  The displacement of a fluid by another less viscous one in a quasi-two
dimensional geometry typically leads to complex fingering patterns. In an
isotropic system, dense-branching growth arises, which is characterized by
repeated tip-splitting of evolving fingers. When anisotropy is present in the
interfacial dynamics, the growth morphology changes to dendritic growth
characterized by regular structures. We introduce anisotropy by engraving a
six-fold symmetric lattice of channels on a Hele-Shaw cell. We show that the
morphology transition in miscible fluids depends not only on the previously
reported degree of anisotropy set by the lattice topography, but also on the
viscosity ratio between the two fluids. Remarkably, the viscosity ratio and the
degree of anisotropy also govern the global features of the dendritic patterns,
inducing a systematic change from six-fold towards twelve-fold symmetric
dendrites. Varying either control parameter provides a new method to tune the
symmetry of complex patterns, which may also have relevance for analogous
phenomena of gradient-driven interfacial dynamics, such as directional
solidification or electrodeposition.
