arXiv subjects

Francisco Vera

Publications and source records attributed to Francisco Vera.

7 recordsLinked to original sources

arXiv2020

Robust triboelectric charging of identical balloons of different radii

Electrification by rubbing different materials is a well known phenomenon with a history that begun more than five centuries BC. However, simple experiments can lead to contradictory or inconsistent results and the history of this phenomena is plagued with non-intuitive results. For example, triboelectric charging by rubbing identical materials is possible. In this work we want to highlight some historical aspects of triboelectricity that could enrich the discussion of electrostatics in an undergraduate physics course. We will focus on the effect of strain on the triboelectric properties of a sample, which we think is not well known to physics teachers. We will show that it is possible to obtain robust polarities by rubbing identical rubber balloons of different radii and we will also show that this charging method can be very useful in introductory physics courses.

physics.ed-ph↗

arXiv2005

The return branch of viscous fingers

We report a simple experiment of two-dimensional pattern formation in a circular Hele-Shaw cell, showing the appearance of a return branch that is equivalent to the upward-connecting leader of lightning. Injecting water from the center into a foam filled cell, we obtained patterns similar to dendrites of two-dimensional dielectric breakdown experiments. When we repeat this experiment allowing the presence of water in the outer (low pressure) region, dendrites grow initially as in a normal experiment, but when a branch is near the outer boundary, the low pressure water begins to penetrate the foam against the pressure field, forming several return branches.

physics.flu-dyn↗

arXiv2005

Complex patterns and tip effect evolution

We studied the formation of complex patterns using a variational principle and a standard energy functional. These patterns evolve by letting the system to search for the optimal configuration of a high conductivity channel, that in one dimension is equivalent to tip effect evolution (evolution towards regions of high electric field).

physics.gen-ph↗

arXiv2002

Understanding the complex patterns of snow crystals

We will show that the complex shapes of snow crystals can be explained from a simple basic mechanism that is also responsible for the appearance of many others structures in nature. We expect that this new physical mechanism, that follows from minimizing the total stored energy, will permit to explain most of the features of snow crystal growth.

nlin.PS↗

arXiv2002

The tortuous behavior of lightning

The complex branched structure of lightning induce scientists to think that dielectric breakdown is a very complicated phenomena, we will show that this is not true and that simulating the structure of lightning is an easy task, but depends strongly on boundary conditions. In this work we will introduce a new way of understanding the origin of this tortuous path that relies on minimizing the total energy stored in the system.

nlin.PS↗

arXiv2000

Direct and indirect exciton mixing in a slightly asymmetric double quantum well

We studied, theoretically, the optical absorption spectra for a slightly asymmetric double quantum well (DQW), in the presence of electric and magnetic fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33} )Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks for the indirect exciton (IX) and left direct exciton (DX) as a function of the external electric field. We show that the presence of a peak near the (DX) peak, attributed to an impurity bound left (DX) in the experimental results, could be a consequence of the non-trivial mixing between excitonic states.

cond-mat.mes-hall↗

arXiv1997

Behavior of excitonic levels in symmetric and asymmetric double quantum wells in a magnetic field

We studied theoretically the excitonic energy levels and the optical absorption spectra for double quantum wells, both symmetric and asymmetric, in the presece of an homogeneous magnetic field. Within the effective mass approach, we expanded the excitonic wave function, in an orthogonal basis formed by products of electron and hole wave functions in the growth direction z, and one particle solutions of the magnetic Hamiltonian in the x-y plane. We applied our method to the case of Al_xGa_{1-x}As, for which we showed how the exciton wave function vary, and how the basis functions are mixed in a non trivial way by the effect of the Coulomb potential. By taking into account all the mixing between the elements in our base, we get anti-crossing between excited excitonic states not reported previously.

cond-mat↗