Easy Demonstration of the Marangoni Effect by Prolonged and Directional Motion: “Soap Boat 2.0”

Abstract

So-called "soap boats" have been known for decades and can be used to demonstrate the Marangoni effect. Inspired by recent scientific work, this paper reports an improved demonstration: a "soap boat 2.0". With this demonstration, a floating object (typically polystyrene foam) can be propelled along the water surface for up to several minutes. The direction of motion (straight, left-handed, or right-handed circles) can be influenced by the boat design. Three easy-to-make and effective boat designs are presented, and fifteen "fuels" have been considered with regard to safety, effectiveness, duration, ease of use, and availability. Some other possible variations of the demonstration are also proposed. The demonstration can be done with any age group, lasts for up to several minutes, and can be turned into a student project of varying length. Readily available materials (e.g., polystyrene foam) and benign household chemicals (e.g., ≥10% 2-propanol ("rubbing alcohol") in water) can be used.

Surfactant-loaded capsules as Marangoni microswimmers at the air–water interface: Symmetry breaking and spontaneous propulsion by surfactant diffusion and advection

“…This contribution is symmetry breaking; inserting this scaling of the concentrations into the relations (23) and (24 for the Marangoni forces, we obtain…”

“…Marangoni boats are moving at the liquid-air interface [22]; typically, they are solid swimmers and operate at the centimeter scale. They are often used as a popular demonstration experiment for the Marangoni effect [23]. As "fuel" serve surface active molecules, which are deposited on the floating swimmer [23] or in which the swimmer is soaked [24][25][26][27][28][29], or the swimmer body itself is made from dissolving surfactant [30].…”

“…As "fuel" serve surface active molecules, which are deposited on the floating swimmer [23] or in which the swimmer is soaked [24][25][26][27][28][29], or the swimmer body itself is made from dissolving surfactant [30]. There are many examples based on DMF (dimethylformamide) [31], alcohol [23,29], soap [29], camphor [24][25][26][27][28]32] or camphene [30] that have also been characterized quantitatively. The surfactant molecules are emitted or dissolved from the swimmer and a radial concentration gradient is established at the air-water interface by diffusion, eventually aided by evaporation for volatile surfactants.…”

From diffusive mass transfer in Stokes flow to low Reynolds number Marangoni boats

“…Propulsion is not caused by surfactants that are anisotropically distributed along the swimmer-liquid interface but by the anisotropic distribution of surfactant at the liquid-air interface along which the soap boat propels [14]. The surfactant molecules at the liquid-air interface are emitted or dissolved from the swimmer; this can be achieved by depositing them on the floating swimmer initially [15], by soaking the swimmer in surfactant [16][17][18][19][20][21], or by using a swimmer body made from dissolving surfactant [22]. There are many examples based on DMF (dimethylformamide) [23], alcohol [15,21], soap [21], camphor [16][17][18][19][20]24] or camphene [22] that have also been investigated quantitatively.…”

“…This means that the direct Marangoni forceFM (see (13)) is only present for explicit symmetry breaking (cS,1 = 0), whereas the total Marangoni forceFM,tot [see (15)] has a logarithmically diverging linear coefficient for largeR, which is identical to the constant flux case. The contribution from explicit symmetry breaking (cS,1) to the direct Marangoni force is always weakened by the presence of Marangoni flows.…”