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Cellularaggregatesareusedasmodelsystemoftumors(1)Whendepositedontofibronectin-coated glassorpolyacrylamide(PAA)gels,theyadhereandspreadbyprotrudingacellularmonolayer("prec ursor film")thatexpandsaroundthelivingdroplet(2,3).Thedynamicsofspreadingresultsfromabalancebetwee nthepullingforcesexertedbythehighlymotilecellsattheperipheryofthefilm,andfrictionforcesassoc iatedwithtwotypesofcellularflows:i)permeation,correspondingtotheentry of thecellsfromtheaggregatesintothe film,andii) slippageasthefilm expands.Wecharacterizethesefiowsbyusingfluorescenttrackingofindividualcellsandparticleima gingvelocimetry(PIV)ofcellpopulations.Wefind thatpermeation islimited toanarrowringofwidthξ(~fewcells)attheedgeoftheaggregateand regulatesthedynamicsofspreading.Furthermore,wefindthatthesubsequentspreadingofthemono layerdependsheavilyonthesubstraterigidity.