Field responsive mechanical metamaterials.

Typically, mechanical metamaterial properties are programmed and set when the architecture is designed and constructed, and do not change in response to shifting environmental conditions or application requirements. We present a new class of architected materials called field responsive mechanical metamaterials (FRMMs) that exhibit dynamic control and on-the-fly tunability enabled by careful design and selection of both material composition and architecture. To demonstrate the FRMM concept, we print complex structures composed of polymeric tubes infilled with magnetorheological fluid suspensions. Modulating remotely applied magnetic fields results in rapid, reversible, and sizable changes of the effective stiffness of our metamaterial motifs

The role of the electric field in the departure of vapor bubbles in microgravity

We present the results of a study aimed at investigating the effects of electric fields on quasi-static bubble departure dynamics during pool boiling of perfluorohexane (FC-72) in microgravity conditions. Analysis was performed through an alternative formulation of the bubble momentum balance in which the contribution of non-uniform electric stress distributions at the bubble interface can be quantified through high-speed video measurements without having to numerically solve the laws of electrostatics. Data used in this study were obtained in the scope of the Multiscale Boiling Project, which included advanced single bubble growth experiments performed aboard the International Space Station. Our results confirm that bubble departure counterintuitively begins before the force resulting from electric stresses starts to pull the bubble up from the wall. When this occurs, it is shown that the shrinking process of the contact line accelerates, in agreement with known theoretical results. It is concluded that the electric force is essentially determined by the electric stress distribution at the bubble cap above the contact area. Furthermore, we show that the electric stress at the bubble interface is also responsible for the increase in bubble internal overpressure, which explains the early departure of the bubble while increasing the intensity of the electric field. The results of this study provide an important step in achieving a more comprehensive understanding of the bubble behavior at the heated surface in the presence of an electric field, which is essential to optimally design electrodes and two-phase heat transfer devices for future space applications

Mechanistic models for pool nucleate boiling heat transfer: input and validation

Correlations for nucleate boiling heat transfer should be improved, or in the long term possibly be replaced, by the development of mechanistic simulations that include the non-uniform spacing and variable characteristics of the nucleation sites and non-linear interactions between the sites. This paper discusses the interactions that should be included in simulations and some lessons from a first attempt to validate a particular simulation against experimental spatio-temporal data for wall temperature. Input data for nucleation site positions and characteristics are a particular problem and the prospects for obtaining this data from measurements that are independent of boiling are discussed

Effects of controlled nucleation on freeze-drying lactose and mannitol aqueous solutions

The lyophilization of lactose and mannitol aqueous solutions was investigated with an emphasis on analyzing the effects of controlled nucleation, temperature of nucleation, and pore size distribution on the freeze-drying process. The experimental procedure involved the depressurization technique of controlled nucleation, in-vial temperature measurements as well as measurements of the chamber pressure, which allowed the analysis of the product batch, loaded in the laboratory lyophilizator. The average pore enlargement was 93 and 58% with the incorporation of the controlled nucleation step in the lyophilization of 6 wt% lactose and 6 wt% mannitol solutions, respectively. Consequently, the primary drying times were lowered from 450 to 500 min in both cases. The pore sizes were determined to be as important as the solid material itself in the scope of the sublimation rates. Namely, the average equivalent diameter of the pores was larger in the dried mannitol cake compared to the lactose cake. However, despite the higher porosity of the dried mannitol cake, the end of the sublimation in the primary drying step was observed approximately 500 min earlier during the lyophilization of the lactose solution with the same initial concentration as the mannitol solution in a comparable freeze-drying protocol. In addition, an increase in mannitol concentration from 3 to 12 wt% was found to substantially extend the time required for the sublimation phase of the lyophilization

Comparison of a mechanistic model for nucleate boiling with experimental spatio-temporal data

Mechanistic numerical simulations have been developed for pool nucleate boiling involving large groups of nucleation sites that are non-uniformly distributed spatially and have different activation superheats. The simulations model the temperature field in the heated wall accurately and use approximations for events in the liquid–vapour space. This paper describes the first attempt to compare the numerical simulations with spatio-temporal experimental data at a similar level of detail. The experimental data were obtained during pool boiling of water at atmospheric pressure on a horizontal, electrically heated stainless steel plate 0.13 mm thick. They consist of wall temperature fields measured on the back of the plate by liquid crystal thermography at a sampling rate of 200 Hz over a period of 30 s. Methods of image analysis have been developed to deduce the time, position, nucleation superheat and size of the cooled area for every bubble nucleation event during this period. The paper discusses the methodology of using some of the experimental data as input for the simulations and the remainder for validation. Because of the high-dimensional dynamics and possibly chaotic nature of nucleate boiling, the validation must be based on statistical properties over a large area and a long period. This preliminary study is restricted to a single heat flux