Soft probing technique to estimate the rolling work of adhesion in nanoscale regime using optical tweezers

Atomic Force Microscopes (AFM) with 10 nm tip is employed to estimate work of adhesion at nano-scale. The AFM tip is pressed against the surface with forces around a few nano-Newtons and retracted back until it breaks from the surface. Thus estimating the work of adhesion due to this technique can be termed as "hard probing" of the surface. Whereas, we propose another configuration in which a spherical particle is trapped near the surface using a linearly polarized light and the particle attaches to the surface by work of adhesion. Here, by moving the surface in tangential direction, the particle is forced into a rolling motion. This motion can be used to estimate work of adhesion and this technique can be called "soft probing". We used the soft probing configuration to estimate rolling work of adhesion of a birefringent 3 μm particle on a glass surface. Further, we have studied the effects of PolydimethylSiloxane (PDMS) which is a hydrophobic surface. This technique is used to probe the rolling work of adhesion of 500 nm nanodiamond bearing Nitrogen-vacancy centers which are birefringent due to the stress in the crystal. These nanodiamonds have a contact diameter as small as 50 nm because of their relatively high Young's modulus. The rolling work of adhesion estimated using our soft probing configuration is about 1 mJ/m², while using the AFM tips to estimate work of adhesion at nanoscale yields about 50 mJ/m².