## 5 Results
### Measurement by the Pipette with Gap
#### 5.1.2 Influence of the Pipette Tip Compression Force on the Measurements  
To investigate the effect of the friction and force of the contact region, the measurement and the numerical simulation were conducted under two different boundary conditions that exhibited different results. The calculated elastic modulus from the measurement by the both pipettes was affected by the force imposed on the silicone surface by the pipette tip wall. All of the previous studies take care of the contact force between pipette and sample cross-section. A transient mount was employed to control the distance of the pipette from the sample surface and a force measurement instrument to control the contact force.  
The following are the results of the 1:1:2 mixture which were measured by the smaller pipette in the two defined boundary conditions. Firstly, the pipette touches the sample surface by imposing various grades of force. The pressure and the displacement amplitude is measured when the pipette is connected to the surface applying 0 mN, 10 mN, 50 mN and 100 mN force on the surface. Secondly, the pipette does not touch the silicone surface and is placed at a very small distance. The results of the measurement are demonstrated in Figure 8. The chart (c) shows the calculated modulus of elasticity when the pipette is connected to the surface applying the four different contact forces and when the pipette placed on the surface with a small gap. The graph clearly shows the contact force even in few (mN) could significantly affect the results. In charts (a) and (b), the measured pressure and displacement was illustrated. The measured pressures of the 10 mN, 50 mN and 100 mN are identical in the connected pipette condition, which is reasonable while the generated displacement was different. The lower displacement was generated when the force is higher. This is because the silicone sample surface became more stiff by higher compression force. The displacement amplitude of the pipette with the gap is much lower according to the lower excitation pressure.  
The pressure of the 0 mN contact force has a considerable shift compared with the three other contact forces in the connected pipette condition. It may be attributed to inaccuracy in measuring the contact force or the distance between the pipette and the surface in the range of absolute zero. The measured pressure of the 0 mN contact force in the graph (a) is for a condition in which the pipette is not connected to the surface but placed in just a few micrometers distance. Therefore, the pressure of the 0 mN contact force is lower than the other contact forces because of a loss in the pressure amplitude caused by the gap. In the plot, (b) the displacement amplitudes of the 10 mN, 50 mN and 100 mN contact force are gradually increasing by growing the force. While the graph of the 0 mN contact force is not following the previous trend, this clearly shows that the pipette tip wall friction and force have a separate influence on the vibration behaviour of the surface enclosed area by the pipette, which will be discussed in Section 5.2.3.  
The results obtained by the bigger pipette is another example that the measured displacement can be affected significantly by the contact force. The pipette in Figure 1b was not fixed to a transient mount and just placed on the sample surface. The pipette's self-weight acts as a contact force in this condition. Therefore, the calculated modulus of elasticity in plot (c) of Figures 7 and 9 are much higher compared to the results of the smaller pipette depicted in Figure 5. As explained earlier in this section, the pipette weight compresses the surface, so the generated displacements decrease in the denominator of Equation (1) and therefore the calculated modulus of elasticity was boosted.