Measuring oceans of activity in one drop of water

“The surface of a water drop is constantly moving, due to what is called Brownian motion, or thermal motion,” said Carmon.

Lab technician using microscope (photo credit: INGIMAGE / ASAP)
Lab technician using microscope
(photo credit: INGIMAGE / ASAP)
Scientists at Haifa’s Technion-Israel Institute of Technology have measured and recorded thermal motion in a water droplet; their work may result in a new kind of medical sensor.
They found that measuring a water droplet with a resolution comparable with the scale of a single atom will reveal that the droplet interface behaves like a miniature stormy sea. The waves in this ocean are generally referred to as “thermal capillary waves,” and they exist even if the droplet is seen, to the naked eye, as being at rest.
Using that knowledge, the researchers developed technology to analyze the thermal capillary dynamics in a drop of water. The advancement could one day lead to a new generation of medical sensors that are able to identify abnormal cells.
The findings by graduate student Shai Maayani and Prof. Tal Carmon of the Technion faculty of mechanical engineering were published in a recent issue of Optica.
The measurement of thermal capillary waves was made possible by turning the water droplet into a device the researchers called an opto-capillary resonator. The device was used to introduce light into a water droplet to record the thermal capillary motion inside it. Being able to accurately measure this activity means that it could also be possible to support a controlled energy exchange between light and capillary waves in the drop.
“The surface of a water drop is constantly moving, due to what is called Brownian motion, or thermal motion,” said Carmon. “Thermal motion on the outer surface of a water droplet impacts many processes, including breaking of a single drop into many smaller droplets.”
The researchers experimented with what are called “capillary oscillations” in a water droplet. These motions are governed by water’s surface tension, the force that gives a drop of water its shape. Water droplets are a fundamental structure of self-contained liquid bounded almost completely by surfaces.
In their experiment, photons (particles of light) were confined to circulate along the equatorial line of the droplet, at a depth of 180 billionth of a meter. Being so close to the drop interface that hosts the thermal capillary waves enabled the recording of this thermal motion of water.
According to the researchers, once inside the water droplet, light circulates up to 1,000 times around its circumference, which helps in measuring the capillary waves. The number of times that light circulates is called “optical finesse” and can be used to monitor the movements down to the size of 1/1000th of the very small wavelength of light. Optocapillary cavities can support a controlled energy exchange between light and capillaries, explained the researchers.
When light waves and water waves co-resonate in certain ways – when they pass through one another – there can be an exchange of energy between the two types of waves. The data from that interaction could be used to develop a new type of sensor. For example, if a biological cell is placed into a water drop the cell’s reaction to waves – whether waves of light, water or sound – can reveal information about the nature of the cell.
“Based on a cell’s reaction to light, water and/or sound waves, it may one day be possible with the optocapillary resonator to determine whether a cell is normal or a malignant cancer cell,” concluded the researchers.
BUILDING A BETTER BOW TIE Bow tie-shaped nanoparticles made of silver may help bring the dream of quantum computing and quantum information processing closer to reality.
These nanostructures, created at the Weizmann Institute of Science and described recently in Nature Communications, greatly simplify the experimental conditions for studying quantum phenomena and may one day be developed into crucial components of quantum devices, according to Prof. Gilad Haran of Weizmann’s chemical physics department.
Working with postdoctoral fellow Dr. Kotni Santhosh, Dr. Ora Bitton of chemical research support and Prof. Lev Chuntonov of the Technion-Israel Institute of Technology, they manufactured two-dimensional bow tie-shaped silver nanoparticles with a minuscule gap of about 20 nanometers (billionths of a meter) in the center.
The researchers then dipped the “bow ties” in a solution containing quantum dots, tiny semiconductor particles that can absorb and emit light, each measuring six to eight nanometers across. In the course of the dipping, some of the quantum dots became trapped in the bowtie gaps.
Under exposure to light, the trapped dots became “coupled” with the bowties – a scientific term referring to the formation of a mixed state in which a photon in the bowtie is “shared” with the quantum dot. The coupling was sufficiently strong to be observed even when the gaps contained a single quantum dot (as opposed to several).
The bow tie nanoparticles could thus be prompted to switch from one state to another – from a state without coupling to quantum dots before exposure to light to the mixed state characterized by strong coupling following such exposure.
Thus the ability to control the coupling of quantum dots may one day be employed in the manufacture of switches for computing or encryption devices relying on quantum phenomena, that is, those operating at the level of photons and single quantum systems such as atoms, molecules or quantum dots.
Because such phenomena open up possibilities unavailable on the macroscopic scale – for example, performing multiple computations simultaneously – quantum devices are expected to be vastly more powerful than today’s electronic computers and encryption systems.
Haran explained: “We’ve made a first step toward creating quantum switches using our coupling method. Much research needs to be done before the method can be incorporated into actual devices, but as a matter of principle, our system is relatively easy to generate and, most importantly, can function at room temperature. We are currently working to fabricate even smaller bowtie particles and to render the coupling stronger and reversible.”
The Weizmann scientists managed to design their bow tie system thanks to advances in nanotechnology – including electron-beam lithography used to fabricate the bow ties and facilitate the introduction of quantum dots into their gaps – and the advent of computational programs providing data analysis that previously required a massive effort on the part of theoreticians.
They also relied on the recently improved understanding of electron oscillations triggered by light in metals, which constitute the physical source of the coupling between the bow tie nanoparticles and the quantum dots. Such oscillations are known to be strongest on the metal surface.
In the new bow tie-shaped particles, the electromagnetic field generated by these oscillations is extremely concentrated because it is focused to the central, narrow portion of the bow tie, much as light is concentrated when focused into a narrow beam.
The high concentration ensures tight control over the coupling, and this control, in turn, is essential for potential future quantum applications. None of the systems built in the past to study quantum interactions between light and matter operated on such a small scale or were able to reduce experiments to the level of individual quantum dots, as was done in the Weizmann study.