Paste extrusion generates a surface lubrication layer

Dense particle-fluid mixtures, or \emph{pastes}, are encountered in the production of various materials, including animal feed, human food, pharmaceuticals, and biomass for bioenergy. The flow behavior of such dense deformable particulate media is poorly understood, as the interplay between applied stresses, particle deformability and interstitial fluids can be very complex. One challenging context is high pressure pipe flow, encountered in extrusion. Despite its widespread use, many questions remain about how during high pressure flow of the paste, the particle-fluid mixture behaves and interacts with boundaries. We show how high pressure paste extrusion induces the formation of a fluid boundary thinner than the particle size. The induced fluid layer emerges from a pressure-induced phase segregation process. The fluid layer is sufficiently thin to affect particle-wall contacts, making the paste friction coefficient tunable. Our results so offer potential pathways for reducing energy consumption and even extrusion product composition and failure.