Acoustic wave interactions with viscous liquids spreading in the acoustic path of a surface acoustic wave sensor

Banerjee, M.K., 1999. Acoustic wave interactions with viscous liquids spreading in the acoustic path of a surface acoustic wave sensor. PhD, Nottingham Trent University.

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Abstract

This thesis introduces a novel sensor system based on surface acoustic waves (SAW's) to investigate the wetting dynamics of partially localised stripes of highly viscous polydimethylsiloxane (PDMS) oil spreading directly in the acoustic path of a surface acoustic wave sensor. SAW's have their energy confined to within one wavelength of the surface and are extremely sensitive and can detect sub-monolayer surface coverages. Therefore SAW's have the potential to study film dynamics in addition to the dynamics of the macroscopic spreading of a liquid. PDMS oils of four different viscosities (100,000 cSt, 30,000 cSt, 10,000 cSt and 1,000 cSt) have been deposited on the substrate surface of a 169 MHz Rayleigh wave sensor operating in pulse mode, to control the characteristic velocity, v* = γ/η, and thus adjust the timescale for the spreading over several orders of magnitude. Simultaneous measurements of SAW transmission and reflection signals and optical interferometry have been conducted directly in the acoustic path of the sensor. The time evolution of the shape of the stripe was followed over a period of up to eight hours. Image processing algorithms and programs have been created and applied to extract the dynamic changes of geometric parameters. The profile of the liquid has been reconstructed and the changes in geometric parameters (contact width, cylindrical cap radius, cylindrical cap height and contact angle) are found to follow well defined power laws. As the stripe spreads across the acoustic path, with an accompanying decrease in cylindrical cap height to conserve volume, the acoustic signals are reflected and transmitted. The acoustic reflection from the stripe of PDMS is found to show a distinct pattern of resonances. The reflection oscillates in amplitude as the advancing front of the fluid reduces the acoustic path length. The transmitted signals, propagating along the solid-liquid interface, are found to be progressively attenuated as the fluid spreads and show asymmetric shaped attenuation resonances in the transmission coefficient. To model the acoustic attenuation, an approach treating the liquids using a Maxwell model of viscoelastic fluids with a single relaxation time has been developed. Mechanisms for the transmission and reflection resonances are discussed. Resonances in the transmission coefficient occur cyclically and approximately correspond to the stripe height matching n γs/4, where n is odd and γs is the shear wavelength of the fluid. The periodic structure of resonance peaks in the reflection signal is discussed using step and triangular shape approximate forms for the stripe geometry.

Item Type:	Thesis
Creators:	Banerjee, M.K.
Date:	1999
ISBN:	9781369313390
Identifiers:	Number Type PQ10183047 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Jeremy Silvester
Date Added:	03 Sep 2020 14:45
Last Modified:	22 Jun 2023 09:46
URI:	https://irep.ntu.ac.uk/id/eprint/40619

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