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We introduce applied shear as a method to control viscous fingering by smoothing the interface between miscible fluids. In the viscous fingering instability, a less viscous fluid displaces a more viscous one through the formation of fingers. The instability, which requires a confined geometry, is often studied in the thin gap of a quasi–two-dimensional Hele-Shaw cell. When the two fluids are miscible, the structures that form in the dimension traversing the gap are important for determining the instability onset. We demonstrate with experiments and simulations that oscillatory translational shear of the confining plates changes the gap-averaged viscosity profile so that it becomes less abrupt at the fingertips. Increasing the amplitude or velocity of the shear delays the instability onset and decreases the finger growth rate. Shear can thus be used to stabilize a pair of miscible fluids against fingering. The results show a direct correlation between a smoother viscosity profile and delayed instability. 

Self-closing assembly is prone to polymorphism due to thermally excited bending fluctuations, which permit the formation of off-target assemblies at the point of self-closure. One way to overcome this source of polymorphism is to use templated growth, a process in which assembly initiates from a precisely defined seed rather than by spontaneous nucleation. We explore this approach to quelling polymorphism in the self-closing assembly of cylindrical tubules assembled from DNA-origami subunits with user-specified inter-subunit binding angles and specific interactions. We develop two strategies to create seeds with precisely defined diameters and helicity: (1) using multicomponent assembly and (2) purifying a specific seed-type from a polymorphic mixture using gel electrophoresis and gel extraction. By tuning the seed and monomer concentrations and adjusting the assembly temperature, we determine the conditions under which tubules grow from the seed while avoiding spontaneous nucleation. We observe that templated tubules tend to follow the guidance of the seed, thereby increasing the selectivity of the target geometry. In addition, we find that by tuning the diameter of the seed, one can template the growth of monodisperse tubules over a range of target diameters, even while using a single monomer type with a single preferred local curvature. Our results demonstrate that employing precisely defined seeds to guide assembly can significantly decrease polymorphism in self-closing assembly in a controllable and economical way.