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Abstract This article examines the Rayleigh–Taylor instability in an rotating inhomogeneous, incompressible fluid with partial viscosity. First, using the modified variational method, we demonstrate the existence of an exponentially growing normal mode in $H^k,k\text{⩾}0$and establish the instability of the linearised problem. Then, we obtain a nonlinear energy estimate for the problem with small initial data. In this process, we employ an innovative method to derive energy estimates for both density and velocity, effectively addressing the challenges posed by partial viscosity. Third, we prove the existence of a classical solution forH³initial data, provided it satisfies a compatibility condition. Finally, by integrating the results of the previous steps, we establish the nonlinear instability of the system in the Hadamard sense. 

Abstract While most studies of biomolecular phase separation have focused on the condensed phase, relatively little is known about the dilute phase. Theory suggests that stable complexes form in the dilute phase of two-component phase-separating systems, impacting phase separation; however, these complexes have not been interrogated experimentally. We show that such complexes indeed exist, using an in vitro reconstitution system of a phase-separated organelle, the algal pyrenoid, consisting of purified proteins Rubisco and EPYC1. Applying fluorescence correlation spectroscopy (FCS) to measure diffusion coefficients, we found that complexes form in the dilute phase with or without condensates present. The majority of these complexes contain exactly one Rubisco molecule. Additionally, we developed a simple analytical model which recapitulates experimental findings and provides molecular insights into the dilute phase organization. Thus, our results demonstrate the existence of protein complexes in the dilute phase, which could play important roles in the stability, dynamics, and regulation of condensates.