Phased-array acoustic levitation to characterize contact electrification

Through wear and friction, tribological contacts account for roughly a quarter of global energy consumption [1]. Contact electrification (CE) is a particularly important contributor to these losses [2], causing intense electrostatic forces through a mechanism that is still largely unknown [3]. To minimize energy losses and wear [2], mitigate risks [4], and even recuperate energy [5], it is essential to develop better tools to probe these processes for a broad range of situations and materials.

Recently, contactless techniques using acoustic levitation have emerged as a promising way to study the fundamental mechanisms of CE [6,7]. While these techniques provide unparalleled precision and repeatability, they have so far been constrained by the use of single-axis, single-source ultrasonic levitators, offering high stability at the expense of flexibility and control.

In this thesis project, a method specifically tailored for measuring CE in situations relevant to industrial and natural processes will be developed. The goal is to remove constraints on sample geometry and material, improve rotational stability, and automate sample loading while lowering costs. To achieve this, the latest developments in phased-array levitators will be used, including acoustic holography [8], high-order modes [9] and high-speed feedback [10]. Through this, we hope to improve modeling [11] and better understand where and how to minimize energy losses [2].

[1] Holmberg et al., Friction 5, 263–284 (2017)
[2] Sayfidinov et al., Sci. Adv. 4, eaau3808 (2018)
[3] Lacks et al., Nat. Rev. Chem. 3, 465 (2019)
[4] Glor, Powder Technol. 135–136, 223-233 (2003)
[5] Wu et al., Nat. Commun. 15, 6558 (2024)
[6] Grosjean et al., Phys. Rev. Lett. 130, 098202 (2023)
[7] Grosjean et al., accepted in Nature
[8] Cox et al., Phys. Rev. Appl. 12, 064055 (2019)
[9] Contreras et al., Ultrasonics 138 (2024) 107230
[10] Hirayama et al., Sci. Adv. 8, eabn7614 (2022)
[11] Grosjean et al., Phys. Rev. Mater. 4, 082602(R) (2020)

The candidate should have a background in physics or closely related fields. Relevant subfields include fluid mechanics, acoustics, electrostatics, and soft matter physics in general. Experience in a lab, programming, and electronics skills are all highly beneficial to the project, which will involve designing and building a new experimental setup. We are looking for curiosity-driven candidates that are motivated, adaptable, and creative. Going from the conception of the experiment all the way to the analysis of the data and communication of the results requires a broad range of interests and a willingness to learn new skills along the way. The candidates must demonstrate good initiative, independence, eagerness to work in a team, and have great written and oral communication skills.

This newly established research group focuses on mesoscale problems in experimental soft matter physics. We use tools such as traps and fields to probe structures and study their properties and behaviors. We typically work at the bottom end of what could be considered macroscopics, where forces such as electrostatics and capillarity become prevalent, but systems are not yet thermal. Our objectives are both to simplify complex problems from nature to isolate the essential physical mechanisms, and observe the emergence of complexity from the combination of simple ingredients.