Easy Demonstration of the Marangoni Effect by Prolonged and

Easy Demonstration of the Marangoni Effect by Prolonged and Directional Motion: “Soap Boat 2.0” ... Publication Date (Web): August 26, 2013 ... Th...
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Easy Demonstration of the Marangoni Effect by Prolonged and Directional Motion: “Soap Boat 2.0” Charles Renney, Ashley Brewer, and Tiddo Jonathan Mooibroek* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, United Kingdom S Supporting Information *

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. KEYWORDS: Elementary/Middle School Science, General Public, High School/Introductory Chemistry, Demonstrations, Physical Chemistry, Hands-On Learning/Manipulatives, Surface Science, Transport Properties, Water/Water Chemistry he “Marangoni effect” refers to the phenomenon that molecules move from areas of high surface tension to areas of low surface tension.1−3 Such a “surface tension gradient” can be achieved by bringing two liquids that have a different surface tension into contact with one another. In nature, the water strider Microvelia excretes a surface-tensionlowering chemical to propel itself along the water surface,4 and in everyday life the Marangoni effect is frequently observed as the “legs” or “tears” of alcoholic drinks.5,6 Over the years, several classroom demonstrations have appeared in this Journal that describe the visualization of the Marangoni effect.5−13 Some demonstrations have reported that soap can act as surface-tension-lowering agent to part a thin layer of talc7 or pepper8 that has been sprinkled on top of water. Soap has also been used in well-known “soap boats”, where a small floating object is propelled along the water surface upon touching the water near the object with a soapimpregnated toothpick.14,15 A disadvantage of these soap-boat demonstrations, however, is that they can be performed only once or twice; a thin layer of soap forms on the surface of the water preventing further propulsion. A number of recent scientific studies have exploited the Marangoni effect to bring about directed and prolonged (autonomous) motion of objects using low-surface-tension water-miscible solvents.16−21 It was thus wondered if these recent developments could be adopted in a classroom demonstration in the form of a “soap boat 2.0”. Surprisingly, there is no practical (or peer-reviewed) manual for such

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© 2013 American Chemical Society and Division of Chemical Education, Inc.

demonstrations or experiments,22−25 nor is there any literature where safety aspects are addressed. Hence, the present paper presents three easy-to-make and effective boat designs, discusses some liquids that could be used as “fuel”, and ends with some suggested variations of this type of demonstration.

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BASIC DEMONSTRATION In principle, any small floating object can be moved by dropping a bit of “fuel” near the object. The object can consist of wood (e.g., a toothpick) or cork, but it was found that polystyrene foam (PSF) is a particularly convenient material because it is readily available, is easy to sculpt, and has a high buoyancy. The fuel can be any (slightly) water-miscible liquid with a lower surface tension than water (see the fuel choice section for details). It is best if students gather around a standard size water tub (greater than 30 × 30 cm) or if a similarly sized translucent container is available, the demonstration can be done in front of a classroom (preferentially with a “large container” boat, see the next section).

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BOAT DESIGNS It was found that the following three boat designs are easy to make and work very well (see Table 1 for a list of required materials): “single shot” boats are propelled forward each time a drop of fuel is dropped near it; “small container” boats, which Published: August 26, 2013 1353

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Table 1. Required Materials for the Three Types of Boats

Figure 2. Making a small container boat: (A) cutting a piece of a polystyrene foam cup, (B) punching a hole at the rear of the foam piece, (C) gluing the foam to paper, and (D) cutting the foam piece to make a boat.

