More than just a party trick, the floating water bridge holds insight into nature and human innovation
Dr. Adam Wexler has spent the past six years of his life studying a 4-cm-long string of water: the so-called “floating water bridge.” The effect is created by applying a high voltage to two containers of water, which causes the water to climb out of the containers and form a bridge in mid-air as the containers are slowly pulled apart. Wexler and his colleagues at Wetsus have discovered that this seemingly simple experiment holds a wealth of information, both for our understanding of the natural world and for guiding future developments in sustainable technology.
“The water bridge is this beautiful and peculiar phenomenon which, despite being discovered more than a century ago, had been largely ignored by the scientific community,” said Wexler, who recently completed his PhD on the subject. “In fact, it was a novelty item passed along from generation to generation as a nerdy party trick that eventually made it to YouTube where two of my promoters, Jakob Woisetschläger and Elmar C. Fuchs, found it. Regardless, anyone who has had the good fortune to see a floating water bridge is instantly mesmerized by the simplicity and grace of what is there before their eyes—a little rope of water hanging in mid-air between two wine glasses.”
Wexler received his PhD for his research on the properties of the water bridge on April 19th, one day after his 42nd birthday. His doctorate was granted by Wageningen University in cooperation with the Graz University of Technology in Austria. Today, he works at Wetsus as both a researcher in Applied Water Physics and as the head of the Arie Zwijnenberg Laboratory for Advanced Microscopy and Optical Metrology.
Although creating a water bridge is fairly straightforward, the science behind it is astonishingly complex. Not only does the floating water bridge provide a deeper fundamental understanding of water, it also offers insight into areas ranging from desalination and zero-waste manufacturing to biochemistry and injury rehabilitation, and even to the question of what it means to be living vs. nonliving. By connecting so many diverse areas, the water bridge is a bridge both physically and metaphorically.
Below, in his own words, Wexler reveals how a tiny string of water holds much more than meets the eye.
On the ordinary and extraordinary properties of the water bridge:
Taking some more time to watch the bridge in action, one is stupefied by the complexity. The water movement is bidirectional, i.e., it simultaneously flows in both directions, and the shape and diameter of the bridge is constantly changing. The bridge also has a lot of strange optical properties like weak birefringence and mixed refractive index, which make it appear somewhat different than “ordinary” water, even to the naked eye!
There is practically nothing ordinary about the water in an active floating bridge, and this is no esoteric experiment, as the strength and shape of the electric field we apply in the water bridge is nearly ubiquitous throughout nature. It turns out that if we examine the electric fields present in nature, such as those in living cells, around soil particles, or in clouds, we find that the field strengths are on the same order of magnitude—megavolts per meter. Which incidentally is the same in the water bridge, not to mention inside many electrochemical and biochemical fuel cells that are now being used to develop the next generation of resource recovery technologies.
Megavolts per meter seems to be this kind of universal constant of field strength in aqueous systems. It’s such an enticing observation that, during my defense, one of my opponents made a point to ask whether I thought this was just a coincidence or indicated some deeper truth that we are as yet unaware. Of course I had to answer the latter, as I am a firm believer that nature is quite deliberate in its construction and there are really no accidents.
On using the water bridge as a tool for understanding water and electric fields:
This electrically excited state of water in the water bridge is likely the most common state encountered in our daily lives, but is often overlooked because it was experimentally inaccessible (and thus unknown!) until our work on the floating water bridge. The liquid state is exceptionally sensitive to perturbations, and the floating water bridge provides a new opportunity to look at how electric fields can influence the dynamics of the hydrogen bonding network.
Developing a clear understanding of the physical and chemical properties of water under the influence of such electric fields is essential to the successful upscaling of many technologies, such as electrochemical and biochemical fuel cells. The problem has been that, until now, it was not possible to reliably measure the influence of electric fields. This is because, unlike in the water bridge, in most other examples where electric fields and water interact, the physical distances are very short, between a few tens of nanometers out to at most a millimeter or two.
