Advanced rheological and tribological methods for cosmetic products

Many different rheological procedures can used to tailor cosmetic products in a way that they meet the desired properties. Simple flow curve measurements reveal the viscosity at shear rates prevailing at processing conditions. Strain sweep give information on the strength of the rest structure and the amount of elasticity. The end of the linear visco-elastic range which is measured in a strain sweep is in a direct correlation to with the yield stress and flow point which need to be overcome in order that a gel like material starts to flow. A frequency sweep can help to gain information on the time dependence of relaxation processes. The higher frequency for example correspond to the application of a cream and lower shear moduli relate to a smoother, creamier less solid consumer experience. Difference at lower frequencies might indicate if a sample in a container flows back to a smooth surface after removing part of the sample or not and how fast such a process is. Indications on long time stability of cosmetic samples can be gained by a so-called temperature swing test in which the temperature is frequently varied between to values and the rheology response over time is followed. While rheology provides information on stability, flowability, temperature behavior, and sensory attributes like volume, smoothness, and firmness of a cosmetic product, tribology simulates more directly the application process of, for example, a cream on the skin. Since the forces and sliding speeds are rather small in cosmetic applications a tribology setup based on a rotational rheometer with its capability of applying small rotational speeds and normal loads as well as measuring small torques is well suited to perform friction measurements of cosmetic materials. A ball on three pins setup was used with a glass ball and pins made of PDMS to mimic the skin. Stribeck curves, i.e. friction factor measurements over an extended range of sliding speeds, have been performed on various cosmetic materials such as skin creams and sun creams. Differences between different samples are identified and discussed. An alternative approach is the use of a bottom layer of artificial skin and rounded pins which slide on top of the skin layer. Various rheological and tribological methods are presented and discussed for different cosmetic applications. Rheology and tribology are shown to be helpful tools to assist the formulation of cosmetic products

Measuring Supermassive Black Hole Properties via Gravitational Radiation from Eccentrically Orbiting Stellar Mass Black Hole Binaries

There may exist stellar-mass binary black holes (BBH) which merge while orbiting nearby a supermassive black hole (SMBH). In such a triple system, the SMBH will modulate the gravitational waveform of the BBH through orbital Doppler shift and de Sitter precession of the angular momentum. Future space-based GW observatories focused on the milli- and decihertz band will be uniquely poised to observe these waveform modulations, as the GW frequency from stellar-mass BBHs varies slowly in this band while modulation effects accumulate. In this work, we apply the Fisher information matrix formalism to estimate how well space-borne GW detectors can measure properties of BBH+SMBH hierarchical triples using the GW from orbiting BBH. We extend previous work by considering the more realistic case of an eccentric orbit around the SMBH, and notably include the effects of orbital pericenter precession. We find that for detector concepts such as LISA, B-DECIGO, and TianGO, we can extract the SMBH mass and semimajor axis of the orbit with a fractional uncertainty below the 0.1% level over a wide range of triple system parameters. Furthermore, we find that the effects of pericenter precession and orbital eccentricity significantly improve our ability to measure this system. We also find that while LISA could measure these systems, the decihertz detector concepts B-DECIGO and TianGO would enable better sensitivity to the triple's parameters.Comment: 12 pages (main text excluding references and appendices), 11 figures, submitted to PR

Extended stribeck curves for food samples

Stribeck curves have long been used for understanding the lubricating behaviour of oils and greases, and in the recent years for applications ranging from ball point inks to synovial fluids. In the current work, an attempt is made to show as to what one can read from Stribeck curves of food samples such as chocolate spread, sauce etc. Additionally, the effect of saliva – human and artificial – on the frictional behaviour has also been studied. The tests were carried out on an MCR Tribometer with a ball-on-three-pin configuration. Polydimethylsiloxane and glass were used to simulate soft contact conditions that exist in the human mouth. Results from the tribological tests are plotted in the form of extended Stribeck curves, wherein, the friction coefficient is plotted as a function of rotational speed. Since the tribometer is capable of speeds as low as a few nanometres per second, it is also possible to observe the build-up of static friction and its transition into the kinetic regime of friction. Results indicate that certain aspects of the Stribeck curve can offer an insight into the correlation between the frictional behaviour of food to their sensory feel