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Surfactant Adsorption Induced Initial Depinning Behavior in Evaporating Water and Nanofluid Sessile Droplets Xin Zhong, and Fei Duan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00288 • Publication Date (Web): 29 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015
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Langmuir
Surfactant Adsorption Induced Initial Depinning Behavior in Evaporating Water and Nanofluid Sessile Droplets Xin Zhong and Fei Duan∗ School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 E-mail:
[email protected] Phone: +65 67905510. Fax: +65 67924062
Abstract A surfactant-induced autophobic effect has been observed to initiate an intense depinning behavior at the initial stage of evaporation in both pure water and nanofluid sessile droplets. The cationic surfactant adsorbing to the negatively charged silicon wafer makes the solid surface more hydrophobic. The autophobing-induced depinning behavior, leading to an enlarged contact angle and a shortened base diameter, only takes place when the surfactant concentration is below its critical micelle concentration (cmc). The initial spreading degree right before the droplet retraction, the retracting velocity of the contact line and the duration of the initial droplet retraction are shown negatively dependent on the surfactant concentration below cmc. An unexpected enhancement in the initial depinning was found in the nanofluid droplets, possibly resulted from the hydrophilic interplay between the graphite nanoparticle deposition ∗
To whom correspondence should be addressed
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and the surfactant molecules. Such promotion of the initial depinning due to the nanoparticle deposition makes the droplet retract even at a surfactant concentration higher than cmc (1.5cmc). The resulted deposition formed with the presence of the depinning behavior has a greatly enhancement for coffee-ring formation as compared with the one free of surfactant, implying that the formation of a coffee ring does not require pinning of the contact line in the entire drying process.
Keywords: depinning, surfactant, autophobing, nanofluids, sessile droplet, evaporation
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Introduction The ubiquitous coffee-ring effect prevailing in both nature and industry forms under prerequisites of a pinning contact line and more intense evaporation at the droplet edge. 1 It is desirable in the printing of repetitive fine lines, 1,2 but undesirable in thin-film formation and micro-coating. Therefore, the manipulation of the particle motion is of significance and has attracted much audience. Hitherto various methods primarily across the aspects of solute, 3 solvent, 4–6 evaporating atmosphere 7 and substrate 8 have been verified to be simple and effective in the effort to control the deposited profile. Among these efforts, the employment of different types of surfactant aiming droplet control primarily relied on its influence on droplet wettability, 9–13 its reduction in particle-particle interaction 14 and the energy for liquid-vapor interfacial deformation, 4 and its uneven distribution on the liquidvapor interface which could lead to a Marangoni flow and the resulted depinning behavior at the ultimate stage of drying. 4,15 On the basis of different governing mechanisms, surfactant could be utilized to either enhance or suppress the coffee-ring effect. 16 Surfactant adsorption at the liquid-vapor interface has received extensive studies by now as it can easily vary the liquid-vapor surface tension, and even the flow regime.
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adsorption of surfactant at the solid surface, on the other hand, has been reported to reduce the wetting of a high energy surface such that the surfactant solution tended to exhibit a non-zero contact angle, which is known as the autophobic effect. 17–20 The necessity for autophobing occurring is that the surfactant molecules transport to and attach to the bare solid-vapor interface with their polar heads. Accordingly the spreading coefficient, the difference of the reduced solid-vapor surface tension over the sum of the reduced solid-liquid and the liquid-vapor surface tensions, was found to be lowered and thus lead to a retreat of the contact line. 21 The tendency of the system to decrease the total energy of the interfaces through surfactant adsorption would either stop the advancing of a liquid or induce a receding of the contact line. 22 The autophobic effect was primarily examined in a system of a vertical solid surface 3 ACS Paragon Plus Environment
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partially immersed in a surfactant solution. 21,23–27 As the substrate was forced to insert into the solution surface, the contact line experienced a stick-slip motion, and then a rapid retreat behavior which was attributed to a decrease in solid-vapor surface tension raised by the surfactant-assemblies at the solid surface external to the contact line never touched by the solution. The phenomena were investigated as well in the other various systems on configurations, 20 types of the surfactant 28–30 and solid surfaces. 28,31 The autophobing were probed by using condensation figures, measuring the thickness of surfactant layers, examining the contact angle and the retracting velocity of the contact line on both clean and pre-coated surfaces, and evaluating the liquid-vapor surface tension. It is well accepted that the autophobing-induced contact line retreat is nearly independent on the surfactant concentration, since the formation of the surfactant monolayer beyond the contact line is kinetically hindered due to the limited mobility of the surfactant molecules across the bare solid surface, although it has directly been observed that the bare solid-vapor area covered by surfactant layer was increased with the loading of surfactant. 20,21,23 Such contact line retreat was found only emerged below 0.45 times of the critical micelle concentration (cmc) of the surfactant for either a liquid in an affinity with a vertical plate or a pure liquid droplet on a horizontal substrate. 21,23 However, questions still remain: Are the duration and velocity of the depinning behavior independent on surfactant inventory in a sessile droplet as well? What would happen if the nanoparticles are added in surfactant solution droplet? Does the coffee-ring effect be suppressed by the autophobing-induced depinning behavior of the nanofluid droplet? Furthermore, it was reported that the advanced distance was always longer at 0.1cmc than that at 1.0cmc right before contact line withdrawal. 21 The dependence of the advanced distance on surfactant concentration over a low range (
