ	fluid density (kg/m3)
σ	standard deviation
Subscripts
i	sampling point or fluid component
m	optimal mixing
max	maximum value
x	axial distance