Ultrasound as a Tool to Infer Particle Deformability in Model Blood Suspensions

Context & Objectives. Blood is a complex fluid due to the coupling between the macroscopic and microscopic scales: blood flow is affected by the spatial arrangement of cells (i.e., the microstructure), which depends on cell interactions and characteristics, while the macroscopic flow in turn modifies this microstructure. Red blood cells (RBCs) are the primary cellular component of blood, as they constitute the vast majority (97%) of the blood cell content and occupy a large volume fraction (called hematocrit) of 35-45% under normal conditions. Consequently, blood rheology is mainly determined by the deformability and aggregation properties of RBCs. For instance, the shear-thinning behavior of blood results from the rupture of RBC aggregates as shear stress increases, and blood viscosity is known to increase with reduced deformability of pathological RBCs. So far, the link between cellular deformability, microstructure and blood rheology remains poorly understood, despite its critical importance for understanding blood flow both in health and disease. 
So far, the connection between the rheology of blood and its microscopic properties at the cellular level (i.e., microstructure and RBC deformability properties) is not well understood, because of the lack of experimental techniques able to yield reliable measurements of the microstructure of RBC suspensions. Blood, being an opaque suspension, limits the use of optical to 2D confined flows [1] or in diluted suspensions [2], which are not physio-pathologically representative. Recently, the LMA and IUSTI laboratories developed a novel anisotropic quantitative ultrasound (QUS) technique [3]. This method measures angular-dependent structure factors to detect the main angular positions of regions depleted in cell pairs within sheared blood (Figure 1). For the first time, it provides a means to probe the anisotropic microstructure of concentrated RBC suspensions under shear flow and detect abnormal alteration in RBC deformability. Before assessing the performance of this QUS approach for pathophysiological human studies, its ability to detect a change in RBC deformability needs to be evaluated and understood.

Figure 1. (a) Sheared suspensions probed by ultrasound for different insonification angles . (b) Pair correlation function g in the shear plane obtained from numerical simulations for deformable RBC suspensions at volume fraction of 30% (collaboration LMA-IMAG-IUSTI). The main direction of the region depleted in cell pairs is indicated in dashed line. (c) Corresponding structure factor S linked to the Fourier transform of g. (d) Measured structure factors as a function of  for sheared suspensions of deformable and rigidified RBCs at volume fraction of 30%.

The aim of this M2 internship is to enlighten the role of deformability of particles/RBCs on the microstructure and rheology of sheared concentrated suspensions in unconfined flow. Optical and ultrasound measurements will be conducted on transparent model suspensions sheared in a Couette rheometer, using the optics as a reference measurement to visualize and quantify the particle spatial arrangement (Figure 2). The transparent (iso-index) model suspensions consist of suspensions of rigid or soft-spheres with various deformabilities (≈200-µm diameter). When compared to RBC suspension, the model suspension is a large-scale experiment with particle radius a=100-μm insonified at 0.5-2.5 MHz corresponding to adimensional ka comprised between 0.2 and 1 (where k is the wavenumber). The microstructure changes obtained by optics will then be correlated to the results of ultrasonic measurements carried out on the same suspensions. Optical measurements will be conducted in collaboration with Laurence Bergougnoux, IUSTI Marseille.

Contact: 
Emilie Franceschini    email : emilie.franceschini@univ-amu.fr

Profiles: Candidates with skills in physics and/or acoustics, a good level on Matlab or Python programming and having an interest in experiments are welcomed to apply. 
Duration of the internship: 4 to 6 months from February 2026              

Grant. 600 € / month 
Possibility of PhD. Yes, according to the ranking at the doctoral school for ministerial scholarship 

[1] Iss C et al. (2019) Self-organization of RBC suspensions under confined 2D flows. Soft Matter 15:2971-2980
[2] Dupire J, Socol M, Viallat A (2012) Full dynamics of a red blood cell in shear flow. Proc Natl Acad Sci USA 109:20808-20813
[3] Lombard O et al. (2020) Ultrasonic backscattering and microstructure in sheared concentrated suspensions. J Acoust Soc Amer 147:1359-1367