Periodic Reporting for period 2 - LubISS (Lubricant impregnated slippery surfaces)
Período documentado: 2019-01-01 hasta 2021-06-30
The LubISS project aims to explore the expansive potential of lubricant impregnated surfaces, focusing on three applications of high societal, environmental, industrial and medical impact: Anti-icing, easy-to-clean and anti-fouling. Textured substrates, which are infiltrated with a lubricant form a new class of functional surfaces, referred to as Lubricant Impregnated Slippery Surfaces (LubISS). The mobility of particular matter on lubricated surfaces greatly reduces the (lateral) adhesion. Therefore, deposited liquids, bacteria or other microorganisms can slide off easily as soon as the surface is tilted by a few degrees. Although capillary forces help to keep the lubricant in place, LubISS face the problem of limited durability. The lubricant needs to be replenished after some time.
The LubISS network has tacked these challenges. For long-term applications of lubricant impregnated surfaces, small interstices are essential, preferentially below 50 nm. This can be achieved by impregnating densely packed nanoparticles with a non-volatile oil. The ice adhesion strength remained very low, even after 50 icing and deicing cycles on the same spot. Lubricant depletion can also be prevented using active methods. The formation of a shear-resistant lubricant film was achieved by flow-induced self-lubrication using an emulsion.
The LubISS network also contributed to a greatly improved understanding of the interactions between lubricant-drop-substrate-air. A generic description of the dependence of the friction force on velocity, interfacial tensions, viscosity of the drop and lubricant was derived. This insight is essential to design easy-to-clean surfaces. We quantitatively, described the shape and temporal evolution of the lubricant underneath an impacting droplet, a si tuation encountered in spray coating or ink-jet printing.
LubISS provided the early stage researchers (ESR) with excellent cross-disciplinary education. It proved very useful to enhance their management, communication and mentorship skills required to become future leaders in academics or industry.
The LubISS network has tacked these challenges. For long-term applications of lubricant impregnated surfaces, small interstices are essential, preferentially below 50 nm. This can be achieved by impregnating densely packed nanoparticles with a non-volatile oil. The ice adhesion strength remained very low, even after 50 icing and deicing cycles on the same spot. Lubricant depletion can also be prevented using active methods. The formation of a shear-resistant lubricant film was achieved by flow-induced self-lubrication using an emulsion.
The LubISS network also contributed to a greatly improved understanding of the interactions between lubricant-drop-substrate-air. A generic description of the dependence of the friction force on velocity, interfacial tensions, viscosity of the drop and lubricant was derived. This insight is essential to design easy-to-clean surfaces. We quantitatively, described the shape and temporal evolution of the lubricant underneath an impacting droplet, a si tuation encountered in spray coating or ink-jet printing.
LubISS provided the early stage researchers (ESR) with excellent cross-disciplinary education. It proved very useful to enhance their management, communication and mentorship skills required to become future leaders in academics or industry.
The main research objectives of our network were:
• Fabrication of lubricant impregnated surfaces
The beneficiaries developed thermally sprayed slippery surfaces. Coatings composed of densely packed nanoparticles were also prepared by spray coating. Both methods are well suited for large-scale production. In another project, micropillar arrays, nanoparticles and silicone structures - such as Silicone Nanofilaments (SNFs) and Silicone Nanorods - were fabricated and tested. The influence of chemical modification of the surfaces was investigated. Good chemical compatibility between the lubricant and the surface reduces dewetting. Typically, a hydrophobic surface is sufficient. Although, coating the rough or porous surface with PDMS brushes is most favourable.
• Experimental characterization of the shape & stability of the lubricating film and of the adhesion of ice, dispersions and microorganisms
The beneficiaries have shown that laser scanning confocal microscopy is well suited to investigate the moving drops on super-liquid repellent surfaces in 3D. Quantitative information on the temporal evolution of the shape of the wetting ridge of moving drops is obtained.
The long-term stability of lubricant impregnated surfaces was tested under flow conditions. A shear-resistant slippery surface can form if an oil-in-water emulsion flows over a micropillar array. It reprensents a novel and promising method to form a lubricant impregnated surface. The filling mechanism seems to be generic and it can be extended to different solid-lubricant combinations. The friction force a drop experiences while sliding off a lubricant infiltrated surface has been measured and modelled. We tested biofouling of surfaces of different topography and wettability (hydrophilic and hydrophobic). We observed that an irregular three-dimensional layer of silicone nanofilaments suppresses bacterial adhesion. Indeed, bacteria hardly adhered to nanofilament coated surfaces that were infiltrated by water. Icephobic characteristics of SLIPS and slippery coatings have been studied under ice accretion–detachment cycles. The findings are compared with hydrophobic surfaces. The baseline data now covers a range of 28 different surfaces in 4 different icing conditions. Notably, lubricant impregnated densely packed nanoparticles show superior long-term anti-icing performance.
