Friction and drag are phenomena that cause energy dissipation and a resulting negative economic effect in many application fields, ranging from mechanics to waterborne transportation. Therefore, design and fabrication of surface coatings with anti-friction and anti-drag properties would represent a huge innovation with many potential positive fallouts. Biomimetic, liquid-repellent coatings have been extensively investigated for their supposed anti-friction[1] and drag reduction[2] properties. However, a vast majority of the studies reported in the literature test these coated surfaces in small, lab-scale facilities in conditions far from those they would face in potential applications. Thus, we focused on the fabrication of liquid-repellent coatings as per different biomimetic approaches followed by the characterization of their anti-friction and anti-drag properties in real operative conditions. The anti-friction properties of a hybrid amphiphobic coating (superhydrophobic + oleophobic) based on nanostructured boehmite AlOOH and fluorolalkylsilanes were first assessed[3]. We deposited the coating on a brass slippers used in axial piston pumps for earth-moving applications, and observed that their wetting properties were effectively modified (from oleophilic to oleophobic with respect to a commonly used lubricant oil). Then, we measured their friction coefficient when rubbing against a swashplate in conditions of hydrodynamic lift. We observed a remarkable reduction in friction coefficient (up to 27% compared to that of uncoated slippers) at pressures up to 100 bars, typical of the starting phases of pump operation. In addition, friction coefficient was still remarkably reduced even after 20 hours of endurance test at constant pressure of 50 bars. On the other hand, the frictional drag reduction properties of different liquid-repellent surfaces were investigated in a high speed water channel. We fabricated water-repellent surfaces on large aluminum plates (48x28 cm2) following two well-known biomimetic approaches, namely the SuperHydrophobic Surface (SHS) approach and the Liquid-Infused Surface (LIS) one. In the experimental setup, water flow in turbulent regime (Re?106) exerted drag on the floating aluminum surfaces and a flexural load cell measured the frictional force. While SHSs showed quick air plastron depletion during the tests and similar drag compared to a reference uncoated panel, LISs showed remarkable drag reduction (up to 16%) in the 1.0÷3.5 m/s velocity range. These first evidences of exceptional anti-friction and drag reduction properties of liquid-repellent coatings foster the need for deeper studies on these subjects and hint at relevant future applications in fields like mechanics and waterborne transportation.
Liquid-repellent coatings for friction and drag reduction in industrial applications
Federico VERONESI;Mariarosa RAIMONDO;Giulio BOVERI
2018
Abstract
Friction and drag are phenomena that cause energy dissipation and a resulting negative economic effect in many application fields, ranging from mechanics to waterborne transportation. Therefore, design and fabrication of surface coatings with anti-friction and anti-drag properties would represent a huge innovation with many potential positive fallouts. Biomimetic, liquid-repellent coatings have been extensively investigated for their supposed anti-friction[1] and drag reduction[2] properties. However, a vast majority of the studies reported in the literature test these coated surfaces in small, lab-scale facilities in conditions far from those they would face in potential applications. Thus, we focused on the fabrication of liquid-repellent coatings as per different biomimetic approaches followed by the characterization of their anti-friction and anti-drag properties in real operative conditions. The anti-friction properties of a hybrid amphiphobic coating (superhydrophobic + oleophobic) based on nanostructured boehmite AlOOH and fluorolalkylsilanes were first assessed[3]. We deposited the coating on a brass slippers used in axial piston pumps for earth-moving applications, and observed that their wetting properties were effectively modified (from oleophilic to oleophobic with respect to a commonly used lubricant oil). Then, we measured their friction coefficient when rubbing against a swashplate in conditions of hydrodynamic lift. We observed a remarkable reduction in friction coefficient (up to 27% compared to that of uncoated slippers) at pressures up to 100 bars, typical of the starting phases of pump operation. In addition, friction coefficient was still remarkably reduced even after 20 hours of endurance test at constant pressure of 50 bars. On the other hand, the frictional drag reduction properties of different liquid-repellent surfaces were investigated in a high speed water channel. We fabricated water-repellent surfaces on large aluminum plates (48x28 cm2) following two well-known biomimetic approaches, namely the SuperHydrophobic Surface (SHS) approach and the Liquid-Infused Surface (LIS) one. In the experimental setup, water flow in turbulent regime (Re?106) exerted drag on the floating aluminum surfaces and a flexural load cell measured the frictional force. While SHSs showed quick air plastron depletion during the tests and similar drag compared to a reference uncoated panel, LISs showed remarkable drag reduction (up to 16%) in the 1.0÷3.5 m/s velocity range. These first evidences of exceptional anti-friction and drag reduction properties of liquid-repellent coatings foster the need for deeper studies on these subjects and hint at relevant future applications in fields like mechanics and waterborne transportation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
