Biofilm density regulates diffusiophoretic colloid penetration

Biofilms are structured communities of microorganisms embedded in an extracellular polymeric substance (EPS) matrix whose dense, polymer-rich architecture strongly hinders the transport of antimicrobial agents and nanoparticle carriers. Diffusiophoresis, the motion of colloidal particles along solute concentration gradients, can enhance penetration into viscoelastic matrices. However, how the changes in biofilm microstructure and density modulate diffusiophoretic delivery of particles is not yet understood. Here, we investigate how biofilm density as characterized by the transmitted light intensity index, Ĩ, modulates diffusiophoretic particle penetration by systematically varying biofilm culture time and particle size and by comparing motile and nonmotile Escherichia coli (E. coli) strains. We find that increasing Ĩ systematically reduces the effective early-time diffusiophoretic mobility, identifying a biofilm-accumulation regime beyond which gradient-enhanced delivery becomes strongly suppressed. Over the range examined here, larger particles penetrate more deeply than smaller ones, indicating that their mobility advantage outweighs the geometric advantage of smaller size. We further observe later-stage reverse particle motion and propose that it results from the combined effects of weakening diffusiophoretic penetration, transient biofilm deformation under the imposed solute gradient, and possible shear-assisted removal of expanded biofilm material near the pore entrance. Together, these results provide a quantitative framework for identifying the range of biofilm accumulation states in which diffusiophoresis can effectively enhance colloidal delivery into biofilms and the denser regime in which biofilm structure suppresses that enhancement.