In this regard, rain can be considered as prolific a sustainable energy source like energy scavenging from solar or wind. Consequently, droplet-driven energy harvesting devices – which offer a simple structure with low-cost materials requirements – have received considerable attention from the global research community.
Apart from energy harvesting applications, the behavior of droplets is an important phenomenon in modern science. Water droplet-related research is increasing significantly in many scientific areas for their wide applications in fundamental sciences such as bioengineering and biotechnology, the fluid mechanism at micro- and nanolevel, and condensation processes.
Existing technologies such as self-cleaning glass or solar cells, heat transformation, air conditioning or ventilation largely depend on water droplets’ characteristics.
Overview and evolution of droplet-based nanogenerators in the literature. a) Number of published papers on droplet-based energy harvesting. b) Subject area of droplet harvesting and corresponding publications. c) Output power from the single electrode mode droplet-based nanogenerators. d) Output power from the freestanding mode droplet-based nanogenerators. e) Output power from the hybridized droplet-based nanogenerators. f) Output power from piezoelectric, acoustic oscillation, and vibrational-dependent droplet-based nanogenerators. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
A recent review in Advanced Energy Materials ("Water Droplet-Based Nanogenerators") comprehensively discusses various methods for harvesting water droplets with their future potential for different applications. The authors report structural device variations with modification according to performance enhancement and application criteria with the latest advancement in fabrication processes and material selections. They also include a performance analysis and address limitations of current device fabrication.
The authors start by discussing the physical properties of water droplets, the chemical interaction of water droplets with dielectric surfaces, their impact on the surface of nanogenerators, surface engineering for droplets-based nanogenerators, solid/liquid interface for energy harvesting, and the significance of water droplets-based nanogenerators in current research.
Based on their working mechanism, i.e. on the way droplets interact with the dielectric materials, several basic types of droplet-based nanogenerators can be categorized.
This category contains most of the reported water droplet-based nanogenerators that are reported in the literature. The triboelectric effect is the build-up of an electric charge between two materials through contact and separation. In this case, the basic working principle of these nanogenerators is based on the contact and separation of a water droplet with the dielectric layers.
The tribovoltaic effect refers to a phenomenon in which quantum energy is released once an atom–atom bond is formed at the dynamic interface of two contacting materials. The tribovoltaic effect can be generated at the liquid-semiconductor interface for instance by moving a water droplet over a silicon surface.
Nanostructured materials can generate electricity on interaction with water, a phenomenon that has been termed the hydrovoltaic effect. Here, The kinetic energy of a flowing droplet on a nanostructured surface can be utilized to generate a streaming potential due to the electrical double layer-based electrokinetic effect.
Based on these three fundamental effects, researchers are extensively studying water droplets-based energy harvesting with different structures, mechanisms, and materials
modification to fabricate ever better-performing nanogenerators.
Depending on their working mechanisms, structural characteristics, and fabrication techniques, water droplets harvesting nanogenerators can be categorized into four major groups, which are discussed in detail by the authors:
–Single electrode-based energy harvesting
–Freestanding mode nanogenerators
–Hybrid structured droplet-based nanogenerators
–Acoustic oscillation, vibrational and jumping droplets harvesting.
The performance of droplets-based nanogenerators is still in the fundamental research phase and most of the devices for harvesting droplets are lacking in higher electrical outputs. As such, researchers' main focus is still on the development of new materials, new structures, and unique approaches to harvest droplets effectively for power generation.
Already it is promising, though, that current outputs from droplets-based generators even at this stage are more than enough for effectively powering up many low-powered electronic devices and sensor systems.
Through constant improvement, these droplets-based nanogenerators will be an alternative to the solar panel in many cases in the near future, especially in those parts of the world that experience frequent rain.