Critical behavior of a two-dimensional complex fluid: Macroscopic and mesoscopic views

Area of Science:

• Surface science
• Physical chemistry
• Soft matter physics

Background:

• Fatty acid Langmuir monolayers exhibit liquid disordered (Ld) and liquid ordered (Lo) phases.
• Understanding phase transitions and critical phenomena in these systems is crucial for materials science.
• Myristic acid (MyA) serves as a model system for studying such transitions.

Purpose of the Study:

• To precisely locate the liquid disordered (Ld) to liquid ordered (Lo) phase transition critical point in myristic acid (MyA) Langmuir monolayers.
• To investigate the macroscopic and mesoscopic behaviors of the monolayer as it approaches the critical point.
• To determine if the phase transition follows established theoretical models like the 2D Ising model.

Main Methods:

• Macroscopic study using surface pressure-area isotherms across a temperature range (20°C-38°C).
• Mesoscopic imaging of domain morphology using Brewster Angle Microscopy (BAM).
• Analysis of domain morphology via Power Spectral Density Function (PSDF) to assess correlation length and scaling behavior.

Main Results:

• The critical point was consistently identified at approximately 38°C using both isotherm and BAM data.
• Monolayer morphology evolved from circular to dendritic domains as the critical point was approached.
• Scaling behavior in the critical regime aligned with the two-dimensional Ising model, with a precritical regime exhibiting fluctuations and unstable domains.

Conclusions:

• The study successfully located the critical point of the MyA Langmuir monolayer phase transition.
• The observed critical behavior exhibits characteristics of both complex and nonequilibrium systems.
• The system demonstrated non-ergodic behavior, with ensemble averages not strictly matching time development.