Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation.

Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( tf), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.

Heat Transfer through a Condensate Droplet on Hydrophobic and Nanostructured Superhydrophobic Surfaces.

Understanding the fundamental mechanisms governing vapor condensation on nonwetting surfaces is crucial to a wide range of energy and water applications. In this paper, we reconcile classical droplet growth modeling barriers by utilizing two-dimensional axisymmetric numerical simulations to study individual droplet heat transfer on nonwetting surfaces (90° < θa < 170°). Incorporation of an appropriate convective boundary condition at the liquid-vapor interface reveals that the majority of heat transfer occurs at the three phase contact line, where the local heat flux can be up to 4 orders of magnitude higher than at the droplet top. Droplet distribution theory is incorporated to show that previous modeling approaches underpredict the overall heat transfer by as much as 300% for dropwise and jumping-droplet condensation. To verify our simulation results, we study condensed water droplet growth using optical and environmental scanning electron microscopy on biphilic samples consisting of hydrophobic and nanostructured superhydrophobic regions, showing excellent agreement with the simulations for both constant base area and constant contact angle growth regimes. Our results demonstrate the importance of resolving local heat transfer effects for the fundamental understanding and high fidelity modeling of phase change heat transfer on nonwetting surfaces.

Management of Cervical Root Fracture in Mandibular Central Incisor: A Case Report.

Root fractures are relatively uncommon injuries, but they represent a complex healing pattern due to concomitant injury to the pulp, periodontal ligament, dentin and cementum. This report presents a case of successful treatment of cervical root fracture in a mandibular central incisor with the help of guttapercha and MTA Filapex sealer.

Nano-lipoidal carriers of isotretinoin with anti-aging potential: formulation, characterization and biochemical evaluation.

BACKGROUND: Treatment of photoaging includes non-prescription cosmeceuticals and prescription products, retinoids. Isotretinoin, an established anti-acne retinoid, is also reported to delay the aging process. However, the drug is reported to be an irritant on skin. PURPOSE: The present study endeavors to explore the potential of a novel set of biocompatible nano-structured systems of isotretinoin in the treatment of photoaging. METHODS: Nano-lipoidal carriers (NLCs) of isotretinoin were developed, characterized and investigated in vivo for anti-aging potential in Laca mice vis-à-vis the marketed products of retinoids. The anti-aging efficacy of NLCs was measured in terms of visual and redox-biochemical parameters in ultraviolet (UV)-irradiated mice. RESULTS: Visual observations revealed that there was no significant change (p < 0.05) w.r.t. erythema, skin sagging and wrinkles in the skin of the animals treated with NLCs formulation compared to the marketed product(s). The malondialdehyde levels were found to be significantly reduced, whereas glutathione levels were increased with the application of NLCs vis-à-vis control and test formulations. The NLCs were able to maintain the normal redox-balance of UV-irradiated skin, and were better tolerated by the animals. CONCLUSION: The study ratifies enhancement in the efficacy of isotretinoin against photoaging and improved skin biocompatibility after its encasement in novel topical dosage forms.