The water bridge is a macroscopic object that provides enough sample volume to use in modern materials characterization experiments like X-ray and neutron scattering, or NMR, ultrafast vibrational, and Raman spectroscopies. These are the tools we and others used to examine whether anything unusual about the way the molecules in the bridge behaved. We also performed an electrochemical analysis because it was a bit puzzling as to why there was so little electrolysis of the water during bridge operation.
It turns out that the water bridge can teach us important lessons not only about electrohydrodynamics but also about water. The floating water bridge is a long-lost piece of the puzzle in understanding why water is such an unusual substance. What we had missed was that, in liquids there exists an intrinsic disequilibrium which continuously drives the system around within a big basin of possible configurations; and water is again exceptional. When we apply the electric field, we perturb the dynamics and change the shape of the attractor basin. This in turn changes the flow of energy through the system and may even liberate stored energy that is inaccessible when the liquid is in the ground state.
On the challenges with models:
The key in my view is that when we are dealing with water in the condensed phase, we find there is a serious limitation imposed by the necessary step of model creation, which we then use to interpret our experimental data. The arbitrary choices we make regarding which spatial and temporal length scales to include are only the beginning of the trouble. We are also faced by the problem of what types of forces to turn on or off. For example, some recent calculations by Prof. John Swain at North Eastern University in Boston showed that, if we use a quantum mechanical description of water to calculate the total energy in the system, we arrive at a value 3.7 times that found with the classical approach!
Before coming to Wetsus, I worked on trying to understand how the macroscopic behavior of the liquid arises from the molecular level—we examined polymer interfaces, colloids, bubbles, and droplets. In all of these experiments, it seemed we kept running up against the question of how to delineate simple but effective assumptions in our interpretations of experiment. When I shifted my focus to the water bridge I found this situation only intensified, and yet there were some in the community who thought everything could be found in textbooks. Lucky for us they were wrong.
On two recent papers, “A floating water bridge produces water with excess charge” in the Journal of Physics D: Applied Physics and “Non-equilibrium thermodynamics and collective vibrational modes of liquid water in an inhomogeneous electric field” in Physical Chemistry Chemical Physics:
The two papers both challenge several long-held assumptions, the first being charge neutrality, and the second the so-called isotropic limit in liquids.
The water bridge allows us to create an electrochemical system whereby we store energy in the two liquid streams produced during bridge operation. This is essentially the same principle working in reverse electrodialysis systems, but here we don’t need a membrane to sort ions, and we further stabilize the produced solutions using injected electrical charge. Wetsus was awarded a patent last year for these findings, and we are now pursuing ways to upscale the technology for practical use. At the moment we are testing a reactor which can produce pure water with different pH values but that does not contain a chemical acid or base.
We also found that if we suddenly disrupt a stable water bridge that has been operating for 30 minutes or more, we measure a residual electrical charge in the two beakers. The charge sign is opposite in the two beakers and is accompanied by a change in the pH as well. If you are worrying that we have violated charge neutrality, when we recombine the two liquid volumes we find that they neutralize each other. We have found a trick for temporarily bending the rules and we are relieved to see that charge is conserved over the entire system. The trick has to do with the way that the water bridge operates, and this brings me to the second paper.
Usually when we perform engineering calculations on a system using water, we assume that the material is essentially uniform in all directions and that any instantaneous fluctuations are countered by a recoil somewhere else in the system, i.e. the isotropic limit. The water bridge shows us that these assumptions are patently false even at macroscopic length scales.
What we have found is that water molecules are easily excited into a collective vibrational mode that extends over a huge number of molecules. This state has the peculiar property of lowering the energy required for the nucleus of the hydrogen atoms (i.e., protons) to become delocalized (or shared) between two adjacent water molecules. This excited vibrational state is shared as a collective mode which stiffens the whole network, increasing the elasticity modulus of the liquid. This is part of why a water bridge can be stretched over a 4-cm gap!
By shaping the electric field, it is possible to spatially vary the physical and thermodynamic properties of water. The liquid is still a continuous system, but now we can direct the flow of energy (both kinetic and potential) and thus achieve local regions with very different physical and chemical properties.
