Monday 11.03.2024 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Electrokinetic interaction between Surface Acoustic Waves and Electrolyte Solutions
Ofer Manor
1Applied Mathematics, Technion – IIT, Haifa; 2Chemical Engineering, Technion – IIT, Haifa
This study commenced several years ago when we asked ourselves how MHz- to GHz-frequency surface acoustic waves (SAWs) that travel in a solid substrate influence the dynamics of ions in neighboring electrolyte solutions. While it is known that electrolyte solutions affect SAWs in piezoelectric materials through the conductivity of the electrolyte, our question followed an observation in the opposite direction: Acousto-capillary flows in micron thick films of water, excited by SAWs, did not show the nice agreement to fluid mechanics theory that silicon oils showed. The difference between oil and water dynamics suggests contributions of ion pressure, also known as electrical double layer (EDL) pressure, to water film dynamics. Moreover, EDLs and SAWs exist at similar time and length scales, so that one expects an interaction. [1]
EDLs of ions are a surface phenomenon. These are nanometer thick clouds of ions that appear at the charged interface between a substrate and an electrolyte solution and give rise to the complexity of biology and to countless industrial processes and products from water desalination to shampoo and super-capacitors. Ions diffuse through the EDL nanometer thickness in nanoseconds: These are ion-specific times for charging or discharging the EDL and are known as the EDL relaxation times. When similar to the periodic times of SAWs, one expects interaction; albeit the SAW travels in solids and the EDL exists in electrolyte solutions. However, both phenomena become entangled through mechanical and (sometimes) electrical field effects.
Ions in the EDL vibrate and may undergo electro-mechanical resonance at the exciting SAW frequency, which results in the leakage of same frequency electrical fields off the EDL; see figure 1. By measuring the electrical leakage, we identify ion-specific relaxation times and hence the intrinsic rate of EDL charge and discharge: We show a new type of spectroscopy in the MHz ultrasonic frequency spectrum that gives ion fingerprints and their dynamics in EDLs and observe that dynamic EDL is much more than a capacitor. [2]
Figure 1: (right) Illustration of a surface acoustic wave (SAW) that travels in a solid substrate and dynamically perturbs ion positions in an electrical double layer (EDL) by generating an evanescent wave in the neighboring electrolyte solution, where red and blue indicate fast and slow ion motions [3]; this results in the leakage of an electrical field,
E→�→
, that we measure using our experimental system (left).
References
[1] O. Dubrovski and O. Manor, Revisiting the electroacoustic phenomenon in the presence of surface acoustic waves, Langmuir, 37, 14679–-14687 (2021)
[2] S. Aremanda and O. Manor, Measurements of Ion Dynamics and Electro-Mechanical Resonance in an Electrical Double Layer near a Surface Acoustic Wave, J. Phys. Chem. C, 127, 20911–20918 (2023)
[3] O. Manor, L.Y. Yeo, and J.R. Friend, The appearance of boundary layers and drift flows due to high-frequency surface waves, J. Fluid Mech., 707, 482–495 (2012)
4/3/2024
Biography: Ofer Manor
Ofer Manor is an Associate Professor at the Department of Chemical Engineering at the Technion in Haifa and is associated with the Interdepartmental Program for Applied Mathematics. He is a Marie Curie Fellow and a recipient of the Henri Gutwirth Research Award.
Research
Ofer uses continuum theory and experiment to study surface phenomena and fluid mechanics, many times in the presence of seismic and ultrasonic waves. He further uses classical density functional theory to investigate structure and properties of complex fluids and surface forces.
Education and positions
Ofer received his BSc diploma from the Wolfson Department of Chemical Engineering in the Technion in 2003. He then accepted an engineering position at Rafael Ltd. (2003-2007) while obtaining his MSc diploma in the field of fluid mechanics at the Technion in 2006. He received his PhD diploma from the department of Mathematics in the field of Colloid Physics at The University of Melbourne, Australia, in 2010, which was followed by postdoctoral fellowships at the Department of Mechanical Engineering at Monash University (2010-2012) and at the Department of Electrical Engineering at RMIT University (2012-2013) in Australia; both postdocs were in the field of acousto-fluidics. Ofer joined the Technion in 2013, where he has been teaching various courses in transport phenomena, numerical analysis, and colloid and surface science.
