Evaporative deposition of mono- and bi-dispersed colloids on a polydimethylsiloxane (PDMS) surface
Introduction
When a liquid, such as water, containing micro/nano-particles or proteins completely evaporates on a solid surface, a deposition pattern is produced. This phenomenon has broad applications in many fields such as ink-jet printing, function materials, colloidal photonic crystal fabrication, DNA/RNA mapping, and disease diagnosis (Larson, 2017, Brutin and Starov, 2018, Dugas et al., 2005, Jeong et al., 2018, Ihnen et al., 2011, Hu et al., 2018, Lin, 2012, Zhao, 2012, Thiele, 2014, Parsa et al., 2018, Gao et al., 2018). Morphology of evaporative deposition depends on a lot of parameters such as physical and chemical properties of solid surface and liquid including surface wettability (Deegan et al., 1997, Lopes and Bonaccurso, 2013, Yu et al., 2017a, Patil et al., 2016), substrate temperature (Patil et al., 2016, Li et al., 2015, Parsa et al., 2015, Zhong and Duan, 2016, Hu et al., 2018), and liquid polarity (Ghosh et al., 2016). Size (Weon and Je, 2010, Lee et al., 2017, Biswas et al., 2010) and geometry (Yunker et al., 2011, Askounis et al., 2015) of particles as well as evaporation environment (Majumder et al., 2012, Mampallil et al., 2012) also play significant roles in deposition formation.
Evaporation of a water droplet on a solid surface begins with a relatively long constant contact radius (CCR) stage due to contact angle hysteresis. When the surface is hydrophilic, evaporation flux near the droplet boundary line is larger than that in the center of the droplet. This results in a capillary compensation flow from inside the droplet to the edge to compensate for mass loss near the contact line and to keep the contact line pinned. This flow will continuously carry micro/nano-particles to the edge and a ring-like structure will be formed near the edge (Deegan et al., 1997). Further research shows another necessary condition for the formation of the ring-like structure is suppression of Marangoni flow (Hu and Larson, 2006).
During the latest decade, researchers have studied the formation mechanism of ring-like depositions from theoretical analysis and numerical simulation. In 2010, Bhardwaj et al. (2010) demonstrated the DLVO interactions in particle deposition and proposed a phase diagram to describe the competition among radial flow, Marangoni flow, and particle transport toward the substrate. Jung et al. (2010), Wong et al. (2011) and Chhasatia and Sun (2011) proposed a self-pinning mechanism of microparticles comprised of electrostatic, van der Waals, drag, and capillary forces. Considering the difference in contact angle when microparticles are introduced into liquid, Weon and Je (2013) assumed a spreading force acting on the outmost microparticles, which become pinned when the spreading force is equal to the capillary force. In 2017, using particle tracking velocimetry, Yu et al. (2017b) observed the quasi-static motion of microparticles near the contact line and presented a new self-pinning mechanism of microparticles wherein the capillary force acts on the outmost microparticles at an inclined angle with respect to the solid surface rather than parallel to it. Using molecular dynamics simulation, Li et al. (2016) studied a single nanoparticle at the contact line and found enhancing the interaction between the nanoparticle and the substrate, increasing hydrophilicity of substrate and decreasing hydrophilicity of nanoparticle was shown to increase the energy barrier of contact line pinning. Xie and Jens (2018) demonstrated the effect of friction on deposition pattern. When no friction is present, a dot-like deposition is produced and a ring-like deposition was observed by increasing friction force.
