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Summary Although mixing nanoparticles with certain biological molecules can result in coronas that afford some control over how engineered nanomaterials interact with living systems, corona formation mechanisms remain enigmatic. Here, we report results from experiments and computer simulations that provide concrete lines of evidence for spontaneous lipid corona formation without active mixing upon attachment to stationary and suspended lipid bilayer membranes. Experiments show that polycation-wrapped particles disrupt the tails of zwitterionic lipids, increase bilayer fluidity, and leave the membrane with reduced ζ potentials. Computer simulations suggest that the contact ion pairing between the lipid head groups and the polycations' ammonium groups leads to the formation of stable, albeit fragmented, lipid bilayer coronas. The mechanistic insight regarding lipid corona formation can be used to improve control over nano-bio interactions and to help understand why some nanomaterial-ligand combinations are detrimental to organisms but others are not.