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1. Acoustically Excited Micro Mass Transport for Remotely Dose-Controlable Drug Releasing

   https://doi.org/10.1109/MEMS46641.2020.9056218  

   Liu, Fang-Wei; Cho, Sung Kwon (January 2020, 2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS))

   null (Ed.)

   Remotely activated drug release strategy with controllable dosage is the key factor of various targeting drug delivery methods as minimal invasive treatment. This article describes mass transport in liquid in microscale with a controllability of releasing amount, which is wirelessly excited by an external acoustic excitation. A liquid droplet (releasing agent, or drug in application) is trapped in the middle of a one-end open microtube which has a ratchet-structure on its inner wall. The droplet is trapped in the tube and neighbored by two gaseous air bubbles on both sides. In the presence of acoustic wave, the air bubbles oscillate and resonate. The air bubble near the tube opening segregates the liquid droplet into smaller ones and transport them on the ratchet-surface wall of the microtube. This mass transport occurs in both directions at similar rates: from the surrounding fluid to the trapped droplet and vice versa. As a result, the overall mass of droplet remains similar. Meanwhile, the other bubble positioned back in the tube sealing side enhances mixing between incoming mass from the surrounding and existing mass in the droplet. This mass transport is significant only when the inner wall of the tube has rachets. The exchanging mass between the surrounding and droplet is monotonically proportional to the excitation period, showing high controllability of mass transport. This mass transport phenomenon possibly provides a new mechanism of in vivo, on-demand, dose controllable drug delivery. 

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2. Heat transfer during droplet impact on a cold superhydrophobic surface via interfacial thermal mapping

   https://doi.org/10.1002/dro2.124  

   Kumar, Vijay; Fu, Qianxi; Szeto, Harrison; Zhu, Yangying (July 2024, Droplet)

   Abstract Undesired heat transfer during droplet impact on cold surfaces can lead to ice formation and damage to renewable infrastructure, among others. To address this, superhydrophobic surfaces aim to minimize the droplet surface interaction thereby, holding promise to greatly limit heat transfer. However, the droplet impact on such surfaces spans only a few milliseconds making it difficult to quantify the heat exchange at the droplet–solid interface. Here, we employ high‐speed infrared thermography and a three‐dimensional transient heat conduction COMSOL model to map the dynamic heat flux distribution during droplet impact on a cold superhydrophobic surface. The comprehensive droplet impact experiments for varying surface temperature, droplet size, and impacting height reveal that the heat transfer effectiveness () scales with the dimensionless maximum spreading radius as , deviating from previous semi‐infinite scaling. Interestingly, despite shorter contact times, droplets impacting from higher heights demonstrate increased heat transfer effectiveness due to expanded contact area. The results suggest that reducing droplet spreading time, as opposed to contact time alone, can be a more effective strategy for minimizing heat transfer. The results presented here highlight the importance of both contact area and contact time on the heat exchange between a droplet and a cold superhydrophobic surface. 

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3. An active self-cleaning surface system for photovoltaic modules using anisotropic ratchet conveyors and mechanical vibration

   https://doi.org/10.1038/s41378-020-00197-z  

   Sun, Di; Böhringer, Karl F. (September 2020, Microsystems & Nanoengineering)

   Abstract 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. 

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4. A Dynamically Responsive Surface with Switchable Wettability for Efficient Evaporation and Self-Cleaning Abilities

   https://doi.org/10.1021/acsaenm.2c00097  

   Parisi, Gregory; Lopez, Alejandro; Narayan, Shankar (October 2022, ACS Applied Engineering Materials)

   Evaporation is crucial in many applications. One of the critical parameters affecting evaporation is surface wettability, which is often tailored using coatings and micro- or nanoscale features on the surface. While this approach has advanced many technologies, the ability to control wettability dynamically can add new functionalities and capabilities that were not possible before. This study demonstrates how a self-cleaning superhydrophobic surface with an equilibrium contact angle of 155° can dynamically change to a superhydrophilic surface with a contact angle near 0°, resulting in drastically different evaporation characteristics. Specifically, we find that the evaporation rate and surface temperature reduction due to the resulting cooling are 3 times higher due to the change in surface wettability. This change in wetting behavior is due to the use of an amino-silane [N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane]-functionalized surface, which is altered in the presence of dilute acetic acid. Upon complete evaporation, the surface reverts to superhydrophobic behavior. This reversible behavior is not seen in traditional nonwetting coatings like perfluorodecyltrichlorosilane and lauric acid. This strategy for dynamic control of wettability and evaporation can lead to advancements in many applications ranging from self-assembly-based fabrication processes to oil–water separation and advanced thermal management technologies. 

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5. Three-dimensional numerical simulation and experimental investigation of boundary-driven streaming in surface acoustic wave microfluidics

   https://doi.org/10.1039/c8lc00589c  

   Chen, Chuyi; Zhang, Steven Peiran; Mao, Zhangming; Nama, Nitesh; Gu, Yuyang; Huang, Po-Hsun; Jing, Yun; Guo, Xiasheng; Costanzo, Francesco; Huang, Tony Jun (November 2018, Lab on a Chip)

   Acoustic streaming has been widely used in microfluidics to manipulate various micro−/nano-objects. In this work, acoustic streaming activated by interdigital transducers (IDT) immersed in highly viscous oil is studied numerically and experimentally. In particular, we developed a modeling strategy termed the “slip velocity method” that enables a 3D simulation of surface acoustic wave microfluidics in a large domain (4 × 4 × 2 mm 3 ) and at a high frequency (23.9 MHz). The experimental and numerical results both show that on top of the oil, all the acoustic streamlines converge at two horizontal stagnation points above the two symmetric sides of the IDT. At these two stagnation points, water droplets floating on the oil can be trapped. Based on these characteristics of the acoustic streaming field, we designed a surface acoustic wave microfluidic device with an integrated IDT array fabricated on a 128° YX LiNbO 3 substrate to perform programmable, contactless droplet manipulation. By activating IDTs accordingly, the water droplets on the oil can be moved to the corresponding traps. With its excellent capability for manipulating droplets in a highly programmable, controllable manner, our surface acoustic wave microfluidic devices are valuable for on-chip contactless sample handling and chemical reactions. 

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