• The wetting properties of materials are largely dictated by the contact angle of the liquid which is in 

contact with the solid surface and the solid-liquid surface tension properties.

• There are plenty of industrial, biological and manufacturing applications which are primarily based 

on phenomena of rapid wetting.1 On the other hand, there are applications which requires poor 

wetting.2

• The scientific community is mostly focused towards the understanding of wetting behavior and 

designing materials or surfaces of desired engineering applications. 

• Due to lack of experimental methods for measuring the contact angle and various

experimental challenges related to small length scale, molecular simulation is one of the

viable option in predicting the preliminary insights of molecular level vision of liquids and

their interfacial properties. 

• it is obvious that the surface properties can modulate the wettability of the drop. The nature of the 

surface can be modified by external stimuli. It would be a great advantage where the surface 

properties can be controlled, resulting in a controlled wetting and dewetting characteristic of the 

nano droplet. 

• In this work, a wetting and dewetting characteristics of a nano water droplet in contact with a 

conductive tip and placed on a perpendicular insulator surface, which is contact with a planar 

electrode layer, has been studied using atomistic molecular simulation. 

Methodology

Figure # 3

Bibliography

• The present simulation system consists of a water nano drop in contact with a conductive tip and 

wetting a perpendicular planar electrode covered by an insulator layer as shown in Fig. 1. 

• At the bottom of the insulator layer two planar platinum electrodes are placed. The Pt layers are 

98 angstrom cubic simulation volume obtained by cutting a face centered cubic platinum crystal 

with lattice parameter 3.9 angstrom along the (1 1 1) crystallographic plane. 

• Below the insulator layer two platinum layers are placed. Basically this simulation system 

represents a typical setup of electrowetting-on-dielectric (EWOD) experiment. 

• In this way the platinum layer consists of 2500 atoms, while the insulator layer counts 1250 

atoms. The counter electrode is a cylindrical platinum rod attached with a hemispherical tip 

which is placed

perpendicular to the planar electrode. The tip consists of 546 Pt atoms. 

• At the beginning of the simulation the tip is inserted inside 2000 water molecules nano drop.

The molecules overlapping the Pt atoms has been removed and the final number of water

molecules inside the drop is 1900. Now a molecular dynamics is performed for the whole system 

by fixing the surface atoms.

1. M. Knoche, "Organosilicone Surfactant Performance in Agricultural Spray Application: A 

Review." Weed Res. 34, 221-239 (1994). 

2. P. Roach, N. J. Shirtcliffe, M. I. Newton, "Progess in Superhydrophobic Surface

Development." Soft Matter 4, 224-240 (2008). 

3. Paneru, M.; Priest, C.; Sedev, R.; J. Ralston, “Electrowetting of Aqueous Solutions of Ionic 

Liquid in Solid-Liquid-Liquid Systems.” J. Phys. Chem. C 114, 8383-8388 (2010). 

4. Jyoti Roy Choudhuri et. al “Dynamic Response in Nanoelectrowetting on a Dielectric” 10, 

8536-8544 (2016).

• Figure 2 shows the effect of surface on the drop.

• Figure 3 shows the variation of the height of the center of mass of the drop with time.

Molecular Kinetic Theory (MKT)

3

:

• It concerns thermally activated displacements of liquid molecules at the three phase contact 

line. Individual molecular jumps occur with a characteristic frequency k0and length λ. The 

relation between the dynamic contact angle and the perimeter velocity (v) is given by 

• kBis the Boltzmann constant and T temperature. For small arguments of sinh, 

• The molecular insight of the wetting of nano drop is presented using Molecular dynamics

simulation.

• The study demonstrates a reversible switching of the droplet’s contact angle regarding the

switchablity of the surface properties.

• The analysis of the droplet relaxation rates shows that the retraction is considerably faster than

the spreading.

• The phenomena is explained liquid/solid friction under the propagating droplet perimeter as

formulated in the molecular kinetic theory.

4

• The study also invites applications in the dynamic control of permeation in

hydrophobic nanopores for nanofluidic devices.

Introduction Results

Figure # 1

Figure # 2

Wetting

Drying

Drying

Wetting

Figure # 3 Figure # 4

Figure # 5 Figure # 6

Wetting Transition of a nano drop on switchable

hydrophobic-hydrophilic surfaces

Dr. Jyoti Roy Choudhuri

Department of Chemistry, BMSIT&M, Bengaluru

Conclusions

v / ms

-1