Liquid droplets move readily under the influence of surface tension gradients on their substrates. Substrates decorated with parallel microgrooves, or striations, presenting the advantage of homogeneous chemical properties yet varying the topological characteristics on either side of a straight-line boundary are considered in this study. The basic type of geometry consists of hydrophobic micro-striations/rails perpendicular to the boundary, with the systematic variation of the width to spacing ratio, thus changing the solid-liquid contact fraction and inducing a well-defined wettability contrast across the boundary. Droplets in the Cassie-Baxter state, straddling the boundary, move along the wettability contrast in order to reduce the overall surface free energy. Results show the importance of average solid fraction and contrasting fraction in a wide range for given geometries across the boundary on droplet motion. A unified criterion for contrasting striated surfaces, which describes the displacement and the velocity of the droplets, is suggested, providing guidelines for droplet manipulation on microstriated/railed surfaces.
In this study the local two-phase heat transfer coefficients were obtained using wall temperature measurements and fluid saturation temperatures in a rectangular high-aspect-ratio (= W ℎ ℎ = 22) polydimethylsiloxane (PDMS) microchannel with a hydraulic diameter of 192 µm. The experiments used FC-72 liquid with a mass flux of 7.37 kg m-2 s-1 and various heat fluxes ranging between 3.34 and 61.95 kW m-2. Fluid saturation temperatures were determined by interpolating pressure measurements obtained with integrated silicon ceramicbased pressure sensors located near the inlet and outlet of the microchannel. The hydrodynamic and flow boiling characteristics of the microchannel were monitored using high frequency and high spatial resolution infrared thermography, with heat transfer coefficients obtained as a function of axial position, lateral position and time and at the inlet, middle and outlet sections of the microchannel. This enabled the effect of heat flux on local temperature, flow boiling heat transfer coefficient distribution and two-phase pressure drop to be determined. These results suggest that the two-phase heat transfer coefficient does not increase monotonically with the heat flux, and actually decreases under certain conditions. This work identifies that the heat transfer coefficient depends on the range of the heat flux and 2 is correlated with vapour-liquid dynamics and liquid film thinning resulting in suspected dryout, observed simultaneously with the temperature measurements. The simultaneous application of high speed thermography and flow visualisation has enabled uniquely detailed information to be obtained that is useful to a more detailed understanding of two-phase flow and bubble dynamics.
A study was undertaken of ethanol droplet evaporation on structured surfaces of pillars (square pillars of variable dimensions and spacing of order microns, and cylindrical of various spacings). On seasoned perflourodecyltrichlorosilane surfaces, significant films were observed extending far beyond the initial contact line for pure ethanol droplets, most prominently for 4 microlitre droplets. On parylene coated surfaces, similar imbibed films were seen for 1.5 microlitre droplets of 50% by volume ethanol-water mixture. This film acts as an additional surface for evaporation and it appears that the droplet then serves as a reservoir for feeding the film until the evaporation process is completed, rather than evaporation being governed by evaporation at the contact line. The droplets with films show higher evaporation rates.
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