“Everything flows?”: elastic effects on startup flows of yield-stress fluids

It is now 30 years since Barnes and Walters published a provocative paper in which they asserted that the yield stress is an experimental artifact. We now know that the situation is far more complicated than understood at the time, and that the mechanics of the solid material prior to yielding must be considered carefully. In this paper, we examine the response of a well-studied “simple” yield-stress material, namely a Carbopol gel that exhibits no thixotropy, and demonstrate the significance of the pre-yielding behavior through a number of elementary measurements

Measuring program fidelity in case management for high risk families. Validation of the Functional Family Parole-Global Rating Measure

Abstract Summary: Program fidelity instruments are a key ingredient for clinical supervision and implementation as well as effectiveness studies. This study examines the factor structure of the Functional Family Parole services Global Rating Measure (FFP-GRM); the program fidelity instrument of Functional Family Parole services for case management in youth parole, child protection and child welfare services. Between October 2012 and February 2015, program fidelity was measured with the FFP-GRM by Functional Family Parole supervisors. Confirmatory factor analysis was performed on 380 cases and internal consistency reliability coefficients were calculated. Findings: Confirmatory factor analyses showed that the 33-item and four-factor model of the FFP-GRM achieved a good fit to the data. Internal validity testing results showed that subscale Cronbach’s a ranged between .82 and .90. Applications: Findings affirm a good fit to the data and a good-to-excellent internal consistency of the FFP-GRM, which is considered sufficient to justify its use. The results are discussed with regard to the use of fidelity instruments for both clinical and research purposes

The rheology of jamming

Traffic jams are a common phenomenon on highways; when there are too many cars on the road the traffic gets stuck. A similar jamming phenomenon also occurs in yield-stress fluids that consist of a dispersion of a material in a liquid, such as suspensions of particles or polymers, foams or emulsions. At high concentrations, these materials behave like solids (like in traffic jams there is no flow), and they only start to flow when enough stress is applied. For example, toothpaste behaves like a solid at rest but it starts to flow when you squeeze it out of the tube. This threshold stress that is needed to initiate flow is called the yield stress, hence the name yield-stress material. It is important to understand these kinds of properties and the flow behavior (rheology) of these materials since they are widely applied in the cosmetic, oil and food industry. We seek to understand the transition from mechanically solid-like to fluid-like behavior on a fundamental level. The associated jamming transition between solid and liquid "states" has similarities to classical phase transitions like those between solid, liquid, gas and plasma phases. However it is not completely clear how general the jamming description is, and to what extent the mechanical behavior of jammed materials can be fully described by considering the jamming transition to be analogous to a classical phase transition. In this thesis we study the flow behavior (the rheology) of a variety of yield-stress materials in the aim of describing, understanding and predicting the rheology of jamming