Fluid dynamic behaviour of dilute dunaliella salina suspensions in contraction-expansion microfluidic devices, potential implications for tubular phototbioreactors

In the last decades, microalgal biotechnology has been triggering increasing attention and several species have been cultivated industrially in photobioreactors (PBRs) of various designs. However, operational costs are currently too high since, in order to guarantee proper mixing levels and prevent algal accumulations, the cells are turbulently pumped and treated as passive particles. Whereas, PBRs should be adapted to the intrinsic features of the cultured organisms; hence, it is paramount to devise a new procedure that would permit experimental data to be acquired directly from the microalgae and use the data for optimising all the phases of the PBR design and utilisation. Therefore, the main objective of this PhD was to investigate experimentally the possibility of exploiting microfluidic devices as complementary tools for PBRs optimisation processes, focusing particular attention on horizontal PBRs equipped with static mixers. The experiments were mainly aimed at exploring the fluid dynamic behaviours of a specific microalga, Dunaliella Salina (DS), considering both living and dead cells while flowing within microchannels characterised by different abrupt expansion-contraction ratios (ECR) geometries (ECR 2-1, ECR 4-1 and ECR 7-1), assuming the employed microchannels as "simplified PBRs" and their microgeometries as static mixers.Initially, the experiments explored the algal behaviours subject to different flow rates in the channels, in order to assess whether better processing conditions may be achieved by optimising the PBR geometrical configuration and therefore addressing the exploitation of microfluidic devices for design purposes. The microgeometries were found to cause the developments of algal plumes that inhibited the wall interactions of the living cells but increased those of the dead ones. These opposite behaviours were due to different spatial distributions of the cells in the plumes. Moreover, the algal plumes also caused the formation of cell free zones which led to more homogeneous irradiance profiles.The second set of experiments investigated the potential use of microchannels for quality control purposes, studying the fluid dynamic experiences of the cells in crucial regions of the channels through targeted image analysis processes and assessing sizes and shapes of the DS cells studied. The results demonstrated that microfluidic devices can be used to detect early shear-induced damage to the cells and the variations of their size and shape distributions during the cultivation process.In the last decades, microalgal biotechnology has been triggering increasing attention and several species have been cultivated industrially in photobioreactors (PBRs) of various designs. However, operational costs are currently too high since, in order to guarantee proper mixing levels and prevent algal accumulations, the cells are turbulently pumped and treated as passive particles. Whereas, PBRs should be adapted to the intrinsic features of the cultured organisms; hence, it is paramount to devise a new procedure that would permit experimental data to be acquired directly from the microalgae and use the data for optimising all the phases of the PBR design and utilisation. Therefore, the main objective of this PhD was to investigate experimentally the possibility of exploiting microfluidic devices as complementary tools for PBRs optimisation processes, focusing particular attention on horizontal PBRs equipped with static mixers. The experiments were mainly aimed at exploring the fluid dynamic behaviours of a specific microalga, Dunaliella Salina (DS), considering both living and dead cells while flowing within microchannels characterised by different abrupt expansion-contraction ratios (ECR) geometries (ECR 2-1, ECR 4-1 and ECR 7-1), assuming the employed microchannels as "simplified PBRs" and their microgeometries as static mixers.Initially, the experiments explored the algal behaviours subject to different flow rates in the channels, in order to assess whether better processing conditions may be achieved by optimising the PBR geometrical configuration and therefore addressing the exploitation of microfluidic devices for design purposes. The microgeometries were found to cause the developments of algal plumes that inhibited the wall interactions of the living cells but increased those of the dead ones. These opposite behaviours were due to different spatial distributions of the cells in the plumes. Moreover, the algal plumes also caused the formation of cell free zones which led to more homogeneous irradiance profiles.The second set of experiments investigated the potential use of microchannels for quality control purposes, studying the fluid dynamic experiences of the cells in crucial regions of the channels through targeted image analysis processes and assessing sizes and shapes of the DS cells studied. The results demonstrated that microfluidic devices can be used to detect early shear-induced damage to the cells and the variations of their size and shape distributions during the cultivation process