[Paper Review] How Nanobubbles Nucleate at a Hydrophobic/Water Interface

Ing‐Shouh Hwang, Chung-Kai Fang+3 more|arXiv (Cornell University)|Dec 25, 2015

Minerals Flotation and Separation Techniques12 references3 citations

TL;DR

This study reveals that nanobubbles nucleate at hydrophobic/water interfaces through a two-step process: first, a thin, fluid wetting layer (~0.3 nm thick) forms, which then transforms into a cap-shaped interfacial nanobubble. The formation of ordered, hydrophilic domains at the perimeter of these fluid regions stabilizes the nanobubble by reducing interfacial tension, explaining its high stability and role in boundary slip and contact angle hysteresis.

ABSTRACT

Experimental investigations of hydrophobic/water interfaces often return controversial results, possibly due to the unknown role of gas accumulation at the interfaces. Here, during advanced atomic force microscopy of the initial evolution of gas-containing structures at a highly ordered pyrolytic graphite/water interface, a fluid phase first appeared as a circular wetting layer ~0.3 nm in thickness and was later transformed into a cap-shaped nanostructure (an interfacial nanobubble). Two-dimensional ordered domains were nucleated and grew over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that interfacial nanobubbles and fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature, explaining their high stability and their roles in boundary slip and bubble nucleation. These ordered domains may be the interfacial hydrophilic gas hydrates and/or the long-sought chemical surface heterogeneities responsible for contact line pinning and contact angle hysteresis. The gradual nucleation and growth of hydrophilic ordered domains renders the original homogeneous hydrophobic/water interface more heterogeneous over time, which would have great consequence for interfacial properties that affect diverse phenomena, including interactions in water, chemical reactions, and the self-assembly and function of biological molecules.

Motivation & Objective

• To resolve controversies in experimental studies of hydrophobic/water interfaces by identifying the role of gas accumulation.
• To investigate the initial nucleation and growth dynamics of gas-containing structures at a highly ordered pyrolytic graphite/water interface.
• To determine the mechanical and structural properties of interfacial nanobubbles and their precursor fluid layers.
• To explore the origin of contact angle hysteresis and boundary slip in hydrophobic systems.
• To clarify the nature of interfacial heterogeneities that affect interfacial phenomena in water.

Proposed method

• Employed advanced atomic force microscopy (AFM) to image the initial evolution of gas-containing structures at a highly ordered pyrolytic graphite/water interface.
• Monitored real-time formation of a fluid wetting layer (~0.3 nm thick) followed by transformation into cap-shaped nanobubbles.
• Tracked the nucleation and growth of two-dimensional ordered domains at the perimeter of fluid regions.
• Analyzed mechanical properties of nanobubbles and fluid layers to infer interfacial tension and liquid-like behavior.
• Correlated the formation of ordered domains with potential roles in contact line pinning and interfacial heterogeneity.
• Used time-resolved AFM to observe the gradual increase in interfacial heterogeneity over time.

Experimental results

Research questions

• RQ1 How do nanobubbles nucleate at hydrophobic/water interfaces at the nanoscale?
• RQ2 What is the role of the initial fluid wetting layer in nanobubble formation?
• RQ3 What causes the high stability of interfacial nanobubbles?
• RQ4 How do ordered domains at the interface influence contact angle hysteresis and boundary slip?
• RQ5 What is the nature of the interfacial heterogeneities responsible for pinning and dynamic hysteresis?

Key findings

• A fluid wetting layer of approximately 0.3 nm thickness forms first at the hydrophobic/water interface before nanobubble nucleation.
• The fluid layer transforms into a cap-shaped interfacial nanobubble, indicating a dynamic nucleation pathway.
• Interfacial nanobubbles and fluid layers exhibit very similar mechanical properties, suggesting low interfacial tension and liquid-like behavior.
• Two-dimensional ordered domains nucleate and grow at the perimeter of fluid regions, eventually confining vertical growth and stabilizing the nanobubble.
• These ordered domains are likely interfacial hydrophilic gas hydrates or chemical surface heterogeneities responsible for contact line pinning.
• The gradual formation of ordered domains increases interfacial heterogeneity over time, significantly affecting interfacial properties and phenomena such as boundary slip and contact angle hysteresis.