A small container boat can also be made from a PSF cup, as shown in Figure 2. A vertical strip is cut from a PSF cup, and a hole punch is used to cut a small hole (±6 mm) for the fuel container at the end of the PSF strip, leaving ±4 mm between the edge of the hole and the rear of the strip. A narrow channel is created from the hole to the end of the strip by cutting with scissors (Figure 2B). A piece of paper is evenly covered with super glue and is glued on the punctured PSF strip while checking that the channel is open for the fuel to flow out (Figure 2C). The paper constitutes the bottom of the fuel container. Finally, after the glue is dry, the vertical strip is cut into the desired shape (Figure 2D), preferentially as small as possible (∼1.0 cm long and 0.7 cm wide). This type of boat can be made larger (e.g., by cutting a bigger hole), but it was found that this design size is easy to make and larger designs can become difficult to propel. For extra buoyancy and stability, a keel (or keels) can be glued onto the bottom of the boat (not shown in the figure, but a video demonstration of a boat with a keel can be found in the Supporting Information). Making small container-type boats (with or without keels) is slightly more difficult than the single shot boat, and the use of super glue restricts this demonstration to children above the age of twelve. Finally, and most enjoyably, large container boats can be made from a small (e.g., 7 × 3 × 2 cm) block of PSF, which can easily be obtained by cutting up PSF packaging material. After sculpting a hull of desired design (e.g., Figure 3A), a fuel reservoir is carved out using a craft knife (Figure 3B,C). A notch is cut into the underside of the vessel at its stern to act as a fuel outlet (Figure 3D) and is used to ensure that the fuel spreads evenly backward to deliver a forward thrust. Finally, a thin needle (typically a thin sewing needle) is used to puncture a hole from the fuel reservoir to the rear outlet (Figure 3E). The hole opening in the fuel outlet should be just above the water surface level. Again, extra buoyancy and stability can be obtained by gluing one or more keels onto the bottom of the boat (Figure 3F). The positioning of the keel(s) can also determine whether the boat will travel in a straight line or rotate in left- or right-handed circles (a video demonstration of such a boat moving in right-handed circles can be found in the Supporting Information). This type of boat is best suited for a classroom demonstration (as it is rather large), but children can also make their own boat. The use of a craft knife and super

a

Premade single shot boats can be obtained by using plastic bread clips or fruit bag clips. These rest on the surface tension rather than float. b Typically an office hole punch has a radius of ±6 mm, but others could also be used. cThe needle should be as thin as possible so that the fuel is deployed as slowly as possible (thin syringe or dissecting needles can also be used). dTypical dimensions of greater than 30 × 30 cm.

can hold a small quantity of fuel, give a slightly prolonged motion (several seconds); and “large container” boats, which can hold more fuel, can move autonomously for up to several minutes. An effective single shot boat can be made by cutting a PSF cup (or any other thin sheet of PSF) into a desired shape (∼2 cm length × 1 cm width), for example, the shapes shown in Figure 1. It is best to cut vertically to retain the cup’s concave or

Figure 1. Single shot boat designs that move to the left (A), right (B), or straight (C) when fuel is deployed.

convex shape across the width of the boat (see also Figure 2A; the concave or convex side can be marked with a pen as a reminder). By shaping the boat appropriately, a leftward (Figure 1A), rightward (Figure 1B), or linear (Figure 1C) direction of motion can be achieved. The fuel is best deployed at the rear cove of the boat (see Supporting Information for a video demonstration). This type of boat most resembles traditional “soap boats” and can easily be made by children around the age of six years. 1354

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durable owing to its low boiling point, high vapor pressure, and low viscosity. The potential fuels NMP, acetone, acetonitrile, and ethyl acetate dissolve polystyrene (entries 3−6). The three water miscible compounds with the most extreme surface tensions (entries 7−9) are impractical. Although they give excellent thrust, HN(iPr)2 (γ = 19.14 mN m−1) has a strong odor and is the most toxic, while tBuOH (γ = 19.96 mN m−1) is a solid below 25 °C. Hardly any propulsion was noticed with DMSO (γ = 42.49 mN m−1). The alcohols listed in entries 10− 16 all have a similar surface tension (γ ≈ 21−25 mN) and are all liquids at 20 °C. Of these alcohols, 1-PrOH, 1-BuOH, 2BuOH, and 3-Me-1-BuOH are less volatile than water (column vp). This means that these liquids might be safer for use outside of fume hoods than others and also last longer. As the viscosity varies greatly (η ≈ 0.5−3.5 mPa s, see column η), the flow rate through a certain size hole of a large container boat can be varied by choosing the appropriate alcohol. It is noteworthy that a particularly high thrust was observed using 1-BuOH (entry 14), 2-BuOH (entry 15), and 3-Me-1-BuOH (entry 16), and that 3-Me-1-BuOH is exceptionally long lasting (high boiling point, low vapor pressure, high viscosity) and relatively benign (LD50 = 5,000 mg/kg). Safety-wise it can be noted that all chemicals except DMSO and NMP are highly flammable and that all are rather benign when used appropriately. That is, a child weighing 20 kg would have to swallow about 20 mL of pure diisopropylamine (the most toxic) to ingest a dose equal to the LD50 value (ρ = 0.722 g/mL). For methanol (ρ = 0.792 g/mL), ethanol (ρ = 0.789 g/ mL), and 3-Me-1-BuOH (ρ = 0.809 g/mL) this is 30, 180, and 124 mL, respectively. Two obvious ways of preventing such accidental ingestion are to limit the quantity given to children to, say, a tenth of this dangerous dose or to dilute the chemical with water (or both). Hexane is nearly insoluble in water, but it was tested whether aqueous dilutions of the alcoholic fuels listed in entries 10−16 of Table 2 were capable of delivering thrust to our three boat designs (see Table 3). For all fully miscible alcohols, ≥40% aqueous solutions were still effective. Interestingly, the fuels with limited solubility were particularly effective in their pure form, and water saturated solutions of

