Analysis of environmental changes has become a great issue nowadays with the increase of pollution and global warming. Surveys about growing populations of species and apparition of new parasites are more and more relevant to preserve our environment and populations needs. The ability to observe for instance the presence of bacteria such as Criptosporidium and Giardia Lamblia in freshwater contributes to prevent people from diseases even in developed countries. The main issue in the observation of such species is that the size of the biological sample should be large enough to be representative of its surrounding environment, meaning that the amount of the scanned sample is several orders superior to the size of the micro species of interest.

A major limitation for optical instruments in performing this kind of analysis is that the magnification required should fit the size of the particles, severely constraining the field of view. As a result, the rate of analysis is often so slow to be compatible with the large samples to analyze.

One way to increase the rate of analysis is to manipulate particles in order to focus them in the field of view of the visualization system.

In this article we use acoustic standing waves to achieve this goal by directing particles in the observation region.

A 1mm × 1mm square section glass channel, excited by a piezo transducer is used. The carrier fluid is water while several types of particles, both synthetic and biological are tested. The dynamic behaviour of particles under the acoustic field has been investigated and the 3D position of each particle is reported.

In order to increase the depth of investigation, we used a Digital Holographic Microscope. This technology permit to scan a larger volume of fluid and also to calculate the 3D position of each detected particle flowing in the channel.

A focusing streamline of as thin as 1/20 of the channel cross section has been successfully achieved.

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