  Although many molecular dynamics simulations have been conducted on slip flow
on graphene, experimental efforts remain very limited and our understanding of
the flow friction on graphene remains far from sufficient. Here, to accurately
measure the slip length in rectangular nanochannels, we develop a 3D capillary
flow model that fully considers the nonuniform cross-section velocity profile,
slip boundary conditions, and the dynamic contact angle. We show that the 3D
analysis is necessary even for a channel with a width/height ratio of 100. We
fabricated graphene nanochannels with 45-nm depth and 5-{\mu}m width, and
measured slip lengths of about 30 to 40 nm using this 3D flow model. We also
reevaluated the slip-length data for graphene obtained from capillary filling
experiments in the literature: 30 nm instead of originally claimed 45 nm for a
25-nm-deep channel, and 47 nm instead of 60 nm for an 8.5-nm-deep channel. We
discover a smaller slip length than existing experimental measurements due to
our full 3D flow analysis considered in our method. This work presents a
rigorous analysis approach while also providing a better understanding of slip
flow in graphene nanochannels, which will benefit further innovation in
nanofluidic applications, including electronics cooling and biomedical chips.
