New opto-acoustic techniques in determining properties of complex suspensions

The objective of the project is to develop new measurement techniques that can be applied to determine properties of complex suspensions. One example a complex suspension is paper pulp, where slender fibres of different sizes, fine material and water are mixed to form a suspension. Other examples are blood, intralipid solution or other biological and biomedical suspensions.

The new technique we are using is the scattering photoacoustic technique (SPA). The SPA technique uses a pulsed laser to inject short bursts of energy into the suspension. The laser energy entering the material will partially be absorbed and partially scattered in the suspension. The absorption of light energy causes a rapid heating and cooling process locally in the suspension. This thermal process is the source for the acoustic, or ultrasonic, waves that will propagate through the suspension. The photons that are not absorbed will scatter through the suspension. Both the scattered photons the acoustic waves carry information on the medium they have been propagating through. The transmitted acoustic waves as well as the scattered photons are received using the same optoacoustic device. This recording of both the acoustical waves and the scattered and/or transmitted photons enables us to simultaneously study the optical and the acoustical properties of the suspension.

The SPA technique has recently proven successful in determining the fibre and fines material concentration respectively in paper pulp. Research is also performed to develop methods to determine the mechanical properties of the suspended fibres in the suspension. A cornerstone of the project is modeling where the acoustic and optical properties as well as the conversion of optical to acoustical energy is modeled from first principles. These models are used in conjunction with experimental results to determine the properties of the suspension. This class of problems, generally known as inverse problems, is an active and growing field of research by the group.