On the water bridge and biochemistry:
The floating water bridge provides us easy experimental access to generating and studying water under the influence of moderately strong (i.e., relative to the direct coulombic interaction between neighboring water molecules) electric fields—the same field strengths that are present in living cells and in the natural environment. From studying the floating water bridge, we have learned that water under such conditions is anything but passive. Water, it seems, is not merely the background for life, but rather it plays an active role in shaping the biochemical machinery upon which life depends.
There are a number of nice animations and even a documentary-length visualization on the inner life of the cell available on the internet, but in all of these videos we only see proteins, lipids, and genetic materials. The water is completely missing from the picture, and this is an unfortunate omission—because we are left with the impression that the cell components interact in a vacuum, which certainly they do not! This reflects a limitation biologists face because we really can’t yet see what water is doing in the cell.
Despite lacking direct visualization, we do know that biomolecules can possess hydrophilic and hydrophobic regions, and that these are arranged in a precision architecture that can shape the dynamics of nearby water molecules. Where steep electric field gradients are also present, the collective vibrational modes found in the bridge would carry this influence deeper into the liquid body. I believe we will find that this selective control of water dynamics is at the heart of enzyme activity, pore selectivity, and even circadian clocks.
On water and life:
When we look at proteins, we find that they have an intimate connection with water, which is the only material flexible enough to fulfill the myriad tasks given by the biochemical engine of life. Water is not only a transient scaffold for life, but also plays an active role in the delivery and polymerization of cytoskeletal components in the cell.
Gaining a better understanding of how life works at its most basic level can lead to a much more clear view of health and healing. It may not be a comfortable admission, but we as yet do not know the difference between the living and non-living states. Many great minds have pondered this question and as yet we only keep confusing the issue rather than clarifying it. I tend to be a systems thinker (I love swarms), and I often wonder how good we would be at making microprocessors if we had no idea about the fundamental science governing transistors.
In my previous life as an injury rehabilitation therapist, I often engaged my colleagues in long discussions about healing, and in the end I concluded we lacked the critical understanding necessary to make any kind of intelligent assertion on why someone would recover while another would not. Beyond all the external factors, there always seemed to be some very bothersome additional factor (e.g., an uncontrolled parameter) that was deterministic in the process.
Now humans are horribly complicated creatures so I don’t want to give the impression that this is all a matter of water… but it was this discussion which ultimately led me to study water, because life (as we know it) is inextricably linked with water. But much in the same way that the electrohydrodynamic theory cannot predict the formation of a floating water bridge, an a priori consideration of the water molecule would never lead one to consider the existence of life.
On the role of water in zero-waste manufacturing:
Along more economically practical lines of thought, when I compare how humans produce goods and materials to how nature does it, the inefficiencies are appalling. For example, there is no dead volume in a cell, but every human built reactor vessel I know of suffers this problem. The generation of side-products in the chemical industry is just an accepted fact of modern plant operation, but for an organism such inefficiencies can be fatal.
Microfluidic reactors provide a leap forward in chemical efficiency and many technologists are getting wise to the importance of reducing our processes to length scales similar to those over which nature works. It’s not just spatial length scales but also temporal scales that matter. Just-in-time manufacturing is a very biologically inspired concept, and in a similar manner, by using the principles we are now learning from the water bridge, we can move this idea to the molecular level.
I can imagine reactors which don’t hold large volumes of caustic materials but rather only produce the ion or pH needed at the place where it is needed. We can achieve this with very high precision and with at least the same processing speeds that common consumer electronics achieve. The miniaturization of the water bridge is already under way and we are closer than ever before to being able to program water to carry out specific reactions at specific locations. For me, it is not as important to achieve better control over the transport of materials in water, but rather to gain mastery over influencing the relationship between water and the material of interest that we must focus our efforts on.
