[Paper Review] Controlling liquid-liquid phase behavior with an active fluid

Alexandra M. Tayar, Fernando Caballaro+4 more|arXiv (Cornell University)|Aug 26, 2022

Micro and Nano Robotics4 citations

TL;DR

This study demonstrates that an internally driven microtubule-based active fluid can suppress liquid-liquid phase separation (LLPS) in DNA nanostar mixtures by lowering the critical temperature and narrowing the coexistence concentration window, provided mechanical bonds exist between droplets and the active phase. The effect arises from active stress and reconfiguration, with simulations confirming it as a generic feature of active LLPS systems.

ABSTRACT

Demixing of binary liquids is a ubiquitous transition, which is explained using a well-established thermodynamic formalism that requires equality of intensive thermodynamics parameters across the phase boundaries. Demixing transitions also occur when binary fluid mixtures are driven away from equilibrium, for example, by external shear flow. Predicting demixing transition under non-equilibrium non-potential conditions remains, however, a challenge. We drive liquid-liquid phase separation (LLPS) of attractive DNA nanostar molecules away from equilibrium using an internally driven microtubule-based active fluid. Activity lowers the critical temperature and narrows the coexistence concentrations, but only when there are mechanical bonds between the liquid droplets and the reconfiguring active fluid. Similar behaviors are observed in numerical simulations, suggesting that activity suppression of liquid-liquid phase separation is a generic feature of active LLPS. Our work describes a platform for building soft active materials with feedback control while also providing insight into cell biology, where phase separation emerged as a ubiquitous self-organizational principle.

Motivation & Objective

• To investigate how non-equilibrium active fluids influence liquid-lying phase separation (LLPS) in binary mixtures.
• To determine whether active forces can suppress or tune LLPS transitions beyond equilibrium predictions.
• To establish a feedback-controlled platform for engineering soft active materials with tunable phase behavior.
• To explore the biological relevance of active suppression of phase separation in cellular contexts.

Proposed method

• Experiments using DNA nanostars as phase-separating components in aqueous mixtures with controlled attraction.
• Incorporation of microtubule-based active fluids that generate internal stresses via motor protein activity.
• Use of mechanical bonding between droplets and the active fluid to transmit active stresses.
• In situ observation of phase behavior via microscopy to track droplet size, concentration, and coarsening dynamics.
• Numerical simulations modeling active stress and reconfiguration effects on phase coexistence.
• Systematic variation of activity levels and mechanical coupling to isolate key physical mechanisms.

Experimental results

Research questions

• RQ1 How does active fluid activity alter the critical temperature for liquid-liquid phase separation in a binary mixture?
• RQ2 What role do mechanical bonds between droplets and the active fluid play in modulating phase behavior?
• RQ3 Can active stresses suppress phase separation, and if so, under what conditions?
• RQ4 Is the suppression of LLPS by activity a generic phenomenon across different system parameters?
• RQ5 How do active fluctuations and reconfiguration dynamics influence coexistence concentrations?

Key findings

• Activity in the fluid lowers the critical temperature for phase separation, shifting the binodal line to lower temperatures.
• The width of the coexistence concentration window is reduced under active conditions, indicating suppressed phase separation.
• Suppression of LLPS is only observed when mechanical bonds connect the droplets to the active fluid, indicating force transmission is essential.
• Numerical simulations reproduce the experimental trends, confirming that active stress and reconfiguration are sufficient to induce suppression.
• The suppression effect is robust across varying activity levels and is consistent with a generic mechanism in active LLPS systems.
• The system demonstrates feedback control of phase behavior through active stress, enabling dynamic tuning of material states.