  Laminar flow prevails in microfluidic channels, and mixing is limited by the
slow diffusive process at the interface between the mixing liquids. Whitesides
and co-workers demonstrated a chaotic mixer by patterning the floor of the
microchannel with obliquely oriented microridges. This creates an anisotropic
resistance to flow, inducing exponential increase in stretching and folding of
fluid elements leading to a dramatic increase in mixing. Rose petals that have
micro papillae decorated with nanofolds, present a unique biomimetic,
superhydrophobic, super adhesive surface. In the present work, a positive and
negative replica of the rose petal surface was created using a simple,
inexpensive soft lithographic technique and these replicas formed the floor of
the microchannel. The floor of the channel naturally presents alternating slip
and no slip boundary conditions for the fluid flow that in turn provides
anisotropic drag reduction. We find that mixing index obtained for positive
replica floor channel is distinctly higher than that of a negative replica
floor channel. The variation in mixing index with Peclet number for the
replicated channels are explained by characterizing their wetting behavior
using contact angle and confocal microscopy. The enhancement in mixing due to
the patterning of the floor is quantified by calculating the percentage change
in mixing in the replicated channels from the corresponding simple Y channel
values. The maximum percentage change is obtained for the positive replica
floor channel (77.2 %) and compares well with chaotic mixer having obliquely
oriented ridges on the floor (74.7 %).
