Bouncing Droplets: A Hands-On Activity To Demonstrate the Properties and Applications of Superhydrophobic Surface Coatings

Abstract

Here we report a hands-on activity addressed to master’s students in Physical Chemistry and Materials Science courses on the properties and applications of superhydrophobic surfaces. This simple and intuitive experience can also be used to teach undergraduate and high school students, thanks to its application-oriented approach and tangible results. Superhydrophobicity was achieved by the functionalization of oxide powders with alkylsilanes and their subsequent deposition on a glass substrate. The film’s superhydrophobicity was assessed by different application tests and compared with the behaviors of model hydrophobic, hydrophilic, and superhydrophilic surfaces. Its antistain properties were tested with both model dye solutions and everyday liquids. Films were fouled with graphite and dye powders to compare the self-cleaning capabilities of the different surfaces, and single droplet transport was achieved by adding magnetic particles. This engaging and adaptable experience introduces students to basic concepts of surface science in an intuitive and tangible way.

Designing and 3D Printing an Improved Method of Measuring Contact Angle in the Middle School Classroom

“…Previously published work has explored the measurement of contact angles in education settings, on samples ranging from hydrophilic to both natural and artificially created hydrophobic surfaces. 8,24−26 However, the measurement of contact angles is typically done either qualitatively with the naked eye (such as by observing the droplet roll off the surface) 26 or using an associated laboratory's contact angle goniometer. 8,24 While there exist some examples of the construction of less complex goniometers, these still require additional components, such as microscopes and lenses, which may be less available to educators.…”

“…Previously published work has explored the measurement of contact angles in education settings, on samples ranging from hydrophilic to both natural and artificially created hydrophobic surfaces. ,− However, the measurement of contact angles is typically done either qualitatively with the naked eye (such as by observing the droplet roll off the surface) or using an associated laboratory’s contact angle goniometer. , While there exist some examples of the construction of less complex goniometers, these still require additional components, such as microscopes and lenses, which may be less available to educators. , With our approach, the measurement of contact angles on various surfaces, including hydrophilic surfaces as well as naturally occurring and artificial hydrophobic materials, can be accomplished wholly within the classroom setting, allowing students the opportunity to collect and analyze quantitative data. Additionally, as this device is a generally applicable tool for contact angle measurements, a wide range of samples with differing liquids and solid surfaces may be observed in addition to water, and thus, it is useful across multiple educational levels as a means to easily explore differing depths of understanding regarding surface chemistry.…”

Thermodynamics of Wettability: A Physical Chemistry Laboratory Experiment

“…30,32 Experiments utilizing commonly found patterned objects such as the surfaces of recordable compact disks (CD-R) have also been described. 31,32 Further laboratory exercises have utilized carbon soot, 33 methyltrichlorosilane, 34 TiO 2 nanoparticles, 35,36 and electroless deposited copper particles 37 to provide texturation. For instance, Kabza and co-workers designed a laboratory exercise that enabled students to calculate the critical surface energy of readily available materials such as glass and polytetrafluoroethylene (PTFE) using Zisman plots.…”

Surface Acoustic Waves to Control Droplet Impact onto Superhydrophobic and Slippery Liquid-Infused Porous Surfaces

“…Understanding droplet impact behavior onto various surfaces is crucial for a wide range of applications, including 3D and inkjet printing, combustion, spray cooling, anti-icing surfaces, agriculture, forensic assay, and coating processes. − Different factors for controlling droplet impact dynamics on various surfaces have been extensively investigated, including the impact regime, contact time (CT), maximum spreading radius, and rebounding angle. , Advances in smart materials, micro and nanoscale structures, and surface fabrication techniques have led to passive surface modification methods that can be used to modify surfaces for different applications. − For example, hydrophilic surfaces have been made to maximize the contact area upon impact, desirable for coating, flash cooling, and ink-jetting applications. , On the other side of the spectrum, superhydrophobic surfaces are used for anti-icing and anti-erosion applications to reduce the solid–liquid contact time during droplet impact. − However, various problems have been reported for these surfaces, such as three-phase contact line (TPCL) pinning, poor mechanical resilience, degradation of wetting properties over time, or low transparency. − Inspired by the Nepenthes pitcher plant structure, slippery liquid-infused porous surfaces (SLIPS) have been developed to achieve highly smooth and pinning-free surfaces. , SLIPS can be manufactured by imbibing porous and superhydrophobic nanostructures with a lubricating liquid (typically oil), which preferentially wets the solid and is immiscible to the contacting liquid of interest . These surfaces benefit self-cleaning and anti-icing applications as they can reject various impacting liquids, not exclusively water-based. ,, In addition, as long as the lubricant is present and coats the top of the porous medium, the SLIPS’s properties are expected to be sustained .…”