Figure 3. Making a large container boat from a piece of polystyrene foam: (A) sculpting a hull-like shape, (B and C) carving a fuel reservoir, (D) carving a fuel outlet, (E) puncturing the reservoir with a needle to make a fuel channel to the outlet, and (F) mounting a keel.

glue also limits this demonstration to children above the age of twelve years.

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FUEL CHOICE Several potential fuels and their physical constants are listed in Table 2. The surface tension (γ) of the liquid determines how much thrust the fuel can deliver. The boiling point (bp), vapor pressure (vp), and viscosity (η) are indicative of how durable a fuel will be; for example, when the vp is high, the liquid will evaporate significantly from the boat’s container, and when the η is low, the fuel will be released quickly. The TWA (timeweighted average) and STEL (short-term exposure limit) values are the concentrations that someone can be exposed to for a long (∼8 h) or short (∼15−30 min) period of time, respectively, without showing any deleterious symptoms. The LD50 values listed are the oral dose by which half of a tested group of rats died and is indicative of the severity of accidental ingestion of the fuel in pure form. Interestingly, the surface tension of Hex (entry 2, γ = 17.89 mN m−1) is the lowest of the liquids listed, and it was found that Hex works reasonably well as a fuel, despite its low solubility (9.57 mg/L at 25 °C).28 However, Hex is not very Table 2. Fuels and Relevant Properties Entry

Substance

γa (mN m−1)

bpb (°C)

vpc (hPa)

ηa (mPa s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Water (H2O) n-Hexane (Hex) N-methylpyrrolidone (NMP) Acetone (OC(CH3)2) Acetonitrile (CH3CN) Ethyl acetate (EtOAc) Diisopropylamine (HN(iPr)2) tert-Butyl alcohol (tBuOH) Dimethyl sulfoxide (DMSO) Methanol (MeOH) Ethanol (EtOH) 1-Propanol (1-PrOH) 2-Propanol (2-PrOH) 1-Butanol (1-BuOH) 2-Butanol (2-BuOH) 3-Methyl-1-butanol (3-Me-1-BuOH)

71.99 17.89 40.70 22.72 28.66 23.39 19.14 19.96 42.49 22.07 21.97 23.32 20.93 24.93 22.54 23.71

100 68.7 202 56.1 81.6 77.1 84.0 82.3 189 64.5 78.2 97.0 82.2 118 99.4 131

32 176 0.40 245 99 97 67 41 0.55 130 60 19 43 5.0 15 3.0

0.890 0.300 1.700 0.306 0.369 0.423 0.393 4.312 1.996 0.544 1.074 1.945 2.038 2.544 3.096 3.692

a

TWA/STELd,e (ppm)

LD50e (mg/kg)

Comment

20/60 25/75 500/1,500 40/60 200/400 5/15 100/150 50/100 200/250 1,000/3,000 200/250 400/500 50/75 100/150 100/125

>90,000 25,000 3,910 5,800 >1,320 5,620 770 2,740 14,500 >1,190 7,060 8,040 5,050 790 2,190 5,000

Not durable Dissolves PSF Dissolves PSF Dissolves PSF Dissolves PSF Odor Solid ≤25 °C Poor propulsion Very volatile Very volatile Less volatile Less volatile Less volatile Less volatile Long lasting

The surface tension and viscosity data are at 25 °C. bThe boiling point data are at standard pressure of 1 bar. cThe vapor pressure data are at 20 °C. TWA is the time-weighted average, and STEL is the short-term exposure limit. eThe toxicity data are from refs 26 and 27.

