在玻璃上，的醋酸和丙酸表現出完全潤濕行為，並且其擴散係數超出了Tanner’s law。然而，在醋酸與丙酸的液滴中加入Silwet L-77三矽氧烷表面活性劑後，潤濕行為將從擴散轉變為自發性隨機運動。通過在兩側塗覆聚矽氮烷所構建之玻璃的線性跑道上，可以實現自發性移動液滴的定向運動。此自發性移動液滴沒有出現接觸線停滯，此現象可歸因於從液滴邊緣延伸的薄膜。因此我們提出了兩種簡單的方法來觀察完全潤濕之液滴和含有Silwet之自發性移動液滴的前軀膜：（1）觀察在液滴附近放置含有 pH 指示劑之水滴的顏色變化，以及（2）通過光學顯微鏡觀察到的非直接接觸相互作用。由於前軀膜的存在，自發性移動的液滴能夠在接觸線之間沒有直接接觸的情況下引起原本靜置的液滴運動。這一有趣的發現可以用由於前體膜和靜態液滴的接觸所造成的Marangoni應力相關的界面張力梯度來解釋。

Acetic and propionic acids on glass show a total wetting behavior with a large exponent beyond Tanner’s law. However, the incorporation of Silwet L-77 surfactant into the droplet of acids changes the wetting behavior from spreading to random motion. On a linear runway constructed by coating polysilazane on glass, the directed motion of the self-propelled droplet can be achieved. The absence of contact line pinning can be attributed to a thin film extending from the edge of the bulk droplet. Two simple methods are proposed to observe the precursor film associated with the spreading droplet and the self-running Silwet-laden droplet: (i) the color change of nearby water droplets containing pH indicator, and (ii) the noncontact interaction observed by optical microscope. Because of the precursor film, the self-running droplet is able to induce motion of a static droplet without the direct contact between their contact lines. This interesting finding may be explained by the interfacial tension gradient associated with Marangoni stress due to the contact of the precursor film and the static droplet.

[1] Young, T., III. An essay on the cohesion of fluids. Philosophical transactions of the royal society of London, 1805(95): p. 65-87.
[2] Wu, C.-J., et al., Superhydrophilicity and spontaneous spreading on zwitterionic surfaces: carboxybetaine and sulfobetaine. RSC advances, 2016. 6(30): p. 24827-24834.
[3] Harkins, W.D. and A. Feldman, Films. The spreading of liquids and the spreading coefficient. Journal of the American Chemical Society, 1922. 44(12): p. 2665-2685.
[4] Ross, S. and P. Becher, The history of the spreading coefficient. Journal of colloid and interface science, 1992. 149(2): p. 575-579.
[5] Tanner, L., The spreading of silicone oil drops on horizontal surfaces. Journal of Physics D: Applied Physics, 1979. 12(9): p. 1473.
[6] He, G. and N. Hadjiconstantinou, A molecular view of Tanner's law: molecular dynamics simulations of droplet spreading. Journal of Fluid Mechanics, 2003. 497: p. 123-132.
[7] Weng, Y.-H., et al., Spreading dynamics of a precursor film of nanodrops on total wetting surfaces. Physical Chemistry Chemical Physics, 2017. 19(40): p. 27786-27794.
[8] De Gennes, P.-G., F. Brochard-Wyart, and D. Quéré, Capillarity and wetting phenomena: drops, bubbles, pearls, waves. Vol. 315. 2004: Springer.
[9] Heslot, F., et al., Experiments on wetting on the scale of nanometers: Influence of the surface energy. Physical review letters, 1990. 65(5): p. 599.
[10] Tiberg, F. and A.-M. Cazabat, Spreading of thin films of ordered nonionic surfactants. Origin of the stepped shape of the spreading precursor. Langmuir, 1994. 10(7): p. 2301-2306.
[11] Villette, S., et al., Ultrathin liquid films. Ellipsometric study and AFM preliminary investigations. Physica A: Statistical Mechanics and its Applications, 1997. 236(1-2): p. 123-129.
[12] Afsar-Siddiqui, A.B., P.F. Luckham, and O.K. Matar, The spreading of surfactant solutions on thin liquid films. Advances in Colloid and interface science, 2003. 106(1-3): p. 183-236.
[13] Kavehpour, H.P., B. Ovryn, and G.H. McKinley, Microscopic and macroscopic structure of the precursor layer in spreading viscous drops. Physical review letters, 2003. 91(19): p. 196104.
[14] Żbik, M.S. and R.L. Frost, PDMS spreading morphological patterns on substrates of different hydrophilicity in air vacuum and water. Journal of colloid and interface science, 2010. 344(2): p. 563-574.
[15] Xu, H., et al., Molecular motion in a spreading precursor film. Physical review letters, 2004. 93(20): p. 206103.
[16] Żbik, M.S. and R.L. Frost, AFM study of forces between silicon oil and hydrophobic–hydrophilic surfaces in aqueous solutions. Journal of colloid and interface science, 2010. 349(2): p. 492-497.
