Hydraulic fracturing is a procedure by which a fracture is initiated and propagates due to pressure (hydraulic loading) applied by a fluid introduced inside the fracture. In this study, we focus on a crack driven by an incompressible Newtonian fluid, injected at a constant rate into an elastic matrix. The injected fluid creates a radial fracture that propagates along a plane. We investigate this type of fracture both theoretically and experimentally. Our experimental apparatus uses a brittle and transparent polyacrylamide hydrogel matrix. Using this medium, we examine the rate of radial crack growth, fracture aperture, shape of the crack tip and internal fluid flow field. Our range of experimental parameters allows us to exhibit two distinct fracturing regimes, and the transition between these, in which the rate of radial crack propagation is dominated by either viscous flow within the fracture or the material toughness. Measurements of the profiles near the crack tip provide additional evidence of the viscosity-dominated and toughness-dominated regimes, and allow us to observe the transition from the viscous to the toughness regime as the crack propagates. Particle image velocimetry measurements show that the flow in the crack is radial, as expected in the viscous regime and in the early stages of the toughness regime. However, at later times in the toughness regime, circulation cells are observed in the flow within the crack that destroy the radial symmetry of the flow field.

In this paper we describe how to construct polyacrylamide hydrogels to study the processes linked with hydraulic fracturing. These transparent, linearly elastic and brittle gels permit fracturing at low pressures and speeds allowing accurate measurements to be obtained. In the context of hydraulic fracturing, the broad range of modulus and fracture energy values that are attainable allow experimental exploration of particular regimes of importance. We also describe how material properties may be deduced from hydraulic fracturing experiments. Lastly, we analyse the fracture surface patterns that emerge from fluid-driven cracks occurring within the medium. These patterns are similar to those that have been observed in other materials and we comment on their fractal-like nature.

The formation of a fracture network is a key process for many geophysical and industrial practices from energy resource recovery to induced seismic management. We focus on the initial stage of a fracture network formation using experiments on the symmetric coalescence of two equal coplanar, fluid-driven, penny-shaped fractures in a brittle elastic medium. Initially, the fractures propagate independently of each other. The fractures then begin to interact and coalesce, forming a bridge between them. Within an intermediate period after the initial contact, most of the fracture growth is localized along this bridge, perpendicular to the line connecting the injection sources. Using light attenuation and particle image velocimetry to measure both the fracture aperture and velocity field, we characterize the growth of this bridge. We model this behavior using a geometric volume conservation argument dependent on the symmetry of the interaction, with a 2D approximation for the bridge. We also verify experimentally the scaling for the bridge growth and the shape of the thickness profile along the bridge. The influence of elasticity and toughness of the solid, injection rate of the fluid, and initial location of the fractures are captured by our scaling.

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In this paper the interfacial characteristics of a liquid flowing over a 1cm2 array of hydrophobic cylindrical micropillars located within a microchannel are investigated. The microchannel was 12mm wide and 32mm long with an average channel height of approximately 83μm. Hydrophobic coating of the channel was achieved via a controlled flow of a trichlorosilane and ethanol solution. A method to remove lodged gas bubbles from a microchannel was successfully demonstrated, while maintaining the favorable Cassie-Baxter wetting state (gas/vapor layer present) of the micropillar structures. This was achieved using degassed water to dissolve low-curvature gas bubbles, while ohmically heating the silicon substrate to reassert and maintain the Cassie-Baxter wetting state of the hydrophobic micropillars. During this experimentation it was discovered that the part wetting and dewetting of a superhydrophobic (SH) surface within a microchannel could be achieved using similar methods. The onset of surface wetting (Wenzel wetting state) was achieved by pumping degassed water through the microchannel. Surface dewetting was then accomplished through substrate heating by the increase in the trapped gas layer pressure, the water vapor pressure and outgassing from the lightly degassed fluid. These reactions force the gas/vapor layer to expand laterally throughout the micropillar array, thus restoring the Cassie-Baxter wetting state. The reported results demonstrate a low-power method for effectively reversing the Wenzel wetting state of a SH surface under microchannel flow conditions and may prove to be a useful technique for manipulating fluid flow within microfluidic devices.