Our dynamic Earth is scattered with cracks. Earthquakes and petroleum fracking make cracks in rocks underground; melting ice forms cracks in glaciers. Freeze-thaw, roots pushing into weathered rock, the shatter from an impact—all these processes create cracks.
The details of the fracturing process remain a mystery. Scientists know that the roughness of a rock or ice surface can affect how fluids flow across it and how fractures travel through it. But what if you had a detailed 3-D movie of fractures in the act of forming, crack by crack?
Catching an actual fracturing event as it happens is tough to do (much less figuring out how to film something underground), but one group of scientists set up and filmed a similar event in their lab using a synthetic material called a brittle hydrogel. This material, comprising mostly water, is transparent, which makes it easy to see cracks as they form.
“I don’t think anyone else has a 3-D movie of a fracture,” said Will Steinhardt, a geophysicist at Harvard University. Steinhardt presented the work in a poster session and an oral session this month at AGU’s Fall Meeting 2018 in Washington, D. C.
Cracks travel through a chunk of this hydrogel in a lab similarly to the way they travel through rock or ice formations in the field. With a high-speed camera and a dye that shines under laser light, the scientists filmed a fracture traveling through this brittle hydrogel in 3-D, seen in the video below.
Lights, Camera, Fracture
As a whole, each fracture Steinhardt studies looks almost like a flattened M&M candy or a bulging coin. Slicing an M&M-shaped fracture lengthwise gives a 2-D view of the pattern of small cracks, called step lines, in the whole fracture. Depending on how many of these smaller cracks are in the sliced area, the surface of the slice may be rough or smooth.
Steinhardt and his graduate adviser Shmuel Rubinstein, an applied physicist at Harvard, wanted to see how a fracture travels in three dimensions and how the interaction of its step lines forms rough surfaces. They chose to study fractures in the transparent brittle hydrogel so they could photograph what happened inside the material as it fractured.
The researchers put a small dent on one side of a chunk of hydrogel to mimic a natural flaw where a fracture might start, like a slight tear in a sheet of paper or an existing crack in rock. They filled the small dent and a connected tube with a dyed fluid that glows under laser light.
When they applied pressure to the fluid in the tube, the hydrogel fractured, with the fluid fanning outward from the initial flaw, as seen in the picture below.
They shined laser light into the gel to make the fluid glow and snapped photos of an area about the size of a small fingernail with a high-speed camera at about 1,000 times per second. By combining these images into a video, the researchers captured the changing shape of the fracture and its pattern of step lines in all their 3-D glory.
Behind the Scenes of Fracture Patterns
The researchers found that they could make more step lines appear in the fracture—and thus create a rougher fracture surface—in one of two ways. Both involved changing properties of the hydrogel.
In the hydrogel, a network of large molecules called polyethylene glycol (PEG) polymer holds the gel’s water in place. Adding another chemical compound called glycerol to the hydrogel increased the total number of large molecules in the gel and made more of the small cracks appear, the scientists found.
The researchers also tried infusing the hydrogel with tiny glass beads that were smaller than the width of a typical human hair. Adding beads also increased the number of step lines that formed in a fracture.
The researchers think that both methods may create more step lines by giving the gel more flaws—the scientists term these “discontinuities”—where the cracks can start.
From their detailed records of the small cracks forming in a fracture over time, the scientists began to figure out what patterns form when two cracks meet at a point. The shape and orientation of two cracks seem to determine whether only one crack stretches past the meeting point, for example, or whether the cracks might simply cross each other.
It’s a “first step in building a comprehensive theory for roughness,” Rubinstein said. By understanding what happens when two step lines meet, the researchers can start piecing together a bigger picture with many step lines interacting to create rough fracture surfaces.
“It’s a new way to study this,” said structural geologist Randy Williams of the University of Wisconsin–Madison. He added that he’d be interested in seeing a comparison to actual rock.
Steinhardt said that comparing their lab-made hydrogel fractures to natural rock fractures is challenging. Many factors affect surface roughness in real-world rocks, making them difficult to compare to controlled experiments. To do a similar study of the relationship between the number of discontinuities in a material and the number of step lines in a fracture, they’d need a range of rocks whose graininess is well understood.
The step to using actual rocks is something “we have wanted to do for a long time,” Steinhardt explained, “but are not exactly sure how.”