Drop impact dynamics on the hydrophobic leaf surface of an aquatic plant: a case study of Pistia stratiotes

Pistia stratiotes is an aquatic plant with a complex structure that allows it to stay afloat. It grows quickly, and in large numbers becomes an undesirable plant as an invasive species. Describing the dynamics of a water drop splash on P. stratiotes leaves can contribute to increasing knowledge of its behavior and finding alternative methods for eradicating it or using it for the benefit of the environment. The non-wettable surface of P. stratiotes presents a complex structure-simple uniseriate trichomes and also ridges and veins. We analyzed the drop impact on a leaf placed on the water surface and recorded it by high-speed cameras. Based on the recordings, quantitative and qualitative analyses were performed. After impacting the leaf, the water drop spread until it reached its maximum surface area accompanied by the ejection of early droplets in the initial stage. Thereafter, three scenarios of water behavior were observed: (i) drop receding and stabilization; (ii) drop receding and ejection of late droplets formed in the later stage as an effect of elastic deformation of the leaf; and (iii) drop breaking apart and ejection of late droplets. The results indicated that the increasing kinetic energy of the impacting drops expressed by the Weber number and the complex leaf surface have an effect on the course of the splash. The simple uniseriate trichomes of the P. stratiotes leaf and the high energy of the falling drops were responsible for the formation and characteristics of the early droplets. The presence of ridges and veins and the leaf's mechanical response had an impact on the occurrence of late droplets.

Splash erosion and surface deformation following a drop impact on the soil with different hydrophobicity levels and moisture content

The splash phenomenon and the scale of the surface deformation of post-fire soils in the variants of various hydrophobicity and moisture content were studied. Splash erosion is the result of the impact of a single water drop and was analysed using high-speed cameras, while the surface deformation was parameterized using a structured light scanner. The extremely water-repellent variant (dry_V) showed distinct differences, expressed primarily in the number of ejected particles, which was 2.5 times higher than in the four soils with lower levels of hydrophobicity. It was also observed that as a result of the drop impact onto an extremely hydrophobic soil surface, a form known as liquid marble was created inside the crater. Soil moisture content determined the manner, scale and dynamics of the splash erosion. In the case of wet soils, the phenomenon proceeded up to five times faster, and as a result of the drop impact, a large number of fine particles were ejected, which reached nearly twice the velocities and three times the displacement distances compared to the dry soil group. However, the particles and/or aggregate splashed on the dry samples were larger, which also translated into the formation of craters up to twice as extensive as those in the wet soils.

Soil Deformation after Water Drop Impact-A Review of the Measurement Methods

Water erosion is an unfavorable phenomenon causing soil degradation. One of the factors causing water erosion is heavy or prolonged rainfall, the first effect of which is the deformation of the soil surface and the formation of microcraters. This paper presents an overview of research methods allowing the study of microcraters as well as the process of their formation. A tabular summary of work on the measurements of various quantities describing the craters is presented. The said quantities are divided into three groups: (i) static quantities, (ii) dynamic quantities, and (iii) dimensionless parameters. The most important measurement methods used to study crater properties, such as (i) basic manual measurement methods, (ii) photography, (iii) high-speed imaging, (iv) profilometers, (v) 3D surface modelling, and (vi) computed tomography (CT) and its possibilities and limitations are discussed. The main challenges and prospects of research on soil surface deformation are also presented.

Behavioural responses of Konik Polski horses to natural, familiar sound of thunderstorm, and unfamiliar similar-sounding sounds of volcanic eruption and sea storms

Background

It is not clear, if modern Konik Polski horses have retained the ability to identify sounds in terms of danger. The aim of the study was to identify differences in their behaviour in response to the reproduction of volcanic eruption and sea storm sounds, assumed to be unfamiliar to these horses, as compared to their response to a thunderclap sound, considered by the horses as potentially dangerous. The study included 13 adult mares of the Konik Polski breed, kept under a free-range system. Their behavioural responses to the reproduction of the three natural sounds with an intensity of over 50 dB, were registered. They were analysed distance of each horse to the central point of the pasture and to the exit from the enclosure, and time and/or frequencies of elements of behaviour categorised as: increased anxiety (walking, trotting and cantering), vigilance (snoring, vocalisation, high head position, high tail position, sticking together), foraging (time of grazing), comfort (playing, examining the surroundings, sniffing), maintenance of hygiene (rubbing against objects, auto- or allogrooming, rolling) and resting. The obtained data were analysed by the Dwass, Steel and Critchlow-Fligner method using the SAS program.

