Droplet behavior on superhydrophobic surfaces: Interfaces, interactions, and transport

The primary objective of the present work is to study droplet dynamics on smooth hydrophobic and textured superhydrophobic surfaces, and to understand the dependence of interfacial interaction mechanisms on surface morphology. ^ A detailed understanding of the dynamics of droplet response to an applied electric field is essential for implementation of electrowetting techniques in various devices. In the first part of the thesis, a systematic study of the transient response in terms of contact angle and contact radius of a sessile droplet on a smooth hydrophobic surface under electrical actuation is presented. A scaling analysis predicts the response time of a droplet during step actuation. It is shown that during time-varying electrical actuation of a droplet, in addition to the primary frequency response at the electrical forcing frequency, the droplet oscillation exhibits sub-harmonic oscillation at half the forcing frequency. ^ The remaining part of the thesis focuses on the design, fabrication and characterization of superhydrophobic surfaces, and droplet behavior on such surfaces. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. The surfaces show enhanced anti-wetting characteristics, and lower contact angle hysteresis compared to single-roughness surfaces. A novel hybrid surface morphology incorporating communicating and non-communicating air gaps is proposed to enhance capillary pressure. The pressure balance during droplet impingement indicates that the effective water hammer is dependent on the surface morphology, and is significantly lower compared to that on smooth surfaces. ^ The last part of the thesis includes evaporative phase change on flat and textured surfaces. An understanding of the evaporation characteristics of the droplet, and accompanying convection flow field on hydrophobic and superhydrophobic surfaces is important to several applications. In this dissertation, droplet evaporation characteristics on unheated and heated hydrophobic and superhydrophobic surfaces with negligible contact angle hysteresis are investigated systematically. A vapor-diffusion-only model is shown to overpredict the rate of evaporation on superhydrophobic surfaces, and the disparity increases with substrate heating. The evaporation characteristics are explained in terms of the evaporative cooling, and vapor buoyancy induced convection. ^ Improved understanding of the convective flow mechanism inside an evaporating droplet can assist in non-intrusive particle manipulation inside a micro-droplet. The recirculating convective flow field inside a water droplet evaporating on hydrophobic and superhydrophobic surfaces is attributed to the thermal buoyancy induced convection. The flow pattern inside the droplet enables understanding of the dependence of flow behavior on the nature of the substrate. High recirculating flow velocity in droplets evaporating on superhydrophobic surfaces is proposed to enable `on-the-spot\u27 mixing in droplets for microfluidics application

Droplet evaporation on heated hydrophobic and superhydrophobic surfaces

The evaporation characteristics of sessile water droplets on smooth hydrophobic and structured superhydrophobic heated surfaces are experimentally investigated. Droplets placed on the hierarchical superhydrophobic surface subtend a very high contact angle (similar to 160 degrees) and demonstrate low roll-off angle (similar to 1 degrees), while the hydrophobic substrate supports corresponding values of 120 degrees and similar to 10 degrees. The substrates are heated to different constant temperatures in the range of 40-60 degrees C, which causes the droplet to evaporate much faster than in the case of natural evaporation without heating. The geometric parameters of the droplet, such as contact angle, contact radius, and volume evolution over time, are experimentally tracked. The droplets are observed to evaporate primarily in a constant-contact-angle mode where the contact line slides along the surface. The measurements are compared with predictions from a model based on diffusion of vapor into the ambient that assumes isothermal conditions. This vapor-diffusion-only model captures the qualitative evaporation characteristics on both test substrates, but reasonable quantitative agreement is achieved only for the hydrophobic surface. The superhydrophobic surface demonstrates significant deviation between the measured evaporation rate and that obtained using the vapor-diffusion-only model, with the difference being amplified as the substrate temperature is increased. A simple model considering thermal diffusion through the droplet is used to highlight the important role of evaporative cooling at the droplet interface in determining the droplet evaporation characteristics on superhydrophobic surfaces

The nonleptonic decays of bb-flavored mesons to SS-wave charmonium and charm meson states

The detection of radially excited heavy meson \\states in recent years and measurement of heavy meson decays, particularly $B_c^+\to J/\psi D_s^+$ and $B_c^+\to J/\psi D_s^{*+}$, by the LHCb and ATLAS Collaborations, have aroused a lot of theoretical interest in the nonleptonic decays of $b$-flavored mesons. In this paper, we study the exclusive two-body nonleptonic $\bar{B}^0$, $\bar{B_s^0}$, $B^-$ and $B_c^-$-meson decays to two vector meson ($V_1(nS)V_2$) states. Assuming the factorization hypothesis, we calculate the weak-decay form factors from the overlapping integrals of meson wave functions, in the framework of the relativistic independent quark (RIQ) model. We find a few dominant decay modes: $B^-\to D^{*0}\rho^-$, $\bar{B^0}\to D^{*+}\rho^-$, $\bar{B_s^0}\to D_s^{*+}\rho^-$, $B^-\to J/\psi K^{*-}$ and $B_c^-\to J/\psi D_s^{*-}$ with predicted branching fractions of 1.54, 1.42, 1.17, 0.53 and 0.52 (in $\%$), which are experimentally accessible. The predicted branching fractions for corresponding decay modes to excited ($2S$) states, obtained in the order ${\cal O }(10^{-3}-10^{-4})$ lie within the detection accuracy of the current experiments at LHCb and Tevatron. The sizeable $CP$-odd fractions predicted for $B_c^-$-meson decay to two charmful states: $D^{*0}D^{*-}_{(s)}$ and $\bar{D}^{*0}D^{*-}_{(s)}$ indicate significant $CP$-violation hinting at the so-called new physics beyond the standard model.Comment: arXiv admin note: text overlap with arXiv:2202.0116