[Paper Review] Surface Nanobubble Nucleation Visualized with TIRF Microscopy

Chon U Chan, Claus‐Dieter Ohl|arXiv (Cornell University)|Apr 12, 2012

Minerals Flotation and Separation Techniques3 citations

TL;DR

This study demonstrates the use of total internal reflection fluorescence (TIRF) microscopy with Rhodamine 6G dye to visualize surface nanobubble nucleation on hydrophilic glass with high temporal and spatial resolution. The technique enables real-time observation of nanobubble formation during water-ethanol-water exchange, revealing that stable nanobubbles nucleate within 4 minutes after water replacement, reaching a surface density of 0.55 bubbles/μm², while transient bubbles form and dissolve during the ethanol phase.

ABSTRACT

Nanobubbles are observed with optical microscopy using the total internal reflection fluorescence (TIRF) excitation. We report on TIRF visualization using Rhodamine 6G at 5$μ\,$M concentration which results to strongly contrasted pictures. The preferential absorption and the high spatial resolution allow to detect nanobubbles with diameters of 230\,nm and above. We present a study of the nucleation dynamics from the water-ethanol-water exchange and report the size distributions. Nanobubble nucleation is observed within 4 min after the exchange, later a stable population of nanobubbles with a surface density of 0.55 bubbles\,/$μ$m$^2$ is formed. Interestingly, unstable, slowly dissolving nanobubbles are observed during the first stage of water-ethanol exchange; only after the ethanol-water exchange stable nanobubbles appear.

Motivation & Objective

• To develop a high-temporal-resolution optical method for studying nanobubble nucleation dynamics on hydrophilic surfaces.
• To overcome the limitations of AFM and IR spectroscopy by enabling real-time observation of nanobubble formation and evolution.
• To investigate the nucleation dynamics and stability of surface nanobubbles during the water-ethanol-water exchange process.
• To quantify nanobubble size distributions and surface density changes over time.
• To provide experimental data on nucleation kinetics and equilibrium formation for improved understanding of nanobubble physics.

Proposed method

• Employed TIRF microscopy using a 532 nm green laser and a high-NA objective (NA 1.49) to excite Rhodamine 6G dye at 5 μM concentration.
• Achieved a penetration depth of ~70 nm and a spatial resolution of 108 nm/pixel, enabling detection of nanobubbles ≥230 nm in diameter.
• Used a microfluidic PDMS channel (1 mm wide, 20 μm high) on a glass cover slip to control liquid exchange and bubble formation.
• Performed water-ethanol-water exchange using syringe pumps at controlled flow rates (1–125 μL/min), with fast initial flow to minimize diffusion at the T-junction.
• Monitored dynamics with a cooled CCD camera at 18 fps (exposure 25–40 ms), achieving 56 ms temporal resolution.
• Normalized fluorescence intensity across frames to track bubble formation and dissolution over time.

Experimental results

Research questions

• RQ1 How does the nucleation of surface nanobubbles evolve during the water-ethanol-water exchange process?
• RQ2 What is the temporal sequence of nanobubble formation and dissolution, and when do stable nanobubbles emerge?
• RQ3 How does the flow rate during liquid exchange affect the nucleation speed and final nanobubble density?
• RQ4 What is the size distribution of detectable nanobubbles, and how does it relate to resolution limits?
• RQ5 What is the surface density of stable nanobubbles after nucleation reaches equilibrium?

Key findings

• Nanobubble nucleation occurs within 4 minutes after the water-ethanol-water exchange is completed, with stable nanobubbles forming after the ethanol phase is replaced.
• A stable population of nanobubbles with a surface density of 0.55 bubbles/μm² is established after nucleation.
• Unstable, rapidly dissolving nanobubbles appear during the ethanol phase and vanish within 40 seconds, indicating transient nucleation.
• At a flow rate of 5 μL/min, nanobubble density rises from 0.1 bubbles/μm² at 2.5 minutes to 0.4 bubbles/μm² at 6.5 minutes, with a rise time (10% to 90% of max) of approximately 240 seconds.
• The size distribution of nanobubbles, resolved down to 230 nm (2 pixels), shows a predominance of larger bubbles, with no resolution of those below the limit.
• The nucleation process is flow-rate dependent: higher flow rates (e.g., 50 μL/min) lead to near-instantaneous nucleation, while lower rates prolong the process.