d

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Table 3. Boat Motion from Different Volume Percentage of Fuels in Water Boat Motiona from Fuel in Water (% v/v) Fuel

100

80

60

40

20

Water Miscibility at 20 °C (% v/v)

Methanol (MeOH) Ethanol (EtOH) Methylated spirit (EtOH/MeOH, 9:1) 1-Propanol (1-PrOH) 2-Propanolb (2-PrOH) 1-Butanol (1-BuOH) 2-Butanol (2-BuOH) 3-Methyl-1-butanol (3-Me-1-BuOH)

Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y (!) Y/Y/Y (!) Y/Y/Y (!)

Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y − Y/Y/Y −

Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y Y/Y/Y − − −

Y/Y/Y Y/Y/± Y/Y/Y Y/Y/Y Y/Y/Y − − −

Y/Y/± Y/±/N Y/±/N Y/Y/Y Y/Y/Y − − −

Miscible Miscible Miscible Miscible Miscible 8.7c 35.9c 3.5c

a

a The data are listed for the single shot, small container, and large container boats, respectively. The boat motion is defined as observed (Y), not observed (N), or hardly observed (±). Exclamation marks indicate particularly rapid propulsion. bPropulsion with all three boat types was still observed when using a 5% (v/v) aqueous solution. cPropulsion with all three boats types was still observed when using a saturated aqueous solution.

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these alcohols were also still able to propel all three types of boats. Based on the data, EtOH seemed a good fuel option (low surface tension, low toxicity, 40% in water still quite effective). EtOH is readily available (e.g., whiskey or vodka); however, the association with alcoholic drinks might not be ideal for education purposes. Methylated spirits (typically 10% methanol in ethanol) are not commonly associated with consumption alcohol and could thus serve as a source. 2-PrOH (rubbing alcohol) is also readily available, not associated with alcoholic drinks, relatively benign, and, remarkably, still effective as a 5− 10% aqueous solution. Although less available (not found in convenience stores), 1-BuOH, 2-BuOH, and especially 3-Me-1BuOH are long-lasting and still safe options owing to their low vapor pressure and high viscosity. In conclusion, the ideal fuel depends on the type of demonstration aimed for and the age group of the children participating in the demonstration. The safest, available, and still effective option is a 10% solution of rubbing alcohol (2PrOH) in water; however, pure 3-Me-1-BuOH is relatively safe but extremely potent and long lasting.

ASSOCIATED CONTENT

S Supporting Information *

Video demonstrations of single shot, small container, and large container Marangoni propelled polystyrene boats using 1butanol as fuel. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Scriven, L. E.; Sternling, C. V. Marangoni Effects. Nature 1960, 187, 186−188. (2) Sternling, C. V.; Scriven, L. E. Interfacial Turbulence Hydrodynamic Instability and the Marangoni Effect. AIChE J. 1959, 5, 514−523. (3) Milliken, W. J.; Stone, H. A.; Leal, L. G. The Effect of Surfactant on the Transient Motion of Newtonian Drops. Phys. Fluids A 1993, 5, 69−79. (4) Bush, J. W. M.; Hu, D. L. Walking on water: Biolocomotion at the interface. In Annual Review of Fluid Mechanics; Annual Reviews: Palo Alto, 2006; Vol. 38, pp 339−369. (5) Silverstein, T. P. Why do alcoholic beverages have “legs”? J. Chem. Educ. 1998, 75, 723−724. (6) Gugliotti, M. Tears of wine. J. Chem. Educ. 2004, 81, 67−68. (7) Ahmad, J. Cleavage Patterns in a Layer of Talc Spread Over Water-Surface. J. Chem. Educ. 1992, 69, 1029−1030. (8) Jasien, P. G.; Barnett, G.; Speckhard, D. Lowering the SurfaceTension of Water - An Illustration of the Scientific Method. J. Chem. Educ. 1993, 70, 251−252. (9) Silverstein, T. P. Polarity, Miscibility, and Surface-Tension of Liquids. J. Chem. Educ. 1993, 70, 253−253. (10) Ahmad, J. Turbulent motion in ethyl acetate-water system. J. Chem. Educ. 2000, 77, 1182−1183. (11) Gugliotti, M.; Baptista, M. S.; Politi, M. J. Surface tension gradients induced by temperature: The thermal Marangoni effect. J. Chem. Educ. 2004, 81, 824−826. (12) Mundell, D. W. Dancing crystals: A dramatic illustration of intermolecular forces. J. Chem. Educ. 2007, 84, 1773−1775. (13) Mundell, D. W. Marangoni Flowers and the Evil Eye: Overhead Presentations of Marangoni Flow. J. Chem. Educ. 2009, 86, 833−836. (14) Vivian, C. Soap boats. In Science experiments & amusement for children; Dover publications Inc.: New York, 1963; p 52. (15) Shalit, N. Magic harbour. In Science magic tricks; Dover Publications Inc.: New York, 1981; p 98.