[17] Beattie, D.A., et al., Molecularly-thin precursor films of imidazolium-based ionic liquids on mica. The Journal of Physical Chemistry C, 2013. 117(45): p. 23676-23684.
[18] Wang, Z. and C. Priest, Impact of nanoscale surface heterogeneity on precursor film growth and macroscopic spreading of [Rmim][NTf2] ionic liquids on mica. Langmuir, 2013. 29(36): p. 11344-11353.
[19] Wang, Z., F. Shi, and C. Zhao, Humidity-accelerated spreading of ionic liquids on a mica surface. RSC advances, 2017. 7(68): p. 42718-42724.
[20] Ivanova, N. and T. Esenbaev, Wetting and dewetting behaviour of hygroscopic liquids: Recent advancements. Current Opinion in Colloid & Interface Science, 2021. 51: p. 101399.
[21] Hardy, W.B., III. The spreading of fluids on glass. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 1919. 38(223): p. 49-55.
[22] Liu, C., et al., Long-range spontaneous droplet self-propulsion on wettability gradient surfaces. Scientific reports, 2017. 7(1): p. 1-8.
[23] Galy, P.E., et al., Self-propelled water drops on bare glass substrates in air: Fast, controllable, and easy transport powered by surfactants. Langmuir, 2020. 36(25): p. 6916-6923.
[24] Cira, N.J., A. Benusiglio, and M. Prakash, Vapour-mediated sensing and motility in two-component droplets. Nature, 2015. 519(7544): p. 446-450.
[25] Hu, S.-W., et al., Directed self-propulsion of droplets on surfaces absent of gradients for cargo transport. Journal of Colloid and Interface Science, 2021. 586: p. 469-478.
[26] Huang, Y.-M., Y.-J. Sheng, and H.-K. Tsao, Peculiar encounter between self-propelled droplet and static droplet: swallow, rerouting, and recoil. Journal of Molecular Liquids, 2022. 347: p. 118378.
[27] Wagner, R., et al., Silicon‐modified surfactants and wetting: I. Synthesis of the single components of Silwet L77 and their spreading performance on a low‐energy solid surface. Applied organometallic chemistry, 1999. 13(9): p. 611-620.
[28] Eş, I., et al., Current advances in biological production of propionic acid. Biotechnology letters, 2017. 39(5): p. 635-645.
[29] Gomes, R.J., et al., Acetic acid bacteria in the food industry: systematics, characteristics and applications. Food technology and biotechnology, 2018. 56(2): p. 139.
[30] Lee, H.-J., et al., Naturally occurring propionic acid in foods marketed in South Korea. Food Control, 2010. 21(2): p. 217-220.
[31] Levine, A. and C. Fellers, Action of acetic acid on food spoilage microörganisms. Journal of bacteriology, 1940. 39(5): p. 499-515.
[32] Sengun, I.Y. and S. Karabiyikli, Importance of acetic acid bacteria in food industry. Food control, 2011. 22(5): p. 647-656.
[33] Suomalainen, T.H. and A.M. Mäyrä-Makinen, Propionic acid bacteria as protective cultures in fermented milks and breads. Le Lait, 1999. 79(1): p. 165-174.
[34] Fondecave, R. and F.B. Wyart, Wetting laws for polymer solutions. EPL (Europhysics Letters), 1997. 37(2): p. 115.
[35] Fondecave, R. and F.B. Wyart, Polymers as dewetting agents. Macromolecules, 1998. 31(26): p. 9305-9315.
[36] Brochard-Wyart, F., R. Fondecave, and M. Boudoussier, Wetting of antagonist mixtures: theleak out'transition. International Journal of Engineering Science, 2000. 38(9-10): p. 1033-1047.
[37] Hu, S.-W., et al., Peculiar Wetting of N, N-dimethylformamide: Expansion, contraction, and self-running. The Journal of Physical Chemistry C, 2019. 123(40): p. 24477-24486.
[38] Nuthalapati, K., Y.-J. Sheng, and H.-K. Tsao, Anomalous interfacial dynamics of pendant droplets of N, N-dimethylformamide containing Silwet. Journal of the Taiwan Institute of Chemical Engineers, 2022. 133: p. 104282.
[39] Collignon, S., J. Friend, and L. Yeo, Planar microfluidic drop splitting and merging. Lab on a Chip, 2015. 15(8): p. 1942-1951.
[40] Wang, W. and T.B. Jones, Moving droplets between closed and open microfluidic systems. Lab on a Chip, 2015. 15(10): p. 2201-2212.
[41] Krumpfer, J.W. and T.J. McCarthy, Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces. Faraday discussions, 2010. 146: p. 103-111.
[42] Luo, M., R. Gupta, and J. Frechette, Modulating contact angle hysteresis to direct fluid droplets along a homogenous surface. ACS applied materials & interfaces, 2012. 4(2): p. 890-896.