Figure 1

A cross-section view of the cylindrical sample cell. The fluid volume is contained between two sapphire windows and a CuBeCo alloy ring (sidewall). The dimensions $(W,D)$ of the cell are indicated. The semicircular liquid-gas meniscus between the two parallel windows is shown. This interface appears dark in the images because the liquid-gas meniscus refracts the normally incident parallel light away from the cell axis.Reuse & Permissions

Figure 2

(a) The cell (from experiment B) is a $D=12\mathrm{mm}$ diameter cylinder made of CuBeCo containing a $W=1.664\mathrm{mm}$ layer of ${\mathrm{SF}}_6$ sandwiched between two sapphire windows (9 mm thick). The convergent lens $L$, with focal point $F$ and the center $C$, projects the out-of-focus grid image on the CCD plane $\Pi$ [panel (b)]. This system can detect a density change of up to $7.8\times {10}^{-5}\mathrm{g}∕{\mathrm{cm}}^3∕\text{pixel}$. The density may then be found by integrating the local density changes from a given reference density. This technique is sufficient to detect steep density gradients in the fluid. It is only capable, however, of detecting 4.3% constant density gradient change across 1 cm length of the cell of ${\mathrm{SF}}_6$. In a liquid-gas mixture, these displacements may be analyzed using the paraxial approximation of geometrical optics. (b) Shown is the defocused grid image recorded by the CCD camera. The evaporation of the liquid film between the sapphire window(s) and the gas bubble leads to a gas-liquid-solid contact line. The part of the grid shadows that is not distorted is to the right of the contact line, indicating the existence of a dry gas-solid interface. When heating is first applied, a receding contact line and a simultaneous change in the bubble/sidewall contact angle, shown above, are seen. The heat transfer through the cell’s copper sidewall evaporates the liquid film, causing this change in the bubble and/or sidewall contact angle. Also shown is the 10 mm diameter reference circle, the center of the cell, the reference line, and the point used in Fig. 9 and Fig. 10.Reuse & Permissions

Figure 3

Temperature ramp applied to cells A and B. The initial ramp has an $8.7\mathrm{mK}∕\mathrm{s}$ rate followed by a 40 min plateau and a $7.4\mathrm{mK}∕\mathrm{s}$ ramp. $T_W$ is the temperature measured in the copper sidewall. Also shown are the times and temperatures for the sequence of images in Figs. 6, 7.Reuse & Permissions

Figure 4

(a) The tilted gas-liquid interface due to liquid evaporation leads to a contact line. (b) The tilt angle induces a significant displacement of the grid shadows. The shadow displacement depends on the tilt angle $\alpha$, the width $w$, and the thickness $t$ of the film (see the text for details). Also shown are $z$, $\Delta z$, $\delta$, and the reference line used in the calculations of the thickness $t$ of the film.Reuse & Permissions

Figure 5

Ray tracing through the cell and the optical system. The tilt angle of the gas-liquid interface is exaggerated for illustration purposes and is shown only on one side for simplicity. An incident ray $I_0$, passing at distance $z_0$ from the optical axes, would remain parallel to the optical axes if the interface is not tilted with respect to the sapphire windows, and would hit the CCD plane at $I_1^{″}$. If there is a tilt angle $\alpha$ of the gas-liquid interface, then the ray starting at $I_0$ would hit the CCD plane at $I_6$ following successive refractions at $I_1$, $I_2$, $I_3$, and $I_4$. The displacement of the shadow $\Delta z$ depends on the tilt angle $\alpha$ of the interface and its thickness $t$.Reuse & Permissions

Figure 6

Shown is a sequence of images from the initial ramp of experiments B [(a)–(d)] and A [(e)–(h)]. In these experiments the compressed bubble touches the sidewall. The initial state of the gas bubble for these two experiments is shown in (a) (experiment B with heating started) and (e) (experiment A before heating). The cell used in experiment A is thicker than the cell in experiment B (see Table ). The liquid-gas meniscus produced a wider dark region in the image [(e)–(h)]. The film contact lines appear when the heating begins and (f) shows it 20 s after heating started. Panels (b)–(c) and (f)–(g) show the contact line with and without the grid present. Panel (i) is an enlargement of (g) and it shows that one of the two contact lines (there is one on each wall) is distorted by a particle deposited on the sapphire windows as indicated by the arrow.Reuse & Permissions

Figure 7

The ramp shown in Fig. 3 is also performed in experiment A. The fast spreading of this bubble appears to be more energetic than the thin bubble in experiment B. Panel (i) is an enlargement of (a) and it shows that one of the two contact lines (there is one on each wall) is distorted by some particles deposited on the sapphire windows as indicated by the arrow. This shows that these thin lines are contact lines in that they can be pinned by a defect while advancing or receding.Reuse & Permissions

Figure 8

Shown are some of the events that occurred in the final portion of a ramp similar to the ramps shown in Fig. 3 in a cell filled with ${\mathrm{CO}}_2$. This cell contains three thermistors and a piston to vary the fluid density between ramps. In this case the fluid was at $\delta \rho =-3.5\%$ below the critical density. Panel (a) shows that a cluster of bubbles has formed at the edge of the cell far below $T_C$. One of these bubbles has coalesced with the large bubble producing a hot plume that travels toward the center of the large bubble. Panel (b), at approximately $T_C$, shows a considerably distorted grid shadow image with critical opalescence at the sidewall in the region just where the bubbles were nucleating. The dynamical activity suddenly stops at about the same time as this layer forms. The particular phase distribution that existed just before the critical temperature was passed appears to be frozen in place. The image in panel (c) is above the critical temperature $T_C$ and the grid shadows in this image shows significant density changes. Slower mass fluxes continue, however, and the system evolves to a uniform supercritical fluid not long after panel (c).Reuse & Permissions

Figure 9

(a) The position of the contact line with respect to the center of the cell versus time (see Fig. 2) after heating begins. (b) The dry area behind the triple contact line and the interior of the dark meniscus region rapidly increases while the contact line moves toward the cell’s center.Reuse & Permissions

Figure 10

The thickness of the wetting film near the triple contact line was computed using the wedge model at the position shown by the point in Fig. 2b (data from experiment B). (a) Estimated thickness of the wetting film versus time calculated at the beginning of the second heating ramp with respect to cell’s center. (b) Estimated spatial profile of the contact line based on the relationship between the distance swept by the contact line and the elapsed time.Reuse & Permissions

Figure 11

Estimated thickness of the wetting film versus time (a) and versus relative temperature (b). The data refer to experiment B and used the linear wedge model. The error bars reflect the uncertainty in the measurements of the contact linewidth.Reuse & Permissions

Figure 12

Dry area [squares in panels (a) and (b)] and the total area of the gas bubble [circles in panels (a) and (b)] versus time [panel (a)] and relative temperature [panel (b)]. The data are from experiment B and both areas were measured with respect to the interior of the dark meniscus. The total area of the gas bubble is constant during heating [circles in panels (a) and (b)]. The ratio of the dry area, behind the contact line, to the total area of the gas bubble, versus time [panel (c)], and versus the relative temperature [panel (d)] is shown.Reuse & Permissions