Eﬀect of Ionic Strength and Surface Charge on Convective Deposition Kedar Joshi, Tanyakorn Muangnapoh, Michael D. Stever, and James F. Gilchrist a) Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States ABSTRACT: Particle-particle and particle-substrate interac- tions play a crucial role in capillary driven convective self- assembly for continuous deposition of particles. This systematic study demonstrates the nontrivial eﬀects of varying surface charge and ionic strength of monosized silica microspheres in water on the quality of the deposited monolayer. Increase in particle surface charge results a broader range of parameters that result in monolayer deposition which can be explained considering the particle-substrate electrostatic repulsion in solution. Resulting changes in the coating morphology and microstructure at diﬀerent solution conditions were observed using confocal microscopy enabling correlation of order to disorder transitions with relative particle stability. These results, in part, may explain similar results seen by Muangnapoh et al., 2013 in vibration-assisted convective deposition. 1. INTRODUCTION Microsphere arrays have tremendous applications in our day to day life. Convective deposition is a well-known technique for depositing microspheres into closed packed structures 1-3 and has proven very useful tool in ﬁelds like coatings for LED’s 4-7 and solar cells, 8 membranes, 9 cell capture devices, 10 and in functionalizing Janus particles. 11 Here the particles are made to form close packed structures using capillary interactions. 12 A meniscus of the suspension is created between a substrate and a blade and by relatively moving a substrate with respect to a blade, the meniscus is stretched into thin ﬁlm. 13 The process consists of two physical processes happening simultaneously. First, convection is driven by evaporation, 3,14 similar to the “coﬀee ring eﬀect”, 15 where ﬂow is generated by the evaporation losses. The other is capillary attraction where particles on liquid interface, were particles are drawn into a close packed structure to minimize the energy, 16-18 similar to that found in the “cheerio eﬀect”. 19 A ﬂux balance equation relating the evaporating liquid to the substrate velocity and particle volume fraction 2 is given as β ε = Φ −Φ V J dN(1 ) c e S (1) Where V c : the crystallization velocity, J e : the evaporation ﬂux, Φ: is the volume fraction of the suspension, S : the evaporation length that scales with the area of the evaporation region, ε: the porosity of deposited crystal, and d: the particle diameter. N: number of particle layers. When N = 1 we get monolayer coating and corresponding velocity (V c ) is referred as monolayer deposition speed. This equation includes an additional interaction parameter, β, originally described as the particle-substrate interaction, arising from the nonuniform distribution of particles through the thickness of the thin ﬁlm. This results in a mismatch between particle and water ﬂux. Particles ﬂowing in the thin ﬁlm are acted upon by diﬀerent forces 20 such as electrostatic repulsion, Van der Waal forces, and buoyancy. All these interactions govern the particles distribution in thin ﬁlm. The following work is done to get more understanding of particle-particle and particle-substrate interactions on monolayer velocity as well as microstructure. Bulk ions concentration as well as charge on colloidal particles can play signiﬁcant role in designing the process. The focus of this paper is on the eﬀect of screening and surface charge on monolayer quality during convective deposition (Figure 1). There are numerous studies of the eﬀects of added electrolytes on self-assembly of particles 21-25 and the eﬀects of diﬀerent surface active groups to change particle-substrate interactions. 26-28 Addition of salt decreases Debye length, thereby further screening the eﬀects of electrostatic repulsion. Rö dner et al. have shown the eﬀects of electrolytes on structural changes of monolayer silica particle ﬁlms. 23 Similar to evaporation-driven deposition, drop drying is aﬀected by DLVO interactions resulting in varying the shape of dried droplets. 24 In convective deposition, using binary suspensions of large and small microspheres has resulted in higher order with less streaking instabilities, although the mechanism is poorly understood. 29,30 Similarly, added lateral vibration during deposition has been shown to increase the degree of order in deposited monolayers as well as increasing the range of velocities in which monolayers can be obtained from essentially a single ideal velocity, as suggested in eq 1, to a Received: August 19, 2015 Revised: October 25, 2015 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.5b03109 Langmuir XXXX, XXX, XXX-XXX