• Theoretical modelling of the static and dynamic properties of the lubricating film, ice adhesion and interaction with particular matter
Using mean field models the dynamics of droplets on solid substrates covered by a thin lubrication layer is investigated. The dynamics of a thin, viscous lubrication layer during and after impact of solid spheres and liquid droplets were studied experimentally, analytically and numerically. Quantitative agreement has been achieved, demonstrating the predictive power of the analytical model and numerical method. Moreover, the beneficiaries simulated droplet dynamics on lubricated slippery interfaces using a Lattice Boltzmann model - both able to simulate multiple components and to handle high-density ratios. The motion of bacteria and particular matter was studied using a hybrid lattice Boltzmann programming method. The hydrodynamic theory of bacteria, cells and self-driven particles was modelled using the concepts of active nematics. The beneficiaries investigated both, the single and collective flow behaviour of swimming organisms, e.g. bacteria, in response to surrounding liquid and surfaces. Furthermore, the network derived a model to describe the growth of frost on lubricated surfaces. The results obtained were consistent with the experimental features, hinting that the important processes were considered.
• Fabrication of lubricant impregnated surfaces
The beneficiaries developed thermally sprayed slippery surfaces. Coatings composed of densely packed nanoparticles were also prepared by spray coating. Both methods are well suited for large-scale production. In another project, micropillar arrays, nanoparticles and silicone structures - such as Silicone Nanofilaments (SNFs) and Silicone Nanorods - were fabricated and tested. The influence of chemical modification of the surfaces was investigated. Good chemical compatibility between the lubricant and the surface reduces dewetting. Typically, a hydrophobic surface is sufficient. Although, coating the rough or porous surface with PDMS brushes is most favourable.
• Experimental characterization of the shape & stability of the lubricating film and of the adhesion of ice, dispersions and microorganisms
The beneficiaries have shown that laser scanning confocal microscopy is well suited to investigate the moving drops on super-liquid repellent surfaces in 3D. Quantitative information on the temporal evolution of the shape of the wetting ridge of moving drops is obtained.
The long-term stability of lubricant impregnated surfaces was tested under flow conditions. A shear-resistant slippery surface can form if an oil-in-water emulsion flows over a micropillar array. It reprensents a novel and promising method to form a lubricant impregnated surface. The filling mechanism seems to be generic and it can be extended to different solid-lubricant combinations. The friction force a drop experiences while sliding off a lubricant infiltrated surface has been measured and modelled. We tested biofouling of surfaces of different topography and wettability (hydrophilic and hydrophobic). We observed that an irregular three-dimensional layer of silicone nanofilaments suppresses bacterial adhesion. Indeed, bacteria hardly adhered to nanofilament coated surfaces that were infiltrated by water. Icephobic characteristics of SLIPS and slippery coatings have been studied under ice accretion–detachment cycles. The findings are compared with hydrophobic surfaces. The baseline data now covers a range of 28 different surfaces in 4 different icing conditions. Notably, lubricant impregnated densely packed nanoparticles show superior long-term anti-icing performance.
• Theoretical modelling of the static and dynamic properties of the lubricating film, ice adhesion and interaction with particular matter
Using mean field models the dynamics of droplets on solid substrates covered by a thin lubrication layer is investigated. The dynamics of a thin, viscous lubrication layer during and after impact of solid spheres and liquid droplets were studied experimentally, analytically and numerically. Quantitative agreement has been achieved, demonstrating the predictive power of the analytical model and numerical method. Moreover, the beneficiaries simulated droplet dynamics on lubricated slippery interfaces using a Lattice Boltzmann model - both able to simulate multiple components and to handle high-density ratios. The motion of bacteria and particular matter was studied using a hybrid lattice Boltzmann programming method. The hydrodynamic theory of bacteria, cells and self-driven particles was modelled using the concepts of active nematics. The beneficiaries investigated both, the single and collective flow behaviour of swimming organisms, e.g. bacteria, in response to surrounding liquid and surfaces. Furthermore, the network derived a model to describe the growth of frost on lubricated surfaces. The results obtained were consistent with the experimental features, hinting that the important processes were considered.
While designing durable and environmentally friendly LubISS, our network greatly contributed to understand the interplay between the physical- and chemical interactions of the surface topography, the lubricating film and the droplet under both static and flow conditions. In particular, we explored the relevance of the interstices. The smaller the interstices, the higher the capillary pressure in the coating, which held the lubricant in place. The height of the wetting ridge results from the balance of the pressure in the ridge and in the coating. Models predicting the adhesion and friction that drops experience on lubricated surfaces were derived, drop impact and drop coalescence dynamics were quantitatively modelled and frost propagation was simulated. In summary we can conclude, that lubricant impregnated surfaces coated with nanofeatures show the desired easy-to-clean and anti-icing properties. The results could only be achieved due to the close interaction between chemists, physicists and material scientists. LubISS was the first world-wide, cooperative research and training initiative to comprehensively address this expanding research field. The knowledge acquired through exchange and expertise holds promise for major breakthroughs and innovations.