VLACHOVÁ, J. Studium parciálního skluzu v kapalinách s využitím vysokofrekvenčních střižných kmitů [online]. Brno: Vysoké učení technické v Brně. Fakulta chemická. 2014.

Posudek vedoucího

Johannsmann, Diethelm

The topic of Ms. Vlachová’s thesis is in the field of contact mechanics. She studied the stiffness of sphere-plate contacts, making use of high-frequency acoustic resonators. Ultrasound is a known tool to study the interfacial stiffness of multi-contact interfaces. Basing the analysis on resonators (rather than propagating waves) is simpler, but conceptually similar. Also, Ms. Vlachová studied individual contacts, rather than multicontact interfaces. The elastic and the dissipative component of the stiffness are extracted from the shifts in resonance frequency and resonance bandwidth of the piezoelectric plates. Crucial to the rather impressive success was an improvement to the setup, which Ms. Vlachová came up with in the beginning of her stay. She put down “tripods”, which are three glass spheres glued to a backing plate. The instrument measures the stiffness of contacts between these three spheres and the resonator surface. In a second step, additional weights are added to this assembly, thereby increasing the normal force. The problem in the past was that the tripod would slightly move on the resonator surface, when the weight was added. The contacts were broken and reestablished, which produced severe problems with the reproducibility. Ms. Vlachová installed a small additional holder for the tripod, which fixes it in place, while she adds the weight. This small device makes all the difference. We now have reproducible data. Ms. Vlachová studied silica/silica contacts and silica/PMMA contacts. For the silica/silica contacts, she studied two sphere sizes, which were 1 mm and 2 mm. All experiments where undertaken in both air dry and water. All experiments were repeated a total of 9 times. The results can be summarized as follows: - The elastic part of the contact stiffness at low amplitudes can be understood in the frame of the Mindlin model, combined with JKR theory. - The dissipative part of the contact stiffness is too large to be explained viscoelasticity of the materials involved. The dissipative component should be attributed to interfacial processes. - At elevated amplitudes, the frequency decreases while the bandwidth increases. Both changes occur in proportion to the amplitude of oscillation, as predicted by the Cattaneo-Mindlin model. The Savkoor model (a competing model) clearly is at variance with experimental data. - The “friction coefficient” is of the order of unity. - The friction coefficient enters the model twice and one can therefore derive two different values (termed mu_Deltaf and mu_DeltaGamma in the thesis) from the experimental data. These two values agree remarkably well. There are two exceptions (silica in air). One has to speculate about reasons. Capillarity might be of influence. In summary, the combination of the Cattaneo-Mindlin model with the JKR model describes all data surprisingly well. I would not have predicted such a good agreement. Ms. Vlachová has joined my group, having little background knowledge of contact mechanics or the QCM. She immersed herself into the topic with much diligence and undertook the experiments with much success. We are writing a publication. Part of the success are numerous repetitions of the experiments, thorough data analysis, which includes fitting and statistics, and a comparison to the literature. Ms. Vlachová went through all of these steps with much dilegence and competence.

Posudek oponenta

doc.Ing.Marian Lehocky, Ph.D.

Popište prosím podrobněji princip měření proliferace buněk pomocí QCM, jak je napsáno na straně 16 dole.

eVSKP id 72761