Ordering of liquid molecules near a solid-liquid interface

In the recent trend of miniaturization of flow devices (MEMS, Lab-on-a-chip), the surface-to-volume ratio will increase. Therefore it will be of growing importance to accurately characterize the surface effects at the solid liquid interface.

It is a well known fact that liquid molecules tend to order near a solid-liquid interface [1]. This is schematically drawn in Figure 1.

With the new ultra-low noise AFM in our group these surface effects are going to be investigated.

Figure 1 Molecules tend to order near a solid-liquid interface.

When a liquid is confined between two solid surfaces, the ordering creates a special effect.

Because of the structuring, the molecular density between the two solid surfaces becomes an oscillatory shaped function of distance. This oscillating density causes oscillatory forces, which can be measured with the AFM cantilever.

The forces as a function of distance are measured using the Brownian motion of the molecules around the cantilever. The thermal noise will cause the cantilever to oscillate at a certain frequency, from which the forces are derived.

Figure 2 When the distance between the solid surfaces is decreased, the oscillatory molecular density shows up.

In this thesis the oscillatory forces are going to be examined for different solid-liquid interfaces. The central question will be: What is the effect of the properties of the solids, the liquid and so the interface on the oscillatory forces?

If you want to know more about this thesis assignment please contact Sissi de Beer (s.j.a.debeer@utwente.nl) or Frieder Mugele (f.mugele@utwente.nl).

[1] Jacob N. Israelachvili, Intermolecular and surface forces, 1991