The Role of Surface Viscosity in the Escape Mechanism of theStenusBeetle

Abstract

Beetles of the species Stenus comma live and hunt close to ponds and rivers, where they occasionally fall on the water surface. To escape this jeopardized state, the beetle developed a strategy relying on the excretion of a secretion containing the substances stenusine and norstenusine. They reduce surface tension and propel the bug to the saving river bank. These substances were synthesized and analyzed with respect to their equilibrium and dynamic adsorption properties at the air-water interface (pH 7, 23 ± 1 °C). The surface dilatational rheological characteristics in a frequency range from 2 to 500 Hz at molar bulk concentrations of 20.6 mmol L(-1) were studied using the oscillating bubble technique. Both alkaloids formed surface viscoelastic adsorption layers. The frequency dependence of the surface dilatational modulus E could successfully be described by the extended Lucassen-van den Tempel model accounting for a nonzero intrinsic surface viscosity κ. The findings confirmed a dual purpose of the spreading alkaloids in the escape mechanism of the Stenus beetle. Next to generating a surface pressure, a transition to surface viscoelastic behavior of the adsorbed layers was observed.

Transfer of Materials from Water to Solid Surfaces Using Liquid Marbles

“…Instead of moving through water, some insects walk and jump on the water surface. − Supported by the surface tension of water, water striders and Stenus beetles − rapidly move on the air–water interface. These insects can move more quickly on the water surface than through the bulk water phase because most of their body is in the air, which has a lower viscosity than that of water.…”

“…Therefore, these insects experience less frictional resistance. Interestingly, the Stenus beetle can escape from predators using the power of Marangoni propulsion . By ejecting a hormone from its rear end, the Stenus beetle generates a surface tension gradient on the water surface and consequently can quickly move over the water surface at a velocity of a tenth of a centimeter per second.…”

“…This flow leads to efficient mass transfer along an interface. Floating objects that are close to the flow or even that cause the surface tension gradient can be powerfully propelled over a water surface. , Examples of the Marangoni propulsion include the Stenus beetle, − camphor crystals, soap boats, depolymerizable plastics, and organic solvent-loaded objects, , all of which move quickly over the water surface by ejecting surface-active compounds that expand over the water surface. Recently, light-induced surface tension gradients have been demonstrated to produce powerful propulsion forces to move small objects. − …”

“…Interestingly, the Stenus beetle can escape from predators using the power of Marangoni propulsion. 7 By ejecting a hormone from its rear end, the Stenus beetle generates a surface tension gradient on the water surface and consequently can quickly move over the water surface at a velocity of a tenth of a centimeter per second. At some point, the beetle reaches the rim of the water surface and can then step onto solid ground to continue its escape.…”

Light-Driven Locomotion of Bubbles

“…For instance, gas generation at the surface of object via catalytic reaction can lead to the powerful propulsion of objects. , In addition, Marangoni flow caused by a surface tension gradient can also work as a driving force to move objects over a liquid surface. , Surface tension gradient is generally generated by dissolution of surface-active chemicals from the objects, which can create concentration gradients of the chemicals at the liquid surface. For example, the Stenus beetle, − soap boats, camphor crystals, organic solvent-loaded objects, , and depolymerizable plastics can move over the water surface via Marangoni flow, which was generated by the ejection of surface-active chemicals that expand over the water surface. Surface tension gradients generated based on temperature gradients, which are induced by light irradiation, have been also utilized to generate powerful propulsion forces to move small objects. ,− Recently, liquid marbles coated by particles with light-to-heat photothermal property were demonstrated to move over water surface by local heating via irradiation with a near-infrared (NIR) laser, which generates a temperature gradient. − …”