Oscillating/Vibrating Surfaces

In this chapter, vibrations of free and sessile droplets and low and high frequencies are discussed. Modes appearing under vertical and lateral vibrations of sessile droplets are examined. Boundary conditions taking place at the triple line are crucial for constituting modes. Both axisymmetric and nonaxisymmetric modes are addressed. Use of the vibration of droplets for the measurement of surface tension of liquids is covered. Vibrated droplets supply valuable information about wetting regimes that take place on flat and rough surfaces and may be successfully used for the study of wetting transitions observed on rough surfaces. Vibrated sessile droplets are useful for the development of microfluidic devices. The mechanisms of displacement of vibrated sessile droplets on ratchet and gradient surfaces are covered. Oscillation of droplets induced by electric and magnetic fields is discussed. An ultrasonic method, high-frequency ultrasonic shear reflectometry, is presented for dealing with high frequencies. This specific nondestructive approach, which can be easily coupled with optical analysis, can give information on complex shear moduli G′ and G″ during droplet evaporation and wetting. Hence, some quantities such as tan(δ)=(G″/G′), which is very useful in rheology for (Sol/Gel) transitions characterization, can be investigated.