111 research outputs found
Vortices in vibrated granular rods
We report the experimental observation of novel vortex patterns in vertically
vibrated granular rods. Above a critical packing fraction, moving ordered
domains of nearly vertical rods spontaneously form and coexist with horizontal
rods. The domains of vertical rods coarsen in time to form large vortices. We
investigate the conditions under which the vortices occur by varying the number
of rods, vibration amplitude and frequency. The size of the vortices increases
with the number of rods. We characterize the growth of the ordered domains by
measuring the area fraction of the ordered regions as a function of time. A
{\em void filling} model is presented to describe the nucleation and growth of
the vertical domains. We track the ends of the vertical rods and obtain the
velocity fields of the vortices. The rotation speed of the rods is observed to
depend on the vibration velocity of the container and on the packing. To
investigate the impact of the direction of driving on the observed phenomena,
we performed experiments with the container vibrated horizontally. Although
vertical domains form, vortices are not observed. We therefore argue that the
motion is generated due to the interaction of the inclination of the rods with
the bottom of a vertically vibrated container. We also perform simple
experiments with a single row of rods in an annulus. These experiments directly
demonstrate that the rod motion is generated when the rods are inclined from
the vertical, and is always in the direction of the inclination.Comment: 6 pages, 10 figure, 2 movies at http://physics.clarku.edu/vortex uses
revtex
A microscopic 2D lattice model of dimer granular compaction with friction
We study by Monte Carlo simulation the compaction dynamics of hard dimers in
2D under the action of gravity, subjected to vertical and horizontal shaking,
considering also the case in which a friction force acts for horizontal
displacements of the dimers. These forces are modeled by introducing effective
probabilities for all kinds of moves of the particles. We analyze the dynamics
for different values of the time during which the shaking is applied to
the system and for different intensities of the forces. It turns out that the
density evolution in time follows a stretched exponential behavior if is
not very large, while a power law tail develops for larger values of .
Moreover, in the absence of friction, a critical value exists which
signals the crossover between two different regimes: for the
asymptotic density scales with a power law of , while for
it reaches logarithmically a maximal saturation value. Such behavior smears out
when a finite friction force is present. In this situation the dynamics is
slower and lower asymptotic densities are attained. In particular, for
significant friction forces, the final density decreases linearly with the
friction coefficient. We also compare the frictionless single tap dynamics to
the sequential tapping dynamics, observing in the latter case an inverse
logarithmic behavior of the density evolution, as found in the experiments.Comment: 10 pages, 15 figures, to be published in Phys. Rev.
Phenomenological glass model for vibratory granular compaction
A model for weakly excited granular media is derived by combining the free
volume argument of Nowak et al. [Phys. Rev. E 57, 1971 (1998)] and the
phenomenological model for supercooled liquids of Adam and Gibbs [J. Chem.
Phys. 43, 139 (1965)]. This is made possible by relating the granular
excitation parameter \Gamma, defined as the peak acceleration of the driving
pulse scaled by gravity, to a temperature-like parameter \eta(\Gamma). The
resulting master equation is formally identical to that of Bouchaud's trap
model for glasses [J. Phys. I 2, 1705 (1992)]. Analytic and simulation results
are shown to compare favourably with a range of known experimental behaviour.
