World Community Grid and researchers at a newly launched multidisciplinary mechanics and innovation center, CNMM, at Tsinghua University are working together to understand the molecular scale properties of a new class of efficient and inexpensive water filter materials, which may help to satisfy demand for cheap, clean drinking water in developing countries.
Clean water is often compared to oil, as a limited resource that has been often squandered over the last decades, and is increasingly expensive to produce. Sources of clean water, in particular underground aquifers, are being depleted at alarming rates in many parts of the world. As the planet's population grows, this situation will only get worse, and may be exacerbated by climate change.
According to a recent special report on water in The Economist, the proportion of people living in countries that are chronically short of water will rise from 8% of the world's population at the turn of the 21st century to 45% by 2050, which by then will represent 4 billion people.
Although every school child knows that our planet is covered mainly in water, most of that water - over 97% - is salt water that can only be transformed into drinking water by an expensive desalination process. Of the roughly 3% of water that is not salty, 70% is frozen at the poles. So with the exception of marine life, all animals on the planet have to survive on less than 1% of the planet's available water.
In parts of the world where water is scarce, and the population density high, lack of access to clean water is a major source of diseases such as diarrhea, which in turn can cause malnutrition. And childhood malnutrition is linked with lifelong health issues that affect people's productivity. Estimates are that the long-term impact of diarrhea may reach 4-5% of GDP in some countries.
As a result, a great number of scientists are focusing their attention on novel ways to produce clean drinking water from contaminated or salty water. Purification of water normally involves several steps, which can be based on principles that are physical (sand filters) chemical (chlorination) or even biological (treatment ponds).
Filtering under pressure
A common type of water purification system relies on pressurizing water in order to force it through membranes with microscopic holes. This is the case of so-called ultrafiltration membranes, used to filter out dissolved substances that might get through larger sand-based filters.
This is also the principle behind the process known as reverse osmosis for producing fresh water from salt water. Reverse osmosis requires external mechanical pressure to counter the osmotic pressure that occurs across semi-permeable membranes which prevent salt flowing through. In the absence of external pressure, fresh water crosses the membrane to dilute the salt water on the other side. By applying high pressure on the salt water side, the water is forced to cross from the salt water side to the fresh water side. Since the membrane prevents salt from crossing, more fresh water is obtained on the low pressure side of the membrane.
Reverse osmosis normally requires pressures of tens of atmospheres to overcome this equilibrium and to keep fresh water flowing through the membrane. Producing such high pressures, and membranes that can withstand them, is expensive. This explains in part why reverse osmosis still only accounts for a very small fraction of drinking water produced around the world.
Nanotechnology to the rescue
Nanotechnology is a buzz word in fields as diverse as electronics, renewable energy and medical diagnostics. And carbon nanotubes - essentially rolled up atomic layers of ordinary graphite, the material used in pencils - are one of the most promising materials in nanotechnology.
One of the most important features of nanotechnology is that, as common objects and devices shrink in size towards the atomic scale, many of their properties can no longer simply be extrapolated from the macro and micro scales, but rather the properties change in radical and often highly beneficial ways. This is the case for water flowing through arrays of nanotubes.
Normally, as the pore size in an ultrafiltration membrane shrinks, so, too, does the rate at which water will flow through the pore. In fact, the rate falls off drastically, roughly as the fourth power of radius of the pore: shrink the pore size by half and the flow ebbs by a factor of 1/16.
But results first published by researchers at the University of Kentucky in the US in 2005 showed that, for flow through membranes made of carbon nanotubes, this was not the case. Indeed, the measured flow rates were many times higher than simple extrapolation from larger pore sizes would have suggested.
Such a dramatic enhancement suggests that great savings could be made in terms of the necessary pressure, and hence the energy involved in pushing water through filters made of nanotubes. Already, many researchers are pursuing this line of inquiry, and attempting to make a new class of low-cost and highly efficient filters this way.
It is always a long way from discovery to practical devices. And one of the important steps on the way is to understand more deeply the physical origins of this enhanced behavior of carbon nanotubes, in order to better exploit it. This is precisely the focus of the research at CNMM, where computer simulations have been used to study the phenomenon at the scale of individual water molecules, using a technique called molecular dynamics.
The story so far
As far back as 1823, it was suggested by the French physicist and engineer, Claude-Louis Navier, that under ideal conditions, it would require a vanishingly small force, or 'shear stress' to make a fluid slip across a solid surface. In other words, the flow would be essentially friction-free. This idea has never been fully verified, but the recently observed enhanced flow rates of water through membranes of carbon nanotubes suggest that some form of this effect may well be at play in these systems.
Using molecular dynamics simulations, the Tsinghua researchers recently found by simulation that there is a logarithmic relationship between the shear stress in the nanotubes and the velocity of the water, which appears to be valid for a wide range of assumptions about the properties of the carbon nanotubes, and the way water might stick to them - the so-called wetting properties of the water-nanotube interface.
This logarithmic relationship appears to hold for slip velocities down to about 1m/s, which is the lower limit of what could be simulated. If this relationship holds at much lower velocities, characteristic of the real conditions in experiments where nanotube water filters have been used, then it could provide a significant clue to why water appears to flow so fast in the nanotubes.
Yet although the lower bound of velocity studied by the Tsinghua group is the lowest of any molecular dynamics study to date, it is still at least one order of magnitude higher than the upper bound of the experimental flow rate range, and several orders of magnitude larger than the flow rates expected in practical devices that would use this effect.
How you and World Community Grid can make a difference
Since compute time scales roughly as the inverse square of the flow rate, the Tsinghua researchers estimate that a compute time of 460 years on a typical desktop computer with a single core processor is required to simulate flow rates comparable with the upper bound measured in experiments. To extend the simulations to velocities of about 1cm/s or less, typical of practical devices, would require another factor of 400 or more in compute time, for a total of 184,000 years. And to simulate a representative range of carbon nanotube pore sizes would require a further factor of 10 to 100, bringing the total compute time to well over a million years.
Scientifically, it is essential to explore this low-velocity region in order to compare simulations directly with experiment, rather than simply trying to extrapolate from higher velocity simulations. Such extrapolation is particularly problematic because of the possibility of non-linear phenomena that may occur at low velocities. For example, stick-slip phenomena occur in solid friction as velocity is reduced, and may be anticipated to play a role in the water layer immediately in contact with the carbon nanotube, since water is known to form an ice-like pattern near the carbon nanotube surface.
Given the very large computing requirements for pursuing this research, which far outstrip the capabilities of the in-house cluster available to the Tsinghua team, World Community Grid and volunteers like you can make a crucial difference, by providing access to far more computing power than researchers could otherwise afford.
The result of this project will not only allow us to test the predictions of Navier, thus contributing to fundamental knowledge about hydrodynamics on the nanoscale, but should also provide insight in how to further optimize fluid flow through carbon nanotube membranes and other forms of nanoscale membranes.
Specifically, the Tsinghua team expects to gain a better physical understanding of optimum pore size as a function of flow rate, which can guide the synthesis and fabrication of future, highly efficient carbon nanotube filter membranes, as well as suggest alternative approaches to making inexpensive water filtration systems.
