Despite the enormous interest in superhydrophobicity for self-cleaning, a clear picture of contaminant removal is missing, in particular, on a single-particle level. Here, we monitor the removal of individual contaminant particles on the micrometer scale by confocal microscopy. We correlate this space-and time-resolved information with measurements of the friction force. The balance of capillary and adhesion force between the drop and the contamination on the substrate determines the friction force of drops during self-cleaning. These friction forces are in the range of micro-Newtons. We show that hydrophilic and hydrophobic particles hardly influence superhydrophobicity provided that the particle size exceeds the pore size or the thickness of the contamination falls below the height of the protrusions. These detailed insights into self-cleaning allow the rational design of superhydrophobic surfaces that resist contamination as demonstrated by outdoor environmental (>200 days) and industrial standardized contamination experiments.

Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Previous understanding is that drop motion is dictated by viscous dissipation and activated dynamics at the contact line. Here we demonstrate that these two effects cannot fully explain the complex paths of sliding or impacting drops. To accurately determine the forces experienced by moving drops, we imaged their trajectory when sliding down a tilted surface, and applied the relevant equations of motion. We found that drop motion on low-permittivity substrates is substantially influenced by electrostatic forces. Our findings confirm that electrostatics must be taken into consideration for the description of the motion of water, aqueous electrolytes and ethylene glycol on hydrophobic surfaces. Our results are relevant for improving the control of drop motion in many applications, including printing, microfluidics, water management and triboelectric nanogenerators.

Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liquid. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11–25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θ A ) and receding (θ R ) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θ A ≈ 87° for velocities of the contact line <20 μm/s. At higher velocities, θ A gradually increased to ∼98°. This value is similar to θ A of a pure polystyrene (PS) film, which we studied for comparison. We associate the gradual increase in θ A to the adaptation process to water: The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory ( 30110544 Langmuir 2018 34 11292 ), we estimated the time constant of this adaptation process to be ≪1 s. For sliding water drops, θ R is ∼10° lower compared to the reference PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45–60 μN, depending on sliding velocity.

Wetting of surfaces depends critically on defects which alter the shape of the drop. However, no experimental verification of forces owing to the three phase contact line deformation at single defects is available. We imaged the contact line of sliding drops on hydrophobic surfaces by video microscopy. From the deformation of the contact line, we calculate the force acting on a sliding drop using an equation going back to Joanny and de Gennes (J. Chem. Phys. 81, 552, 1984). The calculated forces quantitatively agree with directly measured forces acting between model defects and water drops. In addition, both forces quantitatively match with the force calculated by contact angle differences between the defect and the surface. The quantitative agreement even holds for defects reaching a size of 40% of the drop diameter. Our validation for drop’s pinning forces at single defects is an important step towards a general understanding of contact line motion on heterogeneous surfaces.

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