We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7º wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are up to respectively 65 and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer.

No abstract

No abstract

No abstract

The evaporation and dynamics of a multicomponent droplet on a heated chemical patterned surface were presented. Comparing to the evaporation process of a multicomponent droplet on a homogenous surface, it is found that the chemical patterned surface can not only enhance evaporation by elongating the contact line, but also change the evaporation process from three regimes for the homogenous surface including constant contact line (CCL) regime, constant contact angle (CCA) regime and mix mode (MM) to two regimes, i.e. constant contact line (CCL) and moving contact line (MCL) regimes. The mechanism of contact line stepwise movement in MCL regimes in the microscopic range is investigated in detail. In addition, an improved local force model on the contact line was employed for analyzing the critical receding contact angles on homogenous and patterned surfaces. The analysis results agree well for both surfaces, and confirm that the transition from CCL to MCL regimes indicated droplet composition changes from multicomponent to monocomponent, providing an important metric to predict and control the dynamic behavior and composition of a multicomponent droplet using a patterned surface.

No abstract

Liquid perfluorocarbon (PFC) nanodroplets may have a better chance to extravasate through inter-endothelial gaps (400-800 nm) into tumor interstitium for extravascular imaging, which holds promise as an innovative strategy for imaging-guided drug delivery, early diagnosis of cancer and minimally invasive treatment of cancer. Currently available emulsion technologies still face challenges in reducing droplet sizes from the microscale to the nanoscale. To control size and ensure monodispersity of PFC nanodroplets, we developed a flame-shaped glass capillary and polydimethylsiloxane (PDMS) hybrid device that creates a concentric flow of the dispersed phase enclosed by the focusing continuous phase at the cross-junction. Through adjustment of the pressure applied, a stable tip-streaming mode can be obtained for PFC nanodroplet generation. Using this device, we synthesized various kinds of PFC nanodroplets as small as 200 nm in diameter with polydispersity index (PDI) <0.04. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were carried out for the characterization of the PFC nanodroplets. Finally, ultrasound imaging was conducted to demonstrate that the liquid PFC nanodroplets can be used for enhancing the ultrasound contrast upon vaporization.

No abstract