Article Jumping Sundogs, Cat’s Eye and Ferrofluids Alberto Tufaile 1,* , Michael Snyder 2 , Timm A. Vanderelli 3 and Adriana Pedrosa Biscaia Tufaile 1 1 Soft Matter Lab, School of Arts, Sciences and Humanities, University of São Paulo, São Paulo 03828-000, Brazil; [email protected] 2 Technical Space Science Center, 235 Martindale Drive, Morehead State University, Morehead, KY 40531, USA; [email protected] 3 Ferrocell USA, 739 Route 259, Ligonier, PA 15658, USA; [email protected] * Correspondence: [email protected] Received: 20 April 2020; Accepted: 2 July 2020; Published: 8 July 2020 Abstract: We have explored some features of the complex fluids present in Earth’s atmosphere by the observation of some optical phenomena and compared them to the optical phenomena observed in gems and magnetic materials. The main feature of a complex fluid is that it contains polyatomic structures such as polymer molecules or colloidal grains. This paper includes some setups using tabletop experiments, which are intended to show concretely the principles discussed, giving a sense of how well the idealizations treated apply to the atmospheric systems. We have explored sundogs, light pillars, and the halo formation, which involve the existence of a certain structure in the atmospheric medium, resembling the structures observed in some types of gems and ferrofluids. Keywords: sundogs; crown flash; ferrofluids; complex fluids; polarization; cat’s eye effect 1. Introduction The observation of atmospheric optical phenomena has caused a sense of wonder in many people throughout history, such as the sundogs, crown flashes, parhelic circles, light pillars, or the miracle of the sun. All of these phenomena are rare, and we can hardly predict their occurrence. However, the technological development that allows for the availability of cameras and camcorders, associated with the dissemination of the recordings of these phenomena on social networks, allows us to try to give a plausible explanation for these rare events. For example, the crown flash in Figure1 in meteorology is an atmospheric optical phenomenon that consists of a luminous column that moves near dense clouds during the day [1,2]. These images of this atypical event were obtained from a YouTube user’s video (QuadM13) observed at Greenwood, Indiana, USA [3], on the same day that another YouTube user’s video from Shelbyville, Indiana, USA (Chris Mercer) captured another video of this jumping sundog on June 12, 2015 at 6:39 pm CDT [4], with a distance of almost 30 km between the two. Clouds can be an example of a complex fluid [5] that changes constantly due to atmospheric conditions, in which a gas phase contains structures such as tiny ice crystals in motion. The existence of such structure for this kind of system distinguishes a cloud from a simple fluid, with properties which allow self-organization in the presence of an electric field or air flow. In the case of a crown flash or jumping sundogs, one possible explanation for this phenomenon to occur is that sun light is interacting through ice crystals above the clouds in the presence of an oscillating electrical field, while in the case of light pillars, the spatial distribution of particles inside this complex fluid creates the observable light pattern. Condens. Matter 2020, 5, 45; doi:10.3390/condmat5030045 www.mdpi.com/journal/condensedmatter Condens. Matter 2020, 5, 45 2 of 12 Condens. Matter 2020, 5, x FOR PEER REVIEW 3 of 12 Figure 1.1.The The solar solar halo halo is shown is shown in (a) andin (a the) and laser the halo laser obtained halo using obtained ferrofluid using subjected ferroflu toid a subjectedmagnetic field to a is magnetic shown in (fieldb). The is shown jumping in sundog (b). The phenomenon jumping sundog is shown phenomenon in this sequence is ofshown three picturesin this sequence in (c) in states of three 1, 2 and pictures 3, and thein ( dic)ff erentin states values 1, of2 and the magnetic 3, and t fieldhe different can create values a sequence of the of magneticlight patterns, field as can in (d create). For di aff sequenceerent configurations of light patterns of the magnetic, as in (d fields). For 1, different 2 and 3 of configurations these patterns in (ofd), the we canmagnetic emulate field somes 1, patterns 2 and observed 3 of the inse thepatterns crown flash.in (d), we can emulate some patterns observed in the crown flash. 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An important result that can be obtained with all of this is the existence of conditions that allow the emergence of a self-organization that manifests itself through the observation of an optical phenomenon on an atmospheric scale. Condens. Matter 2020, 5, 45 3 of 12 existenceCondens. Matter of conditions 2020, 5, x FOR that PEER allow REVIEW the emergence of a self-organization that manifests itself through3 of the 12 observation of an optical phenomenon on an atmospheric scale. 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