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Microfluidics-based technologies are emerging solutions towards cell separation process. They rely on various physical principles, such as dielectrophoretic force, hydrodynamic force, and acoustic force. In order to take advantage of these approaches for cell sorting, it is crucial to characterize cell biophysical properties, i.e. size, density, compressibility or acoustic contrast factor in the case of acoustophoresis. Various techniques exist to measure those features, including acoustofluidics methods. However, previously described approaches do not allow the determination of all the physical parameters of a given cell. For this study, a 330 μm deep acoustic cavity has been designed, together with an optical technique, to measure the size, the acoustic contrast factor and the density of cells to finally determine their compressibility. A defocusing technique is used to assess the velocity of sedimentation and acoustic focusing of individual cells which can lead to their physical properties using the analytic expression of the Acoustic Radiation Force. This method is used to investigate the evolution of the biophysical properties of Mesenchymal Stromal Cells (MSCs), isolated from two different tissues from different donors, from one passage to the other. Our results highlighted an important heterogeneity of acoustic contrast factor and compressibility among cells from a same donor but also inter-donor. Nevertheless, from passage to passage, the acoustic contrast factor and compressibility of MSCs converge and homogenize at the fourth passage. On the other hand, the density of MSCs remains homogenous from passage to passage and between different donors. Using this technique, it was possible to successfully assess the different biophysical properties of mesenchymal stromal cells using a single setup based on acoustic levitation. The results confirm the necessity to use such a technique to measure the cells properties. It also demonstrates the large heterogeneity of donor/patient-derived cells, in contrast with cultured cells whose properties homogenize during the successive cultures. As a consequence, cell separation processes will be more complex for patient/donor derived cells than for cultured cells.

By means of an ultrasonic technique, the nucleation rate J in depletion experiments has been measured. An array of 8 ultrasonic transducers, fixed along a rock sample. continuously scans the sound velocity and the attenuation in the sample. Due to the great difference in sound velocity between the gas and the liquid, bubbles are easily detected and located by transducers. The sample is also set up with capillary tube at one end for the observation of production by the displacement of a meniscus. The main advantage of ultrasonic technique is the capacity to work at high pressure and temperature. An improvement in the accuracy over the standard techniques is observed. Introduction Many oil reservoirs produce oil by primary depletion that leads to the formation of a gas phase when the pressure falls below the bubble point. Depletion experiments in reservoir pressure and temperature conditions need to be carried out pressure and temperature conditions need to be carried out to determine the influence of parameters such as fluid properties, nature of the rock, depletion rate, etc. The most properties, nature of the rock, depletion rate, etc. The most important parameter governing the nucleation and the critical gas saturation in oil reservoirs is the nucleation rate J expressed by: (1) where Vo is the volume of the fluid contained in the rock sample and At the time necessary for the first bubble to appear under a given supersaturation. In this paper we present an original method to determine the bubble nucleation rate during depletion experiments in rock samples by means of an ultrasonic technique. This technique presents several advantages:the capacity to work at high pressure and temperature without a complicated equipment;the possibility of locating the bubble formed in the sample;the ability of detecting very small volumes u compared to standard methods based on meniscus displacement. Several authors have studied the heterogeneous nucleation by evaluating the nucleation rate. Kennedy and Olson report visual observations of bubbles nucleating at the surface of silica and calcite crystals in a mixture of methane and kerosene. Wood has studied nucleation of oil in a porous medium by the displacement of a meniscus in a capillary tube following the volume increase. Wieland and Kennedy have redesigned the apparatus of Wood to allow greater precision at low supersaturation so that a volume change as precision at low supersaturation so that a volume change as small as 0.05 mm can be detected by electric bellows under pressures in the range of 80 - 100 × 10- Pa. Lubetkin and Blackwell have developed a technique for counting the number of bubbles released out of a supersaturated solution of carbon dioxide in water. However, this technique, which consists in detecting the bursting of the bubbles when they reach the liquid surface, cannot be applied to porous media. Many problems arise with the methods based on volume change measurements. Firstly, this volume change has two different contributions which are difficult to discern: the volume variation of the rock sample due to its elasticity and the volume change due to bubble formation. Secondly, it Is impossible to locate the bubble in the sample. Finally, there are many problems for the visualization of the meniscus in high pressure and temperature experiments. We present in this paper an acoustical technique which overcome these difficulties. P. 551

In recent years, 3D cell culture models such as spheroid or organoid technologies have known important developments. Many studies have shown that 3D cultures exhibit better biomimetic properties compared to 2D cultures. These properties are important for in-vitro modeling systems, as well as for in-vivo cell therapies and tissue engineering approaches. A reliable use of 3D cellular models still requires standardized protocols with well-controlled and reproducible parameters. To address this challenge, a robust and scaffold-free approach is proposed, which relies on multi-trap acoustic levitation. This technology is successfully applied to Mesenchymal Stromal Cells (MSCs) maintained in acoustic levitation over a 24-hour period. During the culture, MSCs spontaneously self-organized from cell sheets to cell spheroids with a characteristic time of about ten hours. Each acoustofluidic chip could contain up to 30 spheroids in acoustic levitation and four chips could be ran in parallel, leading to the production of 120 spheroids per experiment. Various biological characterizations showed that the cells inside the spheroids were viable, maintained the expression of their cell surface markers and had a higher differentiation capacity compared to standard 2D culture conditions. These results open the path to long-time cell culture in acoustic levitation of cell sheets or spheroids for any type of cells.