Fluids can be roughly divided into two categories: ordinary and strange. Water, alcohol, etc. can be roughly counted as ordinary fluids, and when they flow through pipes or are stirred, the effect is more or less in line with expectations; but liquids with high viscosity such as paint, honey, and blood often show some mysterious behaviors that scientists can not solve. The story we're going to tell today is related to a strange fluid behavior that has plagued scientists for more than half a century.
Strange phenomenon in the oil field
In oil fields, in order to extract as much oil as possible, engineers often inject water into the ground as a mixture of a so-called push-extrusion liquid. In the 1960s, rheologist Arthur Metzner and his student Ronald Marshall were doing fieldwork in oil fields and noticed that when these push-extrusion fluids containing long-chain polymers were pumped into the ground at a certain speed, they unexpectedly became more viscous.
Such effects were observed in many similar systems later. However, after decades of in-depth research, scientists still don't know why these liquids become more viscous. This raises a 50-year-old mystery: Why do certain fluids, under pressure, slow down strangely when flowing porous materials, such as soil and sedimentary rock?
Now, a team of researchers from Princeton University has used experiments to find the answer to the puzzle. They published the findings in the journal Science Advances.
Polymer solution
Whether it is a push and extrusion fluid in the oil field, or some other viscous fluid, it can be collectively referred to as a polymer solution. These solutions may contain long and complex molecules, such as those containing dissolved polymers, or containing materials composed of macromolecules having many repetitive subunits.
Many energy, environmental, and industrial processes rely on the flow of these polymer solutions on porous media. Usually, under pressure, the viscosity of polymer solutions decreases and the flow rate increases; but as they pass through porous media, the viscosity increases, resulting in a decrease in flow rate.
At first, scientists thought that this phenomenon might be due to the accumulation of macromolecules in the solution in the pores, like drainage holes blocked by hair. But they soon realized that the problem wasn't that simple, because once the flow rate of the polymer solution fell below the critical threshold, the situation disappeared completely.
Novel experiments
To get to the root of the problem, in the new study, the researchers designed a novel experiment to see what happens when a polymer solution flows through a porous medium.
The entire experimental apparatus consists of a small rectangular chamber, only half the length of the little finger, and the chamber is randomly filled with many tiny borosilicate glass beads. These transparent glass beads act as a porous medium, acting like a transparent artificial rock, allowing researchers to visually observe the movement of the polymer solution in it.
The researchers developed a transparent medium that allowed them to observe the polymer flow. | Image courtesy of David Kelly Crow
In this "artificial rock," the researchers injected a common polymer solution into it. To facilitate the observation of the flow of the solution around the miniature glass beads, they added fluorescent latex particles to the polymer solution in advance.
At the same time, they also specially formulated the polymer solution used in the experiment so that the refractive index of the material could counteract the optical distortion from the glass beads and make the entire device transparent when saturated.
During the experiment, in order to be able to easily examine the flow of liquid through each pore, they focused the microscope on the microwells, or the gaps between the glass beads. The size of these voids is on the scale of 100 microns, similar to the thickness of a human hair.
Surprising find
In this experimental setup, the researchers photographed the movement of the fluid. They observed that as polymer solutions pass through porous media, as the flow rate increases, their flow begins to become chaotic, tumbling and self-circulating in collisions with themselves, creating turbulence. This phenomenon begins in one or two voids, then more and more, and finally fills all the gaps.
A viscous fluid passes through a porous medium and flows smoothly when the flow rate is low (left), but when the flow rate increases (right), the polymer in the fluid causes the fluid to become chaotic and the vortex continues to form, grow, and disappear. | GIF Source: Datta Lab
This phenomenon surprised the researchers because, in general, when a fluid flows at such a rate in such a dense pore, there is no turbulence, but laminar flow, that is, the fluid flows smoothly and steadily.
They found that as these polymer solutions travel through pore space, they stretch, creating forces that can accumulate in different pores and cause turbulence. This effect becomes more pronounced when pushing the solution through at higher pressures.
Through this experiment, the researchers clearly saw and recorded all the unstable regions and found that these regions did affect the transport of the solution as it passed through the medium. Using data collected from experiments, they developed a method to predict how polymer solutions behave in real life.
Wide range of applications
Viscosity is one of the most fundamental properties that describe fluid flow, and this is the first time scientists have linked turbulence to an increase in viscosity in porous media. Historically, scientists have not been able to predict the viscosity of polymer solutions flowing in porous media. New research confirms that such predictions are achievable. Now, we can see exactly what happens when a polymer solution is pumped into the ground or into other opaque porous media. The mystery that has plagued scientists for more than half a century has finally been unraveled.
This discovery will not only contribute to a better understanding of polymer solution flow and chaotic flow, but will also help improve many important processes in the energy, environmental, and industrial sectors, such as guiding practitioners in related fields to prepare the right polymer solutions in different environments and completing tasks with the right pressure.
Because the polymer solution itself is viscous, environmental engineers can inject the polymer solution into highly polluted areas, such as abandoned chemical plants and other industrial plants. The viscous solution helps to remove trace contaminants from the affected soil. In addition, with the right polymer solution, oil can be better extruded from the pores of underground rock, thereby improving the efficiency of oil extraction. What excites researchers most is that turbulence in polymer solutions may be well used to clean contaminated groundwater.
#创作团队:
Author: Light rain
Typography: Wenwen
#参考来源:
https://cbe.princeton.edu/news/fractured-artificial-rock-helps-crack-54-year-old-mystery
https://www.quantamagazine.org/an-injection-of-chaos-solves-decades-old-fluid-mystery-20220104/
https://www.science.org/doi/10.1126/sciadv.abj2619
#图片来源:
Cover image: Dids/Pexels
Photo: GEORGE DESIPRIS/Pexels