How do geckos walk effortlessly along walls and ceilings? How does accumulated rainwater fall from a canopy? The reason is a small force called the Van der Waals (VdW) force that arises when two surfaces are brought into close proximity of each other.
Now, researchers from the Indian Institute of Science Education and Research (IISER), Kolkata, have used oscillation optical tweezers constructed using focused light from lasers to measure the force named after Dutch scientist Johannes Diderik van der Waals.
VdW is of ubiquitous nature with its manifestations found all around us -- starting from how a gecko sticks to a wall to how rainwater accumulated on a canopy falls down in the form of droplets. However, the force acts over a very short spatial range, and dies off when the inter-surface separation is increased to even a few hundreds of nanometres.
Measuring this force is very challenging -- both because it is very small, and because its influence falls off rapidly with distance, Ayan Banerjee, from the Department of Physical Sciences, IISER Kolkata, told PTI.
In the peer-reviewed journal Applied Physics Letters, researchers noted that optical tweezers -- with its capacity to measure forces as small as tens of femto-Newtons (10^-15 N) -- can be a suitable candidate to measure such forces.
"This is what we demonstrate in this paper, where we use an optically trapped probe particle of three microns diameter to come up to 80 nanometres of a second larger particle, and measure how the VdW force influences the motion of the probe," Banerjee said.
Van der Waals force plays a fundamental role in fields as diverse as molecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics.
VdW decreases rapidly as the surfaces move away from each other, and is only very large when the surfaces are almost touching.
However, for tiny objects such as cells, where the forces concerned are themselves very small, the VdW force is of consequence even when the surfaces concerned are hundreds of nanometres away.
This actually is not too small a distance with reference to a cell, where the DNA can only be tens of nanometres long.
Thus, surface forces are of very significant importance in the case of cells since they regulate cell adhesion, and phagocytic engulfment -- a process where a cell surrounds and eats an invader.
The researchers, including Avijit Kundu, Shuvojit Paul, Soumitro Banerjee, noted that there is an enormous interest in understanding the nature of these extremely small forces that act within a cell.
"It is obviously difficult to measure these forces using standard techniques, and this is where optical tweezers come in -- where we use light from a laser focused very strongly using a microscope objective lens to optically confine or trap' small transparent particles, and even move them at will," said Banerjee.
"This small trapped particle thus can act as a probe' to measure forces as small as 10^-15 N," Banerjee said.
About 9.8 Newtons is the force on a one kilogramme object falling freely under gravity.
The researchers wanted to measure how these VdW forces behave as a function of separation between two surfaces.
"Thus, we optically trapped a small -- three micron diameter -- polystyrene particle or probe very close to a bigger silica particle with a diameter of 80 to 100 nanometres and then, actually oscillated the trapped probe using our tweezers very close to the latter," he said.
Both particles were kept in a small droplet of water taken on a microscope cover slip and while the big particle stuck to the cover slip, the probe was floating or diffusing around in the water.
The team trapped it using light -- optical tweezers -- and oscillated it like a pendulum close to the big particle.
As the probe approached the surface, the VdW force pulled it towards the surface, whereas the optical trap pulled it in the opposite direction in a virtual tug of war.
The nature of the particle response was modified.
Basically, the oscillation amplitude was reduced, compared to the amplitude without the surface being present, and the particle could only approach the surface up to where the VdW force balanced the optical trapping force.
In case the researchers had gone too close to the surface, the VdW force would make the particle stick.
From simulations the researchers expected amplitude or the magnitude of change of the particle response given the known nature of the VdW force.
They essentially matched it with the experimental results and determined the value of the VdW force experimentally.
"Ours is a very general technique which may be used to measure the nature of any type of force between two particles or surfaces," Banerjee said.
The work, he said, may be expanded to surface forces due to a cell -- both extracellular (outside) and intracellular (inside).
"For the former, one can find out how a cell membrane influences a particle nearby and also determine whether the influence changes depending on a diseased condition -- in diseases such as malaria, cancer, even due to a heart condition, the membrane elasticity is modified -- and think of this as a diagnostic tool," the researchers noted.
Inside the cell, one can study how surface forces influence the process in which cells phagocyte an invader, and in fact even determine how the intracellular environment behaves.
"As I mentioned, this is a very general technique to measure the influence of any force -- for this particular application we considered the VdW force -- but we may also just oscillate a particle at different locations inside a cell and measure how they differ," Benerjee added.
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