Results

Most of analysed elements increased in response to reproduced sounds and decreased after sounds were stop playing (p < 0.05), however, they were no significant differences in general response to each studied sound.

Conclusions

The responses of horses to similar sounds of both known and unknown origins, i.e. the sound of a thunderstorm, sea storm and volcanic eruption, are similar. The sound stimuli applied were not too stressful for the horses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03314-4.

Experimental investigations of crater formation as a result of high-velocity impacts on sand bed

The formation of craters is an important issue in the investigations of the surface of the earth and other planets. The aim of the study was to check whether the different textures of sand beds affect the size and dynamics of the formation of craters and ejection curtain after high-velocity impacts. The experiments were conducted using an aluminium impactor at two impact speeds (~700 and ~1300 m∙s^-1) and a sand bed composed of either a broad range of sizes (<2.0 mm) or any of the three fractions obtained from it (<0.5, 0.5–1, 1–2 mm). The diameters, depths, wall slope, and rim heights of the resulting craters were measured. The ejecta curtain was characterized by the inclination angle of walls, base diameter, and expansion velocity. The mass of the transferred material and the depth of the impactor penetration were also determined. Additionally, the results were used to calculate dimensionless parameters commonly considered in crater studies (π_V, π₂ and α). The texture of the sand most clearly influenced the diameters of the craters, its effect could also be seen in the case of the distance covered by the ejected material. This information appears to be relevant for future research, providing some rationale to help assess in which aspects of the phenomenon the texture may be important.

Heart rate variability in Konik and purebred Arabian horses in response to different predator vocalisations

The current predation threat of domestic horses is generally low, and horses do not know predators' frightening cues. We studied whether horses still recognise predation threats. The aim of the study was to analyse the emotional response of purebred Arabian horses (Arabian) and Polish Konik horses (Konik) to an Arabian panther (Panthera pardus nimr) (panther) growl and a grey wolf (Canis lupus) (wolf) howl. Panther vocalisation was known to Arabian ancestors, whereas ancestors of Konik knew wolf vocalisation. The response to the howls of golden jackals (Canis aureus) (jackal), which did not prey on equids, was also studied comparatively. Two groups of 10 adult horses of each breed were subject to predator sounds of one predator daily for 5 min during a turn out on pasture. The test was performed for 18 days in total. The sound of each predator was interchangeably featured from one loudspeaker for 3 days followed by four loudspeakers simultaneously to imitate a group of predators for 3 days. The horses' emotional agitation in response to the sounds was measured based on the parameters of heart rate variability (HRV) using telemetric devices. The results showed that the predators' sounds were identified by horses as stressful or neutral. Horses generally retained their anti-predator responses even in the current habitat, which typically lacks predation cues. The results are not always coherent and may demonstrate that the response is somewhat attenuated. The wolf howl elicited a stronger response in Koniks. The panther growl more strongly influenced Arabians, whereas the jackal howl minimally elicited an agitation in the horses. The differentiated response of the two horse breeds to the three predator species suggests that the response is an innate adaptation to the predation risk in the habitat of the breed ancestors. This response occurs regardless of the emotional arousal specific to a breed, and the frightening cue is not the sound per se but the possible attack of predators. Horses display a type of understanding of the sound meaning. Their HRV response seems to be adequate for the threat signalised by the sound.

The intensity of physiological and behavioral responses of horses to predator vocalizations

Background

Predatory attacks on horses can become a problem in some parts of the world, particularly when considering the recovering gray wolf populations. The issue studied was whether horses transformed by humans and placed in stable-pasture environments had retained their natural abilities to respond to predation risk. The objective of the study was to determine the changes in cardiac activity, cortisol concentrations, and behavior of horses in response to the vocalizations of two predators: the gray wolf (Canis lupus), which the horses of the breed studied had coevolved with but not been exposed to recently, and Arabian leopard (Panthera pardus nimr), from which the horses had been mostly isolated. In addition, we hypothesized that a higher proportion of Thoroughbred (TB) horse ancestry in the pedigree would result in higher emotional excitability in response to predator vocalizations. Nineteen horses were divided into groups of 75%, 50% and 25% TB ancestry. The auditory test conducted in a paddock comprised a 10-min prestimulus period, a 5-min stimulus period when one of the predators was heard, and a 10-min poststimulus period without any experimental stimuli.

