[Paper Review] Effect of controlled corrugation on capillary condensation of colloid-polymer mixtures

Andrea Fortini, Matthias Schmidt|arXiv (Cornell University)|Jan 1, 2013

Phase Equilibria and Thermodynamics1 references39 citations

TL;DR

This study uses Monte Carlo simulations to investigate how controlled corrugation of a substrate affects capillary condensation in colloid-polymer mixtures. It finds that increased corrugation amplitude enhances capillary condensation primarily by increasing the fluid-substrate contact area, with theoretical predictions from a modified Kelvin equation showing strong agreement with simulation data.

ABSTRACT

We investigate with Monte Carlo computer simulations the capillary phase behaviour of model colloid-polymer mixtures confined between a flat wall and a corrugated wall. The corrugation is modelled via a sine wave as a function of one of the in-plane coordinates leading to a depletion attraction between colloids and the corrugated wall that is curvature dependent. We find that for increased amplitude of corrugation the region of the phase diagram where capillary condensation occurs becomes larger. We derive a Kelvin equation for this system and compare its predictions to the simulation results. We find good agreement between theory and simulation indicating that the primary reason for the stronger capillary condensation is an increased contact area between the fluid and the corrugated substrate. On the other hand, the colloid adsorption curves at colloid gas-liquid coexistence show that the increased area is not solely responsible for the stronger capillary condensation. Additionally, we analyse the dimensional crossover from a quasi-2D to a quasi-1D system and find that the transition is characterised by the appearance of a metastable phase.

Motivation & Objective

• To understand how substrate corrugation influences capillary phase behavior in colloid-polymer mixtures.
• To determine whether geometric effects such as increased contact area dominate capillary condensation in corrugated systems.
• To derive and validate a modified Kelvin equation for curved, corrugated substrates in confined fluid systems.
• To analyze the dimensional crossover from quasi-2D to quasi-1D confinement and its impact on phase stability.

Proposed method

• Employing Monte Carlo simulations to model colloid-polymer mixtures confined between a flat wall and a sinusoidally corrugated wall.
• Modeling corrugation as a sine wave function in one in-plane coordinate to generate curvature-dependent depletion attractions.
• Deriving a modified Kelvin equation that accounts for the curvature and corrugation amplitude in the system.
• Comparing simulation results for capillary condensation and colloid adsorption with theoretical predictions from the modified Kelvin equation.
• Analyzing the dimensional crossover by varying the system's effective dimensionality from quasi-2D to quasi-1D.
• Identifying the emergence of metastable phases during the dimensional transition through analysis of adsorption and coexistence curves.

Experimental results

Research questions

• RQ1 How does increasing the amplitude of substrate corrugation affect the extent of capillary condensation in colloid-polymer mixtures?
• RQ2 To what extent does the increased fluid-substrate contact area due to corrugation explain the enhanced capillary condensation?
• RQ3 Does the modified Kelvin equation accurately predict capillary condensation in corrugated systems compared to simulation data?
• RQ4 How does the system's dimensional crossover from quasi-2D to quasi-1D influence phase behavior and stability?
• RQ5 What role do metastable phases play in the transition between different dimensional confinement regimes?

Key findings

• Increased corrugation amplitude leads to a larger region of the phase diagram where capillary condensation occurs.
• The modified Kelvin equation derived for this system shows good quantitative agreement with simulation results, validating its predictive power.
• The enhanced capillary condensation is primarily attributed to the increased contact area between the fluid and the corrugated substrate.
• Despite the increased contact area, colloid adsorption data indicate additional contributions beyond geometry, suggesting non-geometric effects in condensation enhancement.
• The dimensional crossover from quasi-2D to quasi-1D confinement is marked by the appearance of a metastable phase in the phase diagram.
• The transition is characterized by a shift in phase coexistence behavior, indicating a significant change in effective interfacial and confinement effects.