We identify active phase separation as a generic demixing phenomenon in nonequilibrium systems with conservation constraints. Examples range from cell polarization to cell populations communicating via chemotaxis, and from self-propelled particle communities to mussels in ecology. We show that system-spanning properties of active phase separation in nonequilibrium systems near onset are described by the classical Cahn-Hilliard (CH) model. This result is rather surprising since the CH equation is famous as a model for phase separation at thermal equilibrium. We introduce a general reduction scheme to establish a unique mathematical link between the generic CH equation and system-specific models for active phase separation. This approach is exemplarily applied to a model for polarization of cells and a model for chemotactic cell communities. For cell polarization, we also estimate the validity range of the CH model.

We studied transgenic mice with cardiac-specific overexpression of H 2 -histamine receptors (H 2 -TG) by using the a-myosin heavy-chain promoter. We wanted to address whether this overexpression would protect the heart against paradigmatic stressors. To this end, we studied isolated atrial preparations in an organ bath under normoxic and hypoxic conditions and after prolonged exposure to high histamine concentrations. Moreover, we assessed cardiac function using echocardiography in mice with cardiac hypertrophy due to overexpression of the catalytic subunit of PP2A (PP2A-TG) in the heart [H 2 -TG Â PP2A-TG = double transgenic (DT)] or H 2 -TG with cardiac systolic failure due to treatment of mice with lipopolysaccharides (LPSs). Furthermore, the effect of ischemia and reperfusion was studied in isolated perfused hearts (Langendorff mode) of H 2 -TG. We detected evidence for the protective role of the overexpressed H 2 -histamine receptors in the contractile dysfunction of DT and isolated atrial preparations subjected to hypoxia. In contrast, we noted the detrimental role of H 2 -histamine receptor overexpression against ischemia (Langendorff perfusion) and LPS-induced systolic heart failure. Hence, the role of H 2 -histamine receptors in the heart is context-sensitive: the results differ between hypoxia (in atrium) and ischemia (perfused whole heart), as well as between genetically induced hypertrophy (DT) and toxin-induced heart failure (LPS). The underlying molecular mechanisms for the protective or detrimental roles of H 2 -histamine receptor overexpression in the mammalian heart remain to be elucidated. SIGNIFICANCE STATEMENTThe beneficial and detrimental effects of the cardiac effects of H 2 -histamine receptors in the heart under stressful conditions, here intended to mimic clinical situations, were studied. The data suggest that depending on the clinically underlying cardiac pathophysiological mechanisms, H 2 -histamine agonists or H 2histamine antagonists might merit further research efforts to improve clinical drug therapy.

We consider a continuum model for motility-induced phase separation (MIPS) of active Brownian particles [J. Chem. Phys. 142, 224149 (2015)]. Using a recently introduced perturbative analysis [Phys.Rev. E 98, 020604(R) (2018)], we show that this continuum model reduces to the classic Cahn-Hilliard (CH) model near the onset of MIPS. This makes MIPS another example of the so-called active phase separation. We further introduce a generalization of the perturbative analysis to the next higher order. This results in a generic higher order extension of the CH model for active phase separation. Our analysis establishes the mathematical link between the basic mean-field MIPS model on the one hand, and the leading order and extended CH models on the other hand. Comparing numerical simulations of the three models, we find that the leading order CH model agrees nearly perfectly with the full continuum model near the onset of MIPS. We also give estimates of the control parameter beyond which the higher order corrections become relevant and compare the extended CH model to recent phenomenological models.

We investigate the orientation of nonlinear stripe patterns in finite domains. Motivated by recent experiments, we introduce a control parameter drop from supercritical inside a domain to subcritical outside without boundary conditions at the domain border. As a result, stripes align perpendicular to shallow control parameter drops. For steeper drops, non-adiabatic effects lead to a surprising orientational transition to parallel stripes with respect to the borders. We demonstrate this effect in terms of the Brusselator model and generic amplitude equations.