Results

The increase in heart rate and saliva cortisol concentration in response to predator vocalizations indicated some level of stress in the horses. The lowered beat-to-beat intervals revealed a decrease in parasympathetic nervous system activity. The behavioral responses were less distinct than the physiological changes. The responses were more pronounced with leopard vocalizations than wolf vocalizations.

Conclusions

The horses responded with weak signs of anxiety when exposed to predator vocalizations. A tendency towards a stronger internal reaction to predators in horses with a higher proportion of TB genes suggested that the response intensity was partly innate. The more pronounced response to leopard than wolf may indicate that horses are more frightened of a threatening sound from an unknown predator than one known by their ancestors. The differing response can be also due to differences in the characteristic of the predators’ vocalizations. Our findings suggested that the present-day horses’ abilities to coexist with predators are weak. Hence, humans should protect horses against predation, especially when introducing them into seminatural locations.

The differences in crown formation during the splash on the thin water layers formed on the saturated soil surface and model surface

Splash is the first stage of a negative phenomenon-soil erosion. The aim of this work was to describe the crown formation quantitatively (as part of the splash erosion) and compare the course of this phenomenon on the thin water film formed on a smooth glass surface and on the surface of saturated soil. The height of the falling water drop was 1.5 m. The observation of the crowns was carried out by high-speed cameras. The static and dynamic parameters of crown formation were analysed. It was found that the crowns formed on the water film covering the saturated soil surface were smaller and the time intervals of their existence were shorter. In addition, the shapes of the crowns were different from those created on the water layer covering the glass surface. These differences can be explained by the slightly different values of surface tension and viscosity of the soil solution, the greater roughness of the soil surface and the lower thickness of the water film on the soil surface.

Sound Wave Energy Resulting from the Impact of Water Drops on the Soil Surface

The splashing of water drops on a soil surface is the first step of water erosion. There have been many investigations into splashing-most are based on recording and analysing images taken with high-speed cameras, or measuring the mass of the soil moved by splashing. Here, we present a new aspect of the splash phenomenon's characterization the measurement of the sound pressure level and the sound energy of the wave that propagates in the air. The measurements were carried out for 10 consecutive water drop impacts on the soil surface. Three soils were tested (Endogleyic Umbrisol, Fluvic Endogleyic Cambisol and Haplic Chernozem) with four initial moisture levels (pressure heads: 0.1 kPa, 1 kPa, 3.16 kPa and 16 kPa). We found that the values of the sound pressure and sound wave energy were dependent on the particle size distribution of the soil, less dependent on the initial pressure head, and practically the same for subsequent water drops (from the first to the tenth drop). The highest sound pressure level (and the greatest variability) was for Endogleyic Umbrisol, which had the highest sand fraction content. The sound pressure for this soil increased from 29 dB to 42 dB with the next incidence of drops falling on the sample The smallest (and the lowest variability) was for Fluvic Endogleyic Cambisol which had the highest clay fraction. For all experiments the sound pressure level ranged from ~27 to ~42 dB and the energy emitted in the form of sound waves was within the range of 0.14 μJ to 5.26 μJ. This was from 0.03 to 1.07% of the energy of the incident drops.

Effect of soil moisture content on the splash phenomenon reproducibility

One of the methods for testing splash (the first phase of water erosion) may be an analysis of photos taken using so-called high-speed cameras. The aim of this study was to determine the reproducibility of measurements using a single drop splash of simulated precipitation. The height from which the drops fell resulted in a splash of 1.5 m. Tests were carried out using two types of soil: Eutric Cambisol (loamy silt) and Orthic Luvisol (sandy loam); three initial pressure heads were applied equal to 16 kPa, 3.1 kPa, and 0.1 kPa. Images for one, five, and 10 drops were recorded at a rate of 2000 frames per second. It was found that (i) the dispersion of soil caused by the striking of the 1st drop was significantly different from the splash impact caused by subsequent drops; (ii) with every drop, the splash phenomenon proceeded more reproducibly, that is, the number of particles of soil and/or water that splashed were increasingly close to each other; (iii) the number of particles that were detached during the splash were strongly correlated with its surface area; and (iv) the higher the water film was on the surface the smaller the width of the crown was.