On‐Demand, Contact‐Less and Loss‐Less Droplet Manipulation via Contact Electrification

Abstract While there are many droplet manipulation techniques, all of them suffer from at least one of the following drawbacks – complex fabrication or complex equipment or liquid loss. In this work, a simple and portable technique is demonstrated that enables on‐demand, contact‐less and loss‐less manipulation of liquid droplets through a combination of contact electrification and slipperiness. In conjunction with numerical simulations, a quantitative analysis is presented to explain the onset of droplet motion. Utilizing the contact electrification technique, contact‐less and loss‐less manipulation of polar and non‐polar liquid droplets on different surface chemistries and geometries is demonstrated. It is envisioned that the technique can pave the way to simple, inexpensive, and portable lab on a chip and point of care devices.


On-demand, Contact-less and Loss-less Droplet Manipulation via Contact Electrification
Wei Wang a,b , Hamed Vahabi c , Arsalan Taassob a , Sreekiran Pillai a , Arun Kumar Kota a,c *

Section 1. Estimation of the adhesion force 𝑭 𝒂𝒅𝒉
We estimated the adhesion force  ℎ based on the contact angle hysteresis as: [1]  ℎ ≈     (cos   − cos   ) (S1) Here,   is the liquid surface tension,   is the width of the triple phase contact line perpendicular to the droplet sliding direction, and   and   are the receding and advancing contact angles of the liquid droplet on the solid surface.When the contact angle hysteresis is low, the shape of the droplet does not deviate significantly from a spherical cap, and   can be computed as: [2]   = 2 sin  ̅ [ 3 (2−3 cos  ̅ −cos 3  ̅ ) ] Here  ̅ is the average contact angle, given as: For a 50 μl droplet of water (  = 72.1 mN/m) on a Cl-PDMS modified glass surface (  = 5.25 mm,   = 98° and   = 103°), from Equations S1-S3, we estimated the adhesion force  ℎ ≈ 32 μN.

Section 2. Estimation of droplet charge 𝒒 𝒅
We estimated the droplet charge   using a Millikan droplet apparatus, based on a balance between the electrostatic force and the hydrodynamic drag experienced by the droplet.A charged water droplet dispensed into an oil bath in a Millikan droplet apparatus experiences a horizontal electrostatic force: [3]   = Here,   is droplet charge,  is applied voltage between the electrodes and  is distance between the electrodes.The water droplet also experiences a horizontal hydrodynamic drag force: [4,5]   = 4      (  From Equation S6, we estimated the droplet surface charge density   ≈ 0.18 nC.

Section S3. Lossless nature of droplet manipulation
We evaluated the lossless nature of our droplet manipulation technique by measuring the droplet volume, substrate mass and droplet contact angles (advancing and receding) as a function of droplet motion on the Cl-PDMS modified glass surfaces.In each experimental cycle, a 50 μl water droplet was placed on the surface, and it was manipulated back and forth across the slippery surface (transport distance ~10 cm per cycle) using a finger-based PTFE actuator.After every 100 cycles, the droplet image was captured and analyzed with ImageJ to determine the droplet volume; the substrate mass was measured; and separately the advancing and receding contact angles of a new water droplet were measured.If there was liquid loss, with increasing cycles, the droplet volume would have decreased, or the substrate mass would have increased, or the advancing and receding contact angles of water would have changed.
However, even after 1000 cycles (total transport distance ~ 100 m), there was no noticeable change in the droplet volume (Figure S1a) or the substrate mass (Figure S1b) or the advancing and receding contact angles of water (Figure S1c).These results confirm that there is no liquid loss associated with our droplet manipulations.

Figure S1 .
Figure S1.Lossless nature of droplet manipulation.a) Droplet volume, b) substrate mass, and c) water contact angles as a function of droplet motion cycles.
is viscosity of the oil,   is droplet radius,   is horizontal velocity of the droplet, is viscosity ratio, and   is viscosity of droplet.At equilibrium,   =   .So, from Equations S4 and S5, we get: