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How water forms ice The various anomalous properties of water have puzzled scientists for decades, and many hypotheses have been put forward to explain their origin. One mystery is the question of what determines the lowest temperature to which water can be cooled before it freezes to ice. Rapid crystallization at low temperatures hampers experimental studies, and simulations are usually prohibitively costly in terms of computer time. Using a simple water model that allows demanding calculations, Emily Moore and Valeria Molinero now show that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water controls the rate and mechanism of ice formation. The structural change also results in a peak in the rate of crystallization at 225 K; below this temperature, ice nuclei form faster than liquid water can equilibrate. This finding explains the observed thermodynamic anomalies, and why homogeneous ice nucleation rates depend on the thermodynamics of water. One of water’s unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature, T H  ≈ 232 K, yet the mechanism of ice crystallization—including the size and structure of critical nuclei—has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase on moving into the supercooled region, according to power laws that would diverge (that is, approach infinity) at ∼225 K (refs 1 , 2 ), so there may be a link between water’s thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below T H . And although atomistic studies have captured water crystallization 3 , high computational costs have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model 4 in the supercooled regime around T H which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanisms of ice formation. The results of the simulations and classical nucleation theory using experimental data suggest that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below T H and well above its glass transition temperature, 136 K. By establishing a relationship between the structural transformation in liquid water and its anomalous thermodynamics and crystallization rate, our findings also provide mechanistic insight into the observed 5 dependence of homogeneous ice nucleation rates on the thermodynamics of water.

Many mammals have been reported to significantly bias their offspring sex ratios, but these deviations from equality have been difficult to understand and are often inconsistent even within the same species. Evolutionary theory predicts a number of scenarios where females should bias their investment to one sex over the other; when fitness returns are sex specific, selection favors the facultative adjustment of offspring sex ratios to the sex with the highest inclusive fitness return. Interpreting sex allocation in mammals within the framework of classic sex allocation theory is challenging given the complex life histories and social interactions of species, hence it is likely that there are multiple mechanisms and selective forces dependent upon maternal and environmental drivers. Here, we show the previously demonstrated condition-dependent sex allocation in bridled nailtail wallabies (Onychogalea fraenata), where females in better condition were more likely to have sons, was reversed under high population densities and elevated maternal stress. In this solitary species, an elevated glucocorticoid stress response under unnaturally high densities likely influences adaptive sex allocation to the dispersive sex, consistent with the local resource competition theory. Managing factors such as population density, maternal stress, and food resources may be used to adjust offspring sex ratios in this endangered species.

As the use of computer-based science simulations in educational environments grows, so too does the need for research on productive use of simulations. This paper presents ways to create effective assignments that accompany an interactive simulation in a variety of educational environments. A framework that supports the creation of assignments with simulations in any environment is provided, as well as a set of heuristics, or strategies, for how to create assignments based on the particular environment and simulation being used. Case studies of use in college and middle school science classes are provided to illustrate implementation of the heuristics, and how the heuristics can be used to promote productive use of a simulation.

One of water's unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature TH{\\approx}232 K, yet the mechanism of ice crystallization - including the size and structure of critical nuclei - has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase upon moving into the supercooled region according to a power law that would diverge at Ts{\\approx}225 K,(1,2) so there may be a link between water's thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below TH. And while atomistic studies have captured water crystallization(3), the computational costs involved have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model(4) in the supercooled regime around TH, which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanism of ice formation. The simulations reveal that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below TH and well above its glass transition temperature Tg{\\approx}136 K. By providing a relationship between the structural transformation in liquid water, its anomalous thermodynamics and its crystallization rate, this work provides a microscopic foundation to the experimental finding that the thermodynamics of water determines the rates of homogeneous nucleation of ice.(5)

We build on theoretical foundations of tool-mediated learning, tool design, and human computer interaction to develop a framework for implicit scaffolding in learning environments. Implicit scaffolding employs affordances, constraints, cueing, and feedback in order to frame and scaffold student exploration without explicit guidance, and it is a particularly useful design framework for interactive simulations in science and mathematics. A key purpose of implicit scaffolding is to support a range of educational goals including affect, process, and content. In particular, the use of implicit scaffolding creates learning environments that are productive for content learning and are able to simultaneously support the affective goals of student agency and ownership over the learning process - goals that may not be addressed in more directed learning environments. We describe how the framework is applied in the context of the Energy Skate Park: Basics simulation, a simulation aimed at middle school student learning of energy concepts. Interview data provides an exemplar of the process by which implicit scaffolding can support productive student exploration with a computer simulation. While we present this framework for implicit scaffolding in the context of computer simulations, the framework can be extended and adapted to apply to a range of tool-mediated learning environments.

Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, its anomalies and phase transitions with comparable or better accuracy than the most popular atomistic models of water, at less than 1% of the computational cost. We conclude that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water. The speedup in computing time provided by mW makes it particularly useful for the study of slow processes in deeply supercooled water, the mechanism of ice nucleation, wetting-drying transitions, and as a realistic water model for coarse-grained simulations of biomolecules and complex materials.

I failed my A levels first time because I was a bit `psychedelic'. I got chucked out of the sixth form -which I hated anyway as it was an all boys grammar school - and took a year out to retake my A levels at college. I did English, sociology and politics. I applied to Essex University because you didn't need high grades to get in - I thought I'd do badly in my A levels again, as the first time round I got three Fs. But I ended up getting two As and a B!

My time at primary was the happiest part of my school life. I wasn't very good at being at school. I like to think that it was because I'm independent, creative and free-spirited. That's the positive side. The negative is that I was a pain in the arse! Booth wasn't a wholly pleasant experience. I loathed the teacher I had in my last-but-one year. She used the `slipper' on us and frightened me so much that I couldn't sleep at nights and used to wet the bed. I ran away from school all the time, because I was terrified of her. So it was a great relief to get into the classroom of my favourite teacher, Mr (Paul) Bilsborough, in my last year at primary! He was the first male teacher I'd had -in fact, apart from the headteacher, he was the only one in the school. His approach was quite butch and he seemed really solid, very clear and uncomplicated. He was quite strict and pushed us academically. He was also really into sport, especially football, which I loathed. I'm sure having such a `nancy-boy' in the class wasn't what he was after, and in fact I think he thought I was a ghastly child - he's not one of those sentimental teachers.

It was clear from the first that Marion Studholme was really special. She had not long retired from singing as a high soprano and arrived at the Royal College of Music with all sorts of techniques to pass on and strong ideas. Singing is a very physical thing and you need to have a rapport with your teacher. I felt Marion and I were on the same wavelength. She spoke very much in imagery - as a singer you can't see your instrument so you have to talk about what it feels like. If your teacher is not on your wavelength then it can be difficult to understand what they're on about. You need someone who understands the way you tackle a role and a character. With opera singing, or any singing, it's not just a matter of singing the notes, you're interpreting them and you're putting through that character what you think . . . how you think that character would behave. When you sing, what you're thinking comes through in the sound. Certain ways of thinking help you get the best sound. Nobody had spoken to me about that before Marion. I think of her as the beginning of it all.

Mr (Basil) Edwards was legendary - a larger than life character. He taught me English in my first year and in the sixth form, by which time I had become that weird teenage mixture of very bolshie and full of myself yet unconfident at the same time. He must have heard the same stuff coming out of teenagers' mouths year after year. But he always made us feel that what we were saying was interesting and worthwhile.He kept alive his pleasure in young people and their ideas and aspirations. He had tremendous energy and enthusiasm for everything - from railways to coins - and that has remained undiminished. English with Mr Edwards was a broad arts education, really. We never knew what we'd be doing - we could be looking at sculpture or a film or a book. And the emphasis was always on debate and analysis. He encouraged vigorous free thinkers.