Spinodal Dynamics in Non-Equilibrium Compression of Two Self-Avoiding Polymer Chains

Two polymers under 1D confinement can phase separate as a consequence of chain interconnectivity and entropy maximization, and this demixing may be one mechanism leading to bulk chromosomal segregation in bacteria. While there has been considerable theoretical effort to explore this problem, few experiments have attempted to directly investigate segregation/mixing behaviour of two self-avoiding chains in model geometries. In this experiment, two differentially labeled nanochannel confined DNA molecules are compressed against a barrier using hydrodynamic flow. The differential labeling enables us to quantify the concentration profile of each chain along the channel independently. The two DNA molecules will mix given that the applied forcing is strong enough, but instead of simple mixing, we observe a complex phenomena whereby the chains interpenetrate but form alternating, fluctuating bands in which one chain has higher concentration relative to the other. We interpret this phenomenon as a spinodal decomposition of the two polymers and rationalize it using a model based on a 1D convective Cahn-Hilliard equation, a classic model describing spinodal decomposition in driven binary systems. Simulations for such a model yield striking similarity to the experimental observations.