Kiran ACHARYA will defend her PhD thesis entitled “Pressure Sensing and Fluid Transport in Lymphatic-Inspired Compliant Channels” on Monday, August 24, 2026, at 10:00 a.m. in Pierre Cotton Room, Institut Fresnel, St. Jerome Campus, Marseille.
The presentation and the slides will be in English.
Jury Composition :
– Cécile BARON, IRPHE, CNRS, Marseille, President
– Baptiste DARBOIS-TEXIER, Laboratoire FAST, CNRS, Orsay, Examiner
– Gaëlle RECHER, LP2N, CNRS, Bordeaux, Reviewer
– Stéphane DORBOLO, CESAM, FNRS, Univ. Liège, Reviewer
– Serge MONNERET, Institut Fresnel, CNRS, Marseille, Supervisor
– Martin BRANDENBOURGER, IRPHE, CNRS, Marseille, Co-Supervisor
– Thomas CHAIGNE, Institut Fresnel, CNRS, Marseille, Invited Member
Abstract : Collecting lymphatic vessels transport lymph against adverse pressure gradients through a series of contractile segments, called lymphangions, each bounded by a pair of passive valves. Dysfunction of these valves leads to diseases for which current treatments are primarily palliative and do not address the underlying cause as it is difficult to understand the reason for the failure. Despite its clinical relevance, the fluid mechanics of lymph transport at the vessel scale remain poorly characterized as this system is robust in its functioning and rarely fails, attracting far less attention compared to the blood circulatory system. Further, the access to the vessels in-vivo is also difficult as they are embedded and hard to image due to tissue scattering. Therefore, this thesis takes the approach of physical experimental models that are simplified, accessible systems made of compliant polymer material, reproducing the essential mechanics of the lymphatic vessel at controlled scales. It identifies key gaps in understanding the functioning of these lymphangions and addresses two of them: the need to measure non-invasively pressure change in micrometric channels and the need to understand the collective interaction of valves in collecting lymphatics. Quadriwave Lateral Shearing Interferometry (QLSI) is used to infer pressure with high sensitivity inside a cylindrical PDMS microchannel from phase-sensitive optical measurements of wall deformation. Upon the application of internal pressures, the technique recovers both the channel outer diameter and the refractive index contrast between substrate and fluid simultaneously from a single phase image. The measured deformation is described analytically by a Neo-Hookean model for a thick-walled cylinder and confirmed numerically by COMSOL finite element simulation. All three approaches are in close agreement, establishing QLSI as a quantitative, non-invasive pressure sensor for soft microfluidic channels. The technique further resolves the transient wall response to pressure steps with sub-micron precision at 0.5 s time resolution. Having established that wall deformation encodes pressure in a compliant channel, the second part of this thesis focuses on the other mechanical feature of the lymphatic pump which is the valves that rectify flow. In a second series of experiments, an array of flexible polymer leaflets inside a rigid millimetric-sized channel is shown to produce flow asymmetry that grows with leaflet density. These observations are well described by a quadratic pressure-flow relation. A flexible PDMS channel with the same leaflet geometry, subjected to cyclic external compression, generates sustained unidirectional net transport. This unit is then scaled up to 3 and 5 units to characterize the role of channel length. When normalized by the imposed volume and plotted against the visco-elasto number (a ratio of viscous drag to leaflet elastic restoring force), the fluid transport shows a clear optimum independently of the system size. This work demonstrates that optical phase measurements in compliant microchannels can serve as a quantitative pressure diagnostic, and that collective leaflet arrays under cyclic compression reproduce key features of lymphatic pumping. Together, these two contributions advance a common direction which is development of experimental model systems for studying the mechanics of lymphatic fluid transport.
