Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface

Gas nuclei in water are typically too small to detect directly. They expand into bubbles under negative pressure, a process called cavitation. Prior research has shown that cavitation can be triggered by high-intensity focused ultrasound (HIFU). However, the role of particle surface properties in gas nucleus formation remains unclear. No prior work had resolved how particle hydrophobicity affects cavitation behavior. This gap motivated a closer look at how surface chemistry influences bubble formation. It was already known that hydrophobic materials interact differently with water than hydrophilic ones. That uncertainty drove the need to investigate how these differences might affect acoustic aggregation. The study aimed to clarify how surface properties influence cavitation and aggregation. This uncertainty highlights the need for more detailed studies on particle-surface interactions in acoustic fields.

This study aimed to examine how gas nuclei form and behave in suspensions of hydrophilic and hydrophobic silica particles. The researchers wanted to determine whether particle surface properties affect cavitation thresholds and aggregation outcomes. They focused on whether hydrophobic surfaces promote gas nucleus formation more than hydrophilic ones. The motivation was to explore how these differences might be used for selective particle aggregation. They hypothesized that hydrophobic particles would support stable bubble formation under ultrasound. The goal was to test this hypothesis using high-intensity focused ultrasound and high-speed imaging. The researchers also wanted to analyze cavitation thresholds and acoustic forces to explain the observed effects. This approach could lead to new methods for particle separation in industrial settings.

The researchers used high-intensity focused ultrasound (HIFU) to measure transient cavitation thresholds in silica particle suspensions. They prepared suspensions of hydrophilic and hydrophobic silica particles in water. Gas nuclei formation was tracked using high-speed camera imaging. They observed cavitation bubbles under negative pressure conditions. The aggregation process was captured using a charge-coupled device (CCD) camera. They analyzed how 200 kHz ultrasonic standing waves (USW) affected particle behavior. The cavitation thresholds were compared between the two types of suspensions. The acoustic radiation forces were calculated to explain the aggregation mechanism.

Gas nuclei formed only on the surface of hydrophobic silica particles. These nuclei grew and coalesced into stable bubbles under 200 kHz USW. Aggregation occurred only in hydrophobic particle suspensions, not in hydrophilic ones. The cavitation threshold was lower for hydrophobic particles than hydrophilic ones. The CCD camera captured the aggregation process in real time. The high-speed camera confirmed bubble formation on hydrophobic surfaces. The study found that acoustic radiation forces were stronger on hydrophobic particles. These findings suggest that surface chemistry controls cavitation and aggregation behavior.

The study showed that gas nuclei form selectively on hydrophobic particle surfaces. These nuclei grew into stable bubbles under 200 kHz USW, leading to aggregation. The aggregation process was observed using a CCD camera. The results suggest that hydrophobic surfaces promote bubble formation and aggregation. The cavitation threshold was lower for hydrophobic particles than for hydrophilic ones. The acoustic radiation forces were stronger on hydrophobic particles. This method could be used for selective particle aggregation in industrial applications. The findings support the use of acoustic methods for separating hydrophobic materials.