Superhydrophobic metrology and applications

Superhydrophobic surfaces repel water and exhibit useful properties like self-cleaning, anti-fogging, anti-icing, and anti-fouling. Current characterization techniques have trouble grading the superhydrophobicity of samples due to limits in force and optical resolution. Thus, more suitable measurement techniques are needed to grade superhydrophobic surfaces. The current standard method, contact angle goniometer (CAG), is compared to two force-based measurement techniques: oscillating droplet tribometer (ODT) and micropipette force sensor (MFS). In addition, the properties of superhydrophobic surfaces are explored for lubrication. Publication I compares the sensitivity of the MFS and CAG on measuring superhydrophobic surfaces. In this publication, MFS measures the contact angle hysteresis force and CAG measures the contact angles of superhydrophobic surfaces. For superhydrophobic surfaces, the MFS can easily distinguish even slight differences between the samples, while the CAG cannot differentiate the samples from each other. Publication II studies the sensitivity of the ODT and CAG on measuring superhydrophobic surfaces. In this publication, the viscous and contact angle hysteresis force is measured using ODT and the contact angles with CAG. For the measured superhydrophobic surfaces, the ODT can differentiate all the samples and even find slight differences in heat treated nanostructured copper samples, while the CAG cannot distinguish the samples from each other. Publication III uses the MFS in an oscillating mode, which is based on the model of the ODT. In this publication, levitating carbonated water and MilliQ water are used to study the viscous losses in the air layer between the droplet and the surface in superhydrophobic surfaces. A mathematical model is constructed to explain the losses in the system, and it is validated using experimental data. Publication IV explores the lubrication properties of a the superhydrophobic surface. A slippery air-water bilayer forms between superhydrophobic surface and a water layer. This bilayer is used to lower dissipation forces between two solids. These dissipation forces are measured using oscillating tribometer and tilted plane. Extreme level of lubrication is demonstrated at low velocities (v<1 m/s) and pressures (50 Pa) using this system with friction coefficients at the order of 0.001, which is on par with the state-of-the-art lubrication methods. This thesis demonstrates the accuracy of ODT and MFS for characterizing superhydrophobic surfaces and the limited suitability of CAG for characterizing superhydrophobic surfaces. The need for these type of measurement devices will increase, as more superhydrophobic surfaces and application enter the market. These accurate measurement devices will be critical for utilizing and commercializing the useful properties of superhydrophobic surfaces.