Pattern formation during the evaporation of a colloidal nanoliter drop: a numerical and experimental study

Abstract

An efficient way to precisely pattern particles on solid surfaces is to dispense and evaporate colloidal drops, as for bioassays. The dried deposits often exhibit complex structures exemplified by the coffee ring pattern, where most particles have accumulated at the periphery of the deposit.In this work, the formation of deposits during the drying of nanoliter colloidal drops on a flat substrate is investigated numerically and experimentally. A finite-element numerical model is developed that solves the Navier-Stokes, heat and mass transport equations in a Lagrangian framework. The diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet-air interface. Laplace stresses and thermal Marangoni stresses are accounted for. The particle concentration is tracked by solving a continuum advection-diffusion equation. Wetting line motion and the interaction of the free surface of the drop with the growing deposit are modeled based on criteria on wetting angles. Numerical results for evaporation times and flow field are in very good agreement with published experimental and theoretical results. We also performed transient visualization experiments of water and isopropanol drops loaded with polystyrene microsphere evaporating on respectively glass and polydimethylsiloxane substrates. Measured evaporation times, deposit shape and sizes, and flow fields are in very good agreement with the numerical results. Different V dimensionless velocity vector V = (U.V) X Concentration of particles [kg of particles/kg of solution] z axial coordinate [m] Z dimensionless axial coordinate [ 0 max, / r z ]

Biomolecular interactions control the shape of stains from drying droplets of complex fluids

“…Figure 1 demonstrates that, in both cases, the particles first gather into forming a large peripheral ring. This indicates strong contribution from radial flow and that the drying process is driven by evaporation at the wetting line (20), (21). As the water evaporates, particles the distribution profiles show uniformly distributed beads around the center for both biotin-and streptavidin-coated solutions.…”

Surface Properties of Vitally Important Ions: Sessile Desiccated Drop Studies

“…14, a) started to grow at the three-phase boundary and proceeded on the surface of a hydrophilic "pillow" towards the drop center. This suggests that at least Na + cations were not initially present on the surface of the drop; instead, they were gradually brought there by the ascending thermocapillary Marangoni flows [59][60][61]. When Na + was replaced by K + in the mixture, the crystallization process changed: after initial deposition of KCl along the three-phase boundary further crystallization of KCl proceeded simultaneously over the entire drop surface (Fig.…”

Fluid dynamics topics in bloodstain pattern analysis: Comparative review and research opportunities

“…For instance, Hu and Larson [165,166] modeled the evaporation of a drop of pure liquid with a pinned wetting line. Simulations of the formation of stains have recently been performed [167][168][169][170]; this requires solving the motion of the liquid and of many suspended solid particles. This is a non-trivial task, because of, for example, the interaction of the particles with the wetting line [170].…”

“…Area of Convergence, Directionality [13] [ 5,97] Area of Origin [13,19,68 Backspatter Pattern; Forward Spatter Pattern area of origin, weapon [48] [5, 47,110,191] Blood Clot [6,162] [ 160 , 162] Bloodstain pattern area of origin, weapon [13,35,76 [ 166,170,196] Perimeter Stain time between drip and wipe-off, [155,197,198] [ 155,188] [188] [155,163,197,198] Flow Pattern [12,162,199] [12, 162,185,199] [185] [12] Impact Pattern weapon, motion; directionality [78][79][80]128] [5] [72,128,136] Insect Stain [157,200] Mist Pattern weapon, directionality [81, 154 , 186 Spatter Stain [201] [126, [15...…”