Liquid Substrates Neumanns Construction

liquid A wets liquid B and spreads totally on the free surface of B, as exemplified by PDMS on water. 

The advantage of liquid substrates is that they are smooth on an atomic scale and chemically homogeneous. Furthermore, one can readily measure the three interfacial tensions 7, 7b sjid, 7aBj Si luxury not available with a solid substrate. The wettability can easily be adjusted by selecting liquids that are more or less polar. 

The price to pay is that the substrate is not rigid and that it flows. When studying the dynamics of wetting, it becomes necessary to take into account the flows induced in the substrate as liquid A spreads or dewets (chapter 7). ° 

To avoid contamination effects, it is a good idea to select liquids with a low surface tension. A possible choice is the PDMS/fluoroalkylsiloxane pair (the latter is a fluorinated derivative of PDMS). These two liquids are immiscible. PDMS, which happens to be lighter, does not wet the fluorinated siloxane. 

If the goal is to work with systems with very low viscosity, we recommend either water, which does not spread on CCI4 or CHCI3, or alkanes on water. 

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Fig. 1.3. Equilibrium contact angles at a triple line at rest, (a) Solid substrate Young s construction. (b) Liquid substrate Neumann s triangle. If the line moves, the dynamical contact angle is 9d-If 6d < E, the line recedes in the B direction and if > fe, the line advances in the A direction.

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