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OTHER VARIATIONS The above demonstrations are naturally open to the teacher’s own variations, but a few fun variations are presented here. To visualize the Marangoni effect even more, the fuel can by colored with a dye. For example, acridine orange dissolves in ethanol and water. Simple food dyes that contain emulsifier (typically polysorbate 80) cannot be used, as emulsifiers act like soap in that they form a small layer on top of the water, thereby quenching propulsion. Alternatively, a glass container can be used and placed on top of an overhead projector to visualize the differences in refraction index (which are also typically visible with the naked eye). Another, though far less effective, variation is to use the thermal Marangoni effect, for example, by using ice water in the tub and warm water as fuel. The surface tension gradient will be much less (γwater = 75.6 mN m−1 at 0.01 °C and 59.9 mN m−1 at 95 °C),29 but it was found that with the single shot boats some propulsion is still observable. It is also possible to organize races or competitions between students by letting them choose (and argue) a certain fuel (e.g., hexane versus methanol versus 80% 3-methyl-1-butanol in water). Finally, it is possible to propel a hollow polysterene block of significant proportions 14 × 14 × 6 cm, which could be exploited in a school project with older children. 1356

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(16) Chen, Y.-J.; Nagamine, Y.; Yoshikawa, K. Self-propelled motion of a droplet induced by Marangoni-driven spreading. Phys. Rev. E 2009, 80, 016303. (17) Zhao, G.; Pumera, M. Liquid-Liquid Interface Motion of a Capsule Motor Powered by the Interlayer Marangoni Effect. J. Phys. Chem. B 2012, 116, 10960−10963. (18) Jin, H.; Marmur, A.; Ikkala, O.; Ras, R. H. A. Vapour-driven Marangoni propulsion: continuous, prolonged and tunable motion. Chem. Sci. 2012, 3, 2526−2529. (19) Lauga, E.; Davis, A. M. J. Viscous Marangoni propulsion. J. Fluid Mech. 2012, 705, 120−133. (20) Sharma, R.; Chang, S. T.; Velev, O. D. Gel-Based Self-Propelling Particles Get Programmed To Dance. Langmuir 2012, 28, 10128− 10135. (21) Wang, Y.; Liu, X.; Li, X.; Wu, J.; Long, Y.; Zhao, N.; Xu, J. Directional and Path-Finding Motion of Polymer Hydrogels Driven by Liquid Mixing. Langmuir 2012, 28, 11276−11280. (22) Party Cruise. http://www.prism-magazine.org/jan12/first-look. cfm (accessed August 2013). (23) Water, Water Everywhere. http://www.whoi.edu/fileserver. do?id=30772&pt=2&p=35789 (accessed August 2013). (24) Spike’s Science Projects - Surface Tension Boats. http:// spikesworld.spike-jamie.com/science/liquids/c121-20-boats.html (accessed August 2013). (25) Walton, G. Propulsion process for lightweight miniature toy boats. US 5213616-A, 1993. (26) Handbook of Chemistry and Physics, 93rd ed.; Haynes, W. M., Lide, D. R., Bruno, T. J., Eds.; CRC Press: Boca Raton, FL, 2012− 2013. (27) MSDS files obtained from http://www.sigmaaldrich.com or http://www.acros.be (accessed August 2013). (28) Gutmann, V.; Resch, G. Hydrophobic interactions in aqueous solutions: Their operation in living systems. J. Phys. Org. Chem. 1997, 10, 335−342. (29) Vargaftik, N. B.; Volkov, B. N.; Voljak, L. D. International Tables of the Surface-Tension of Water. J. Phys. Chem. Ref. Data 1983, 12, 817−820.

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