Search for: All records

An elastic spring network is an example of evolvable matter. It can be pruned to couple separated pairs of nodes so that when a strain is applied to one of them, the other responds either in-phase or out-of-phase. This produces two pruned networks, with incompatible functions, that are nearly identical but differ from each other by a set of “mutations” each of which removes or adds a single bond in the network. We generate ensembles of network pairs that differ by a fixed number,M, of discrete mutations and evaluate allM! mutational paths between the in- and out-of-phase behaviors up toM $=$ 14. With a threshold response for the network to be considered sufficiently fit for either function, so that nonfunctional networks are disallowed, only some mutational pathways are viable. We find that there is a surprisingly high critical response threshold above which no evolutionarily viable path exists between the two networks. The few remaining pathways at this critical value dictate much of the behavior along the evolutionary trajectory. The effect of multiple mutations is epistatic, that is, the impact of a mutation is not invariant but depends on what other mutations have already occurred. In most cases, the mutations break up into two distinct classes based on epistasis. The analysis clarifies how the number of mutations and the position of a mutation along the pathway affect the evolutionary outcome. 

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.