This includes the logarithmic densification and power spectrum of fluctuations
under constant \eta, the annealing curve when \eta is varied cyclically in
time, and memory effects observed for a discontinuous shift in \eta. Finally,
we discuss the physical interpretation of the model parameters and suggest
further experiments for this class of systems.Comment: 2 references added; some figure labels tweaked. To appear in PR
Women and postfertilization effects of birth control: consistency of beliefs, intentions and reported use
BACKGROUND: This study assesses the consistency of responses among women regarding their beliefs about the mechanisms of actions of birth control methods, beliefs about when human life begins, the intention to use or not use birth control methods that they believe may act after fertilization or implantation, and their reported use of specific methods. METHODS: A questionnaire was administered in family practice and obstetrics and gynecology clinics in Salt Lake City, Utah, and Tulsa, Oklahoma. Participants included women ages 18–50 presenting for any reason and women under age 18 presenting for family planning or pregnancy care. Analyses were based on key questions addressing beliefs about whether specific birth control methods may act after fertilization, beliefs about when human life begins, intention to use a method that may act after fertilization, and reported use of specific methods. The questionnaire contained no information about the mechanism of action of any method of birth control. Responses were considered inconsistent if actual use contradicted intentions, if one intention contradicted another, or if intentions contradicted beliefs. RESULTS: Of all respondents, 38% gave consistent responses about intention to not use or to stop use of any birth control method that acted after fertilization, while 4% gave inconsistent responses. The corresponding percentages for birth control methods that work after implantation were 64% consistent and 2% inconsistent. Of all respondents, 34% reported they believed that life begins at fertilization and would not use any birth control method that acts after fertilization (a consistent response), while 3% reported they believed that life begins at fertilization but would use a birth control method that acts after fertilization (inconsistent). For specific methods of birth control, less than 1% of women gave inconsistent responses. A majority of women (68% or greater) responded accurately about the mechanism of action of condoms, abstinence, sterilization, and abortion, but a substantial percentage of women (between 19% and 57%) were uncertain about the mechanisms of action of oral contraceptives, intrauterine devices (IUDs), Depo-Provera, or natural family planning. CONCLUSION: Women who believe that life begins at fertilization may not intend to use a birth control method that could have postfertilization effects. More research is needed to understand the relative importance of postfertilization effects for women in other populations, and in relation to other properties of and priorities for birth control methods. However, many women were uncertain about the mechanisms of action of specific methods. To respect the principles of informed consent, some women may need more education about what is known and not known about the mechanisms of action of birth control methods
Predicting youth participation in urban agriculture in Malaysia: insights from the theory of planned behavior and the functional approach to volunteer motivation
This study examines factors associated with the decision of Malaysian youth to participate in a voluntary urban agriculture program. Urban agriculture has generated significant interest in developing countries to address concerns over food security, growing urbanization and employment. While an abundance of data shows attracting the participation of young people in traditional agriculture has become a challenge for many countries, few empirical studies have been conducted on youth motivation to participate in urban agriculture programs, particularly in non-Western settings. Drawing on the theories of planned behavior and the functional approach to volunteer motivation, we surveyed 890 students from a public university in Malaysia about their intention to join a new urban agriculture program. Hierarchical regression findings indicated that the strongest predictor of participation was students’ attitude toward urban agriculture, followed by subjective norms, career motives and perceived barriers to participation. The findings from this study may provide useful information to the university program planners in Malaysia in identifying mechanisms for future students’ involvement in the program
Mental Health Service Needs of a Latino Population: A Community-Based Participatory Research Project
Studying Adolescent Male Sexuality: Where Are We?
This article critically reviews the literature about adolescent males’ sexuality in order to describe the state of the science and to identify promising concepts and research designs that have the potential to guide the next generation of research. A critique was conducted on 94 peer-reviewed studies of sexual behaviors that included a sample of adolescent males; 11 scholarly texts and 2 dissertations. Most studies lacked a theoretical foundation and had cross-sectional designs. For those studies with a theoretical base, 3 perspectives were most often used to guide research: cognitive, biological, or social-environmental. Studies frequently relied on older adolescents or young adult males to report behaviors during early adolescence. Male-only samples were infrequent. Findings include (a) the measurement of sexual activity is frequently limited to coitus and does not explore other forms of “sex”; (b) cognitive factors have been limited to knowledge, attitudes, and intent; (c) little is known about younger males based on their own self-reports; (d) little is known about the normative sexuality development of gay adolescent males; and (e) longitudinal studies did not take into account the complexities of biological, social, and emotional development in interaction with other influences. Research on adolescent sexuality generally is about sexual activity, with little research that includes cognitive competency or young males’ sense of self as a sexual being. The purpose of the paper is to critically review the literature about male sexuality in order to describe the state of the science as well as to identify potential directions to guide the next generation of adolescent male sexual being research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45299/1/10964_2005_Article_5762.pd
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