This review focuses on self-cleaning surfaces, from passive bio-inspired surface modification including superhydrophobic, superomniphobic, and superhydrophilic surfaces, to active micro-electro-mechanical systems (MEMS) and digital microfluidic systems. We describe models and designs for nature-inspired self-cleaning schemes as well as novel engineering approaches, and we discuss examples of how MEMS/microfluidic systems integrate with functional surfaces to dislodge dust or undesired liquid residues. Meanwhile, we also examine “waterless” surface cleaning systems including electrodynamic screens and gecko seta-inspired tapes. The paper summarizes the state of the art in self-cleaning surfaces, introduces available cleaning mechanisms, describes established fabrication processes and provides practical application examples.

The purpose of this work is to develop an active self-cleaning system that removes contaminants from a solar module surface by means of an automatic, water-saving, and labor-free process. The output efficiency of a solar module can be degraded over time by dust accumulation on top of the cover glass, which is often referred to as “soiling”. This paper focuses on creating an active self-cleaning surface system using a combination of microsized features and mechanical vibration. The features, which are termed anisotropic ratchet conveyors (ARCs), consist of hydrophilic curved rungs on a hydrophobic background. Two different ARC systems have been designed and fabricated with self-assembled monolayer (SAM) silane and fluoropolymer thin film (Cytop). Fabrication processes were established to fabricate these two systems, including patterning Cytop without degrading the original Cytop hydrophobicity. Water droplet transport characteristics, including anisotropic driving force, droplet resonance mode, cleaning mechanisms, and system power consumption, were studied with the help of a high-speed camera and custom-made test benches. The droplet can be transported on the ARC surface at a speed of 27 mm/s and can clean a variety of dust particles, either water-soluble or insoluble. Optical transmission was measured to show that Cytop can improve transmittance by 2.5~3.5% across the entire visible wavelength range. Real-time demonstrations of droplet transport and surface cleaning were performed, in which the solar modules achieved a 23 percentage-point gain after cleaning.

We present a piezoelectric transducer for standing wave surface acoustic wave nebulization (SW-SAWN) patterned with anisotropic ratchet conveyors (ARCs) to automate the sample preparation and droplet delivery.

We report a digital microfluidic device to transport aqueous droplets on an open surface in air using electrowetting-on-dielectric (EWOD) with anisotropic ratchet conveyors (ARCs). ARCs are micro-sized periodic semicircular hydrophilic regions on a hydrophobic background, providing anisotropic wettability. SiN_x and Cytop are used as the dielectric layer between the water droplet and working electrodes. By adopting parylene as a stencil mask, hydrophilic patterning on the hydrophobic Cytop thin film layer is achieved without the loss of Cytop hydrophobicity. While the traditional EWOD platform requires the control of multiple electrodes to transport the droplet, our system utilizes only two controlling electrodes. We demonstrate that 15 μl water droplets are transported at a speed of 13 mm/s under 60 V_peak sinusoid AC signal at 50 Hz. Droplet transport at 20 Hz is also presented, demonstrating that the system can operate within a range of frequencies.