Session: Research Posters

Paper Number: 172819

Establishment of Crystalline Structures in Monodispersed Foam Utilizing Acoustic Manipulation 

Crystalline ordering imparts mechanical uniformity, structural integrity, and predictable transport properties, which are traits that are essential for applications in metamaterials, tissue scaffolds, and programmable soft materials. The formation of crystalline structures in monodispersed foams presents a significant challenge due to the inherent instability of fluid-based bubble systems and the difficulty in achieving long-range order in soft matter. Traditional patterning methods rely on gravitational settling, surfactant-mediated chemical stabilization, or mechanical constraints, each carrying limitations in terms of speed, cytotoxicity, or scalability. This study introduces acoustic manipulation as a viable and efficient method to induce rapid crystallization in monodispersed foams without the need for toxic stabilizers or lengthy settling times.

Monodispersed foams, composed of gas bubbles of uniform size dispersed in a continuous liquid phase, offer substantial promise for applications ranging from thermal insulation and metamaterials to biomedical scaffolds and controlled reaction media. However, achieving ordered three-dimensional bubble arrays within these foams remains challenging. Existing methods for bubble stabilization and patterning, including chemical surfactants and gas saturation with compounds such as perfluorohexane, often introduce environmental concerns and biological incompatibility. In contrast, the use of acoustic fields allows for the physical manipulation of bubble arrays using standing wave patterns that encourage ordered packing, especially in the form of hexagonal close-packed structures.

To evaluate this approach, we developed a microfluidic flow-focusing device capable of producing highly uniform microbubbles using room air supplied under regulated pressure and a continuous aqueous phase composed of deionized water, glycerol, and sodium dodecyl sulfate. Microbubbles were collected and exposed to controlled acoustic fields generated by a piezoelectric actuator driven by a function generator and amplifier. A high-speed camera mounted to an inverted microscope recorded the dynamic behavior of bubbles during and after acoustic excitation.

The results indicate that bubble size decreases consistently with increased gas pressure and fluid flow rate, aligning with capillary number theory. The foam produced was visibly monodispersed, with a low polydispersity index suitable for structured assembly. Upon acoustic excitation in the 70 to 80 kilohertz range, rapid organization of bubbles into crystalline structures was observed. Notably, crystallization occurred within seconds of acoustic excitation, which is significantly faster than gravitational or chemical patterning methods reported in the literature. Furthermore, the bubble lattices exhibited subgrain shifting and alignment behavior, suggesting the presence of dynamic self-assembly mechanisms during acoustic exposure.

This study demonstrates that acoustic manipulation can serve as an effective method for bubble patterning, offering rapid assembly, structural stability after manipulation, and chemical independence. By eliminating reliance on toxic or environmentally harmful agents, this approach enhances the potential for biomedical applications, including tissue-engineered scaffolds designed for optimal cell infiltration and vascularization. Additionally, these structured foams have promising implications for creating advanced thermal and acoustic metamaterials, lightweight structural materials, and programmable reactive systems within lab-on-chip platforms.

Future research will focus on scaling the system to enable bulk crystallization over larger volumes, with particular emphasis on developing porous bioscaffolds suitable for cell seeding or suspending living cells during foam generation. Integrating this acoustic patterning technique into layered manufacturing workflows such as foam-based three-dimensional printing will be explored to fabricate complex tissue-mimicking structures. Additional studies will investigate tuning acoustic parameters to achieve alternative bubble arrangements and assess the longevity and mechanical stability of crystalline foams. This approach provides a versatile, rapid, and environmentally friendly method for structuring soft matter, with broad implications for advanced materials and engineered systems that rely on precisely ordered bubble architectures.

Presenting Author: Blake Acree The University of Memphis

Presenting Author Biography: Blake Acree is a Ph.D. student in Mechanical Engineering at the University of Memphis. His research focuses on bioscaffolds, microfluidics, and bubble generation, with an emphasis on soft matter patterning and tissue engineering applications. He is a member of the university's Microfluidics Laboratory, where he explores the use of acoustic and fluidic systems for the formation of structured foams. In addition to his academic work, Blake coordinates the robotics program for the West Tennessee STEM Hub, supporting K–12 STEM outreach and hands-on engineering education for the Mid-South.

Authors:

Blake Acree The University of Memphis
Yuan Gao University of Memphis