Using Magnetic Field to Measure Detachment Force between a Nonmagnetic Droplet and Fibers

The ability to predict and/or measure the force needed to detach a droplet from a fiber (or an assembly of fibers) is important in designing efficient droplet–air or droplet–fluid separation media for a variety of applications. This paper reports on the use of magnetic force to measure the force of detachment for nonmagnetic droplets for the first time. This is accomplished by adding a small amount of ferrofluid (the secondary fluid) to the original nonmagnetic droplet (the primary fluid) to create a compound droplet with the ferrofluid nesting inside or cloaking the nonmagnetic droplet. Either way, the secondary ferrofluid can be used to induce a body force to the resulting compound droplet and thereby detach it from the fiber(s). The recorded detachment force can be used directly (the case of nesting ferrofluid) or after scaling (the case of cloaking ferrofluid) to obtain the force of detachment for the original nonmagnetic droplet. The novelty of the proposed method lies in the fact that it circumvents the need for using an external object, an airflow, or a centrifugal device for force measurement. The accuracy of our measurements was examined through comparison with validated numerical simulations as well as experimental data in the literature and good agreement was observed.