Printing Small Dots from Large Drops

Abstract

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol−gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.

Tunable Coffee Ring Formation on Polycarbonate Nanofiber Film for Sensitive SERS Detection of Phenylalanine in Urine

“…The larger the contact angle, the smaller is the coffee ring. The contact angle is dependent on the roughness 36,37 and hydrophobicity 38 of the surface of the solid support. Moreover, the chemical composition of the droplet influences its surface tension 39,40 and causes variation in the contact angle.…”

Controlled Uniform Coating from the Interplay of Marangoni Flows and Surface-Adsorbed Macromolecules

“…These processes are complex because of physicochemical dynamics that arise from Marangoni effects [2, 5-10, 12, 21, 22] and particle deposition mechanisms [11,12,14,23]. In fact, although a binary mixture is used quite often to achieve uniform particle deposition from droplets smaller than 100 µm [4,18,19], to our best knowledge such coatings have not been achieved for larger droplets. Furthermore, while the wetting and dewetting behaviors of binary mixture drops have been investigated [24,25], the relation between the deposition pattern and the evaporatively driven flow field in a binary mixture droplet is incomplete (Table S1, Supporting Information (SI)) [26].In this Letter, to achieve a uniform coating, we identify key characteristics of a multicomponent solution, which consists of a binary mixture, surface-active surfactant, and surface-adsorbed polymer.…”

Drying of Ethanol/Water Droplets Containing Silica Nanoparticles

“…As noted above, we are also interested in solvent blends rather than pure water. We show that fresh hydrophobic fumed silica rapidly forms a 3-dimensional viscoelastic network below the gasliquid interface of an evaporating ethanol/water droplet, suppressing Marangoni flow and finally leading to a uniform deposit; effective control was obtained with a loading of hydrophobic fumed silica NPs (0.2-0.5 vol.%) that is much lower than in our previous study in which 23 wt.% laponite NPs were used to control the deposition of particles in water droplets 13,15 .…”

“…1,2 Suppression of the CRE is of great interest due to the broad applications of inkjet printing from traditional graphics to functional devices. [3][4][5][6][7][8][9][10][11][12] Various strategies have been employed to control the CRE: (i) suppression of capillary flow via solgel transition; [13][14][15][16] (ii) prevention of contact-line pinning via substrate treatment to cause the contact line to retract, negating the outward capillary flow; [17][18][19][20] (iii) particle assembly at the gasliquid interface via rapid evaporation 21,22 and/or adsorption of the particles to the gasliquid interface through manipulation of particle charge 23 , amphiphilicity 24 or shape [25][26][27][28] ; (iv) enhancement of particlesubstrate interactions; 29,30 (v) recirculation of particles at the three-phase contact line via thermal Marangoni flow (induced by the 4 temperature gradient at the gasliquid interface arising from evaporation) 31 or solutal Marangoni flow (induced by concentration gradients at the gasliquid interface arising from preferential evaporation of one component) 7,[32][33][34][35][36][37] ; (vi) in-situ generation of particles from emulsions in which the disperse phase is more volatile than the continuous phase.…”