Movement of uncharged
particles in the electrostatic
field
Electrophoretic mobility of oil
• Oil droplets in pure water show different mobilities depending on the pH
• Attributed to the adsorption of 𝑂𝐻−
or 𝐻3 𝑂+
surface charge
• Point of zero charge is at pH=6, but at that point there is negative
mobility
𝐸
+ If
pH is neutral or high
If pH is low
+ ++
+
+
+
++
+
+ +
+
+
+
+
+
+
+
+ +
+
Electrophoretic mobility of oil
• Heptane droplet/slab in water
• Hydropobic wall in water
• Complete absence of ions
• Electric field
Electrophoretic mobility of oil
• Oil droplets migrate in the negative field direction
• Slab moves tangential to the interface
• Electrostatic potential of oil phase? positive
𝐸
+ -
𝐸
Electrophoretic mobility of oil
• Mobility contributions:
electrophoretical force + frictional force
• Interface roughness is required for electrophoretic
force
Electrophoretic mobility of oil - Conclusions
• Mobility in the absence of ions should be zero, but simulations show
negative mobility
• Electrophoresis does not only reflect charge or electrophoretic
potential of the droplet
• Mobility is attributed to partial charges of the atoms
• If true, electrophoretic experiments could me misleading
Electrophoretic mobility of oil - Explanation
• Mobility only appears when using simple truncation method for LJ
forces.
Water flow on carbon nanotubes
• Water oriented along one direction by aplying an external
field
• Flux in the direction of the aligned water dipoles
Water flow on carbon nanotubes
• When using shift approach
instead of truncated forces for
LJ, mobilities dissapear.
• In LAMMPS, flux dissapears for
smaller rc