Between giant oscillations and uniform distribution of droplets: The role of varying lumen of channels in microfluidic networks

Abstract

The simplest microfluidic network (a loop) comprises two parallel channels with a common inlet and a common outlet. Recent studies that assumed a constant cross section of the channels along their length have shown that the sequence of droplets entering the left (L) or right (R) arm of the loop can present either a uniform distribution of choices (e.g., RLRLRL...) or long sequences of repeated choices (RRR...LLL), with all the intermediate permutations being dynamically equivalent and virtually equally probable to be observed. We use experiments and computer simulations to show that even small variation of the cross section along channels completely shifts the dynamics either into the strong preference for highly grouped patterns (RRR...LLL) that generate system-size oscillations in flow or just the opposite-to patterns that distribute the droplets homogeneously between the arms of the loop. We also show the importance of noise in the process of self-organization of the spatiotemporal patterns of droplets. Our results provide guidelines for rational design of systems that reproducibly produce either grouped or homogeneous sequences of droplets flowing in microfluidic networks.

Red blood cell phase separation in symmetric and asymmetric microchannel networks: effect of capillary dilation and inflow velocity

“…Although these authors used droplets (rather than RBCs) and significantly wider micro-channels, the results are applicable to the microcirculation due to the dynamic similarity of the systems. These studies provided evidence that small changes of cross-section13 or length20 along specific channels can enable/disable specific patterns of droplets in a network. Nevertheless, it should be kept in mind that the physics of droplets is inherently different from RBCs due to intrinsic mechanical properties of the RBC membrane able to support shear29.…”

“…The basic mechanisms have been extensively investigated in single bifurcations6891011. In networks of bifurcating (diverging and converging) vessels, RBC dynamics becomes more complex and often counter-intuitive5121314151617. Computational simulations818 of blood rheology in microcirculatory networks of the rat mesentery have shown that blood flow can undergo spontaneous, self-sustained oscillations in capillaries under steady conditions without any active biological regulation.…”

“…Computational simulations818 of blood rheology in microcirculatory networks of the rat mesentery have shown that blood flow can undergo spontaneous, self-sustained oscillations in capillaries under steady conditions without any active biological regulation. Recent studies with droplets131920 and RBCs1221 suggested that the topology of the network can lead to dynamic changes of the hematocrit ( Ht ) within the network. Computational models of idealized networks12 showed that RBC distribution and velocities are irregular and heterogeneous even in uniform networks (uniform vessel length and diameter) although the flow and the RBC density at the inlet were maintained constant.…”

“…Strictly theoretical studies of RBC dynamics in microvascular networks are rare28. Cybulski and Garstecki1319 and Amon et al 20. were the first to provide a detailed theoretical explanation for the heterogeneous dynamics of droplets in in vitro networks.…”

Fixing the direction of droplets in a bifurcating microfluidic junction

“…The various observations confirmed that in the bifurcating junction a single short droplet is directed into the channel with the higher flow velocity (Parthiban and Khan 2013;Wang and Vanapalli 2014;Cybulski et al 2015). As the droplet changes the resistance of the flow in a channel, the distribution of flows between the two outputs will depend on the current number of droplets in each of the branches.…”