Capillary force plays an important role during particle sedimentation. Larger microparticles are driven from the edge to the interior due to capillary force and no ring-like structure is formed (Weon and Je, 2010). When microparticles with different radii are introduced into the liquid, particles will have different motions driven by capillary force, and a more complex deposition pattern may be obtained. Under certain conditions, particles may be sorted using capillary force. Using dielectrophoresis, Jung and Kwak (2007) successfully separated and detected micro/nanoparticles and biological cells. They further studied evaporation of colloidal droplets containing micro/nano-particles on a hydrophilic surface and found the nanoparticles moved toward the contact line while the microparticles were dragged toward the center of the droplet under the action of the liquid surface tension force (Jung et al., 2009). Chhasatia and Sun (2011) studied the influence of substrate wettability on evaporative deposition of bi-dispersed colloid. The authors found micro-and nanoparticles mixed well on hydrophobic substrates and particle separation was incomplete on hydrophilic substrates. Monteux and Lequeux (2011) concluded a particle-free thin film produced at the edge of a droplet could be used for separating colloidal mixtures. This film persists until the evaporation process is complete and is controlled by the contact angle and particle size. Devlin et al. (2016) investigated gravitational effect on particle deposition pattern and proposed a new model considering this effect. Zhong et al. (2017) analyzed evaporation of colloidal droplets containing bi-dispersed nanoparticles. Parsa et al. (2017) and Patil et al. (2018) separately studied the effect of substrate temperature on bi-dispersed colloid deposition patterns. Iqbal et al. (2018) investigated the effect of surface wettability on evaporative deposition of bi-dispersed colloids. A centralized deposition pattern of particles was observed on a hydrophobic surface. On a hydrophilic surface, smaller particles (0.2 and 1.0 µm in diameter) were shown to form an outer ring at the contact line and larger particles (3.0 and 6.0 µm in diameter) were dragged inward to form an inner ring or small mass depending on particle size. Grosshans et al. (2016) experimentally and numerically studied evaporation of single bi-component mannitol-water droplets by changing temperature, relative humidity, droplet size and initial mannitol mass fraction to predict the droplet heating, evaporation, and formation of mannitol layer.
In this paper, evaporative depositions of mono- and bi-dispersed colloids on PDMS were studied. We found ring-like depositions of 1.21 μm silica microparticles and nearly uniform depositions of polystyrene (PS) microparticles were obtained from mono-dispersed colloids. Microparticle sedimentation speed is discussed and deposition of bi-dispersed colloids are analyzed based on the action of capillary force.
Section snippets
Material and methods
PS colloidal suspensions with particle diameters of 1.58 µm (PS04N, initial concentration: 10 wt%), 6.10 µm (PS06N, initial concentration: 10.38 wt%) and 20.33 µm (PS07N, initial concentration: 10.1 wt%) and 1.21 µm diameter silica colloidal suspension (SS04N, initial concentration: 10 wt%) were obtained from Bangs Laboratories, Fisher, USA. Mono-dispersed colloids with 1.21 µm silica and 1.58 µm PS were extracted and diluted to 0.02, 0.08, 0.32 and 1.32 wt% particle concentrations for studying
Evaporation of bi-dispersed colloids
Figs. 2 and 3 show the evolution of nominal contact radius and contact angle versus normalized time (tf is the total evaporation time) of bi-dispersed colloid evaporation with particle concentrations of 0.05 wt% and 0.25 wt%, respectively. All experiments proceeded with a CCR mode prior to a constant contact angle mode (CCA) and ended with a mixed mode. At low particle concentration (0.05 wt%), no change in the duration of CCR stage with all bi-dispersed colloids was observed. At high particle
Settling of microparticles
Microparticles inside an evaporating droplet experience two kinds of forces before settling on the PDMS surface: drag force (Majumder et al., 2012, Landau and Lifshitz, 1987) (, where R is the radius of the microparticle, is the dynamic viscosity of water and is the velocity of the evaporative flow) and the net gravitational force (, where is the gravitational acceleration, and are the densities of the microparticle and water, respectively). Assuming the two
Conclusions
Evaporation of mono- and bi- dispersed colloidal droplets on PDMS was studied. Settling speed was determined to greatly influence the deposition pattern. 1.58 μm PS microparticles, which have slower settling speed compared with 1.21 μm silica microparticles, produced nearly uniform depositions of PS microparticles at low concentration while a ring-like deposition of silica microparticles was formed at particle concentrations ranging from 0.02 to 0.32 wt%. The width of the ring was shown to
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the revision manuscript entitled “Evaporative deposition of mono- and bi-dispersed colloids on a polydimethylsiloxane (PDMS) surface”.
Acknowledgements
The work is financially supported by the National Natural Science Foundation of China (Grant No. 11572114), and the opening fund of State Key Laboratory of Nonlinear Mechanics (LNM). We also thank Prof. Quanzi Yuan from State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences and Prof. Fengchao Wang from University of Science and Technology of China for helpful discussions. Besides, we thank ScienceDocs Inc. (https://www.sciencedocs.com) for language
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