In recent years, the study of thermoelastic waves generated by lasers has been undertaken by several researchers because the technique provides an efficient non-contact technique for generation and detection of ultrasonic waves. Laser-generated ultrasonic waves have diverse applications ranging from material characterization to defect characterization. Transient ultrasonic guided waves generated by a pulsed laser in anisotropic infinite plate are investigated in this article. An exact analytical method is adopted for this purpose. The governing equations and boundary conditions are first transformed from spatial-time domain into wavenumber-frequency domain using Fourier Transform. After solving these equations and satisfying the boundary conditions in the wavenumber-frequency domain, the Cauchy's residue theorem is used to get the response in the spatial domain and then the numerical integration is used to eventually obtain the response in time domain. Results for dispersion and transient guided waves in infinite silicon nitride (Si₃N₄) plates are presented. Numerical results show that pulsed laser excites mainly the lowest Lamb modes, namely, the lowest symmetric (S ₀) and antisymmetric (A₀) modes. They also show that the transient response is dominated by the antisymmetric mode A₀ which shows dispersion characteristic. This study provides a quantitative model for laser generated ultrasonic waves in an anisotropic plate and can be used for non-destructive evaluation.

Water droplet cleaning of a dusty hydrophobic surface is examined. Environmental dust are used in the experiments and cloaking velocity of a dust layer by a droplet fluid is measured and hemi-wicking conditions for the dust layer are analyzed adopting the pores media wick structure approach. A droplet motion on dusty and inclined hydrophobic surface is analyzed using a high speed digital imaging system. Influences of dust layer thickness, droplet volume, and surface inclination angle on the mechanisms of dust removal by a rolling droplet are evaluated. The findings revealed that dust cloaking velocity decreases exponentially with time. The droplet fluid can cloak the dust layer during its transition on the dusty surface. The transition period of droplet wetted length on the dusty surface remains longer than the cloaking time of the dust layer by the droplet fluid. Translational velocity of rolling droplet is affected by the dust layer thickness, which becomes apparent for small volume droplets. Small volume droplet (20 µL) terminates on the thick dust layer (150 µm) at low surface inclination angle (1°). The quantity of dust picked up by the rolling droplet increases as the surface inclination angle increases. The amount of dust residues remaining on the rolling droplet path is relatively larger for the thick dust layer (150 µm) as compared to its counterpart of thin dust layer (50 µm).

Water droplet impacting onto a hydrophobic surface is considered and the influence of the surface wetting state on the droplet dynamics is examined. Pressure variation in the impacting droplet is predicted numerically using the level set model. The droplet spreading and the retraction on the hydrophobic surface are assessed for various wetting states of the hydrophobic surface. Experiment is carried out to validate the predictions of the droplet shape and the restitution coefficient. It is found that predictions of impacting droplet shape and the restitution coefficient agree with those obtained from the experiment. The local pressure peaks formed in the droplet fluid, particularly in the retraction period, causes alteration of the droplet vertical height and the shape. Droplet spreading is influenced by the wetting state of the hydrophobic surface; hence, increasing contact angle of the hydrophobic surface lowers the spreading diameter of the droplet on the surface. The transition time of the droplet changes with the wetting state of the hydrophobic surface such that increasing droplet contact angle reduces the transition time of the droplet on the surface. The droplet remains almost round after the first bounding for large contact angle hydrophobic surface.