Indian scientists measure Van Der Waals force, use laser

Scientists at Indian Institute of Science Education and Research (IISER), Kolkata, have measured Van der Waals forces -- that allows geckos to walk effortlessly along walls, and rainwater to accumulate and fall from a capy -- using optical tweezers. Van der Waals (VdW) force -- named after Dutch scientist Johannes Diderik van der Waals -- arises when two surfaces are brought into close proximity of each or, and IISER Kolkata researchers measured it using oscillation optical tweezers constructed using focused light from lasers. 

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 capy falls down in form of droplets. However, force acts over a very short spatial range and dies off when inter-surface separation is increased to even a few hundreds of nametres. Measuring this force is very challenging -- both because it is very small, and because its influence falls off rapidly with distance, Ayan Banerjee, from Department of Physical Sciences, IISER Kolkata, told PTI.

Researchers te

In peer-reviewed journal Applied Physics Letters, researchers ted 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 nametres of a second larger particle and measure how VdW force influences motion of probe," Banerjee said.

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About Van der Waals

Van der Waals force plays a fundamental role in fields as diverse as molecular chemistry, structural biology, polymer science, natechlogy, surface science, and condensed matter physics. VdW decreases rapidly as surfaces move away from each or, and is only very large when surfaces are almost touching. However, for tiny objects such as cells, where forces concerned are mselves very small, VdW force is of consequence even when surfaces concerned are hundreds of nametres away. This actually is t too small a distance with reference to a cell, where DNA can only be tens of nametres long. Thus, surface forces are of very significant importance in case of cells since y regulate cell hesion, and phagocytic engulfment -- a process where a cell surrounds and eats an inver.

Measuring Van der Waals force

researchers, including Avijit Kundu, Shuvojit Paul, Soumitro Banerjee, ted that re is an ermous interest in understanding nature of se extremely small forces that act within a cell. "It is obviously difficult to measure se 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 m 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 force on a one-kilogram object falling freely under gravity. researchers wanted to measure how se 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 nametres and n, actually oscillated trapped probe using our tweezers very close to latter," he said. Both particles were kept in a small droplet of water taken on a microscope coverslip and while big particle stuck to coverslip, probe was floating or diffusing around in water. team trapped it using light -- optical tweezers, and oscillated it like a pendulum close to big particle. As probe approached surface, VdW force pulled it towards surface, whereas optical trap pulled it in opposite direction in a virtual tug of war. nature of particle response was modified.

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Basically, oscillation amplitude was reduced, compared to amplitude without surface being present, and particle could only approach surface up to where VdW force balanced optical trapping force. In case researchers h gone too close to surface, VdW force would make particle stick. 

On technique 

From simulations, researchers expected amplitude or magnitude of change of particle response given kwn nature of VdW force. y essentially matched it with experimental results and determined value of VdW force experimentally. "Ours is a very general technique which may be used to measure nature of any of force between two particles or surfaces," Banerjee said. work, he said, maybe expanded to surface forces due to a cell -- both extracellular (outside) and intracellular (inside). "For former, one can find out how a cell membrane influences a particle nearby and also determine wher influence changes depending on a diseased condition -- in diseases such as malaria, cancer, even due to a heart condition,   membrane elasticity is modified -- and think of this as a diagstic tool," researchers ted.

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Inside cell, one can study how surface forces influence process in which cells phagocyte an inver, and in fact, even determine how intracellular environment behaves. "As I mentioned, this is a very general technique to measure influence of any force -- for this particular application we considered VdW force -- but we may also just oscillate a particle at different locations inside a cell and measure how y differ," Banerjee ded. 

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