Needle-free drop deposition: the role of elastic membranes

Abstract

Contact angle measurement of low-energy surfaces (superhydrophobic, superoleophobic, etc.) with needle–drop assembly is critical for characterizing such substrates. However, it is extremely difficult to detach the needle from the drop when it is brought into contact with a characterizing substrate and often one has to report contact angles with the needle attached to the drop itself. To overcome this challenge, here we present a new technique to achieve a ‘needle-free’ drop by bringing the drop into contact with an additional elastic membrane, kept between the needle–drop assembly and the characterizing substrate. The detachment of the drop from the needle is achieved by retracting the needle–drop assembly at a finite speed and allowing the drop to receive the elastic energy of the soft flexible membrane. Such interaction of the drop with the elastic membrane allows the drop to be repelled from the elastic membrane and further be deposited on a characterizing substrate. The repelling behavior of the drop can be controlled by appropriately selecting the mechanical and wetting properties of this additional elastic membrane. This technique not only provides a needle-free drop deposition that is independent of the physical properties of the liquid and the needle but it also allows achievement of drop sizes independent of the needle diameter. Experimental analysis and theoretical investigations suggest that the mechanical properties of the elastic membrane, particularly its elasticity, play an important role towards the success of the drop deposition technique.