On rethinking desalination:
I love the example of reverse osmosis desalination. Here we have a few percent of salt in a massive volume of water which entirely ruins the usefulness of the stuff, so we push 96+% of the material through some very tiny pores expending huge amounts of energy to get rid of this minor fraction of salt. Wouldn’t it be much better if we could ‘tell’ the water to drop the salt? I don’t know how to do that, but I know there are plenty of organisms that do and with much better efficiencies. For me the missing part in our understanding is that we still largely consider water as a passive material. That view has to change, and this is part of my mission to make more researchers in the world aware of the lessons learned from the floating water bridge. I think it will take a large concerted effort to embrace these insights and that this will be a driving force for transforming our technologies into something which resembles those found in nature.
On winning the Marcel Mulder prize, which is awarded to the Wetsus researcher(s) who made the most exceptional performance in the field of water technology in the past year:
2015 was the first year that the prize was split, and I shared it with my colleague Jouke Dykstra. I think this is not only a sign that Wetsus is growing, but that the quality of the work we are doing remains of the highest caliber. I received the award for my work on developing specialized optical spectroscopy instrumentation and for advancing our understanding of the interactions between water molecules—in particular the disequilibrium state over the mesoscale. It was great to be recognized for my contributions over the years, but without the help of many hard-working people behind the scenes I would not have been so successful.
While I was standing on stage at the Wetsus congress looking out into the crowd it was quite humbling to see so many smiling faces and to know that so many people had contributed to that moment. I received many compliments afterwards and many commented that I really deserved it. The best thing I can do with so much positive energy is to turn around and spread enthusiasm amongst my colleagues to really go for it. I think that we have a special opportunity to make a lasting and significant contribution to the future. It truly baffles me that there are still so many people on this planet who lack basic sanitation and drinking water! We can and should do more to alleviate this.
On becoming the head of the Arie Zwijnenberg Laboratory for Advanced Microscopy and Optical Metrology at Wetsus:
This laboratory is unique in that it is the only optical facility in Europe, and as far as I am aware, the world, that is dedicated to the application of optical science to advancing water technology. This is an excellent example of how the collaborative and cross-disciplinary environment within Wetsus functions. I have the opportunity to learn about the questions my colleagues are working to answer and to consider how we can cooperate in bringing new solutions and perspectives from one discipline to another.
The diversity of research topics at Wetsus pushes the bounds of both science and engineering. I am encouraged and supported to get involved not only with my fellow researchers but also with industry partners to build a stronger network and to improve the ease with which discoveries find their way into the real world. It is a very positive environment and we have a lot of freedom to explore and innovate.
Wetsus and our members benefit from the top research facilities and the diverse expert staff, but mostly from the interpersonal relationships that this unique environment forges. I have made life-long friends here and have a lot of respect for and fun with my colleagues. I know these connections will continue to influence what we do in the future no matter where life takes us.
On the future of water science and technology:
I think water science and technology has the ability to transform human technology in general. We have been locked in a stone-age framework for millennia—we like to build very complex but rigid systems and this goes completely against the way of nature. The natural world is fluid. Everything is always moving, changing, adapting. This kind of flexible, self-optimizing technology is what we need to move towards and we won’t get there by doing what we have always done. At Wetsus we have the opportunity to forge new scientific understandings and then immediately apply them to real-world engineering problems. We cannot only surpass our current limits of capability but also hopefully avoid the pitfalls of deploying technology we don’t understand.
When we examine the technological history, we find that all too often we must expend tremendous resources to later remediate the damage caused by advancing technologies without properly understanding the mechanisms and interactions between human and natural systems. Challenges like nuclear waste, heavy metals contamination, persistent organic pollutants, antibiotic resistance, aquatic microplastics, biodiversity collapse, and climate change all have roots in the actions of our forbearers’ haste to revolutionize industry and to prioritize the metric of human gain over environmental stability. Today the pressure to advance is even stronger, and our responsibility as the current generation of leaders to carefully vet our choices is likewise amplified. This is what has given rise to the sustainability movement and here Wetsus is a front-runner.
Water is a unifying force on this planet, and while the technological challenges are great, the human and societal norms pose the biggest obstacles to change. The excellent outreach and the drive to deploy new technologies, often to those areas where it is most needed, is another important way that Wetsus makes a difference.