Self-Cleaning Mechanism: Why Nanotexture and Hydrophobicity Matter.

Dust particles can adhere to surfaces, thereby decreasing the efficiency of diverse processes, such as light absorption by solar panels. It is well known that superhydrophobicity reduces the friction between water droplets and the surface, thus allowing water drops to slide/roll and detach (clean) particles from surfaces. However, the forces that attach and detach particles from surfaces during the self-cleaning mechanism and the effect of nanotextures on these forces are not fully understood. To shed light on these forces and the effect of nanotexture on them, we prepared four Si-based samples (relevant to solar panels): (1) smooth or (2) nanotextured hydrophilic surfaces and (3) smooth or (4) nanotextured hydrophobic surfaces. In agreement with previous publications, it is shown that the efficiency of particle removal increases with hydrophobicity. Furthermore, nanotexture enhances the hydrophobicity, whereby particle removal is further increased. Specifically, hydrophilic particle removal increased from ∼41%, from hydrophilic smooth Si wafers to 98% from superhydrophobic Si-based nanotextured surfaces. However, the reason for the increased particle removal is not low friction between the droplets and the superhydrophobic surfaces; it is the reduction of the adhesion force between the particle and the surface and the altered geometry of the water-particle-air line tension acting on the particles on superhydrophobic surfaces, which increases the force that can detach particles from the surfaces. The experimental methods we used and the criterion for particle removal we derived can be implemented to engineer self-cleaning surfaces using other surfaces and dust particles, exhibiting different chemistries and/or textures.

Effect of Temperature on the Structure, Electrical Resistivity, and Charge Capacitance of Supported Lipid Bilayers.

Supported lipid bilayers with incorporated membrane proteins have promising potential for diverse applications, such as filtration processes, drug delivery, and biosensors. For these applications, the continuity (lack of defects), electrical resistivity, and charge capacitance of the lipid bilayers are crucial. Here, we highlight the effects of temperature changes and the rate of temperature changes on the vertical and lateral expansion and contraction of lipid bilayers, which in turn affect the lipid bilayer resistivity and capacitance. We focused on lipid bilayers that consist of 50 mol % dimyristoyl- sn-glycero-3-phosphocholine (zwitterionic lipid) and 50 mol % dimyristoyl-3-trimethylammonium-propane (positively charged lipid) lipids. This lipid mixture is known to self-assemble into a continuous lipid bilayer on silicon wafers. It is shown experimentally and explained theoretically that slow cooling (e.g., -0.4 °C min-1) increases the resistivity significantly and reduces the capacitance of lipid bilayers, and these trends are reversed by heating. However, fast cooling (∼ -10 °C min-1 or faster) damages the membrane and reduces the resistivity and capacitance of lipid bilayers to practically zero. Importantly, the addition of 50 mol % cholesterol to lipid bilayers prevents the resistivity and capacitance reduction after fast cooling. It is argued that the ratio of lipid diffusion coefficient to thermal expansion/contraction rate (proportional to the heating/cooling rate) is the crucial parameter that determines the effects of temperature changes on lipids bilayers. A high ratio (fast lipid diffusion) increases the lipid bilayer resistivity and decreases the capacitance upon cooling and vice versa. Similar trends are expected for lipid membranes that consist of other lipids or lipidlike mixtures.