Dense Suspension Splat: Monolayer Spreading and Hole Formation after Impact

Abstract

We use experiments and minimal numerical models to investigate the rapidly expanding monolayer formed by the impact of a dense suspension drop against a smooth solid surface. The expansion creates a lace-like pattern of particle clusters separated by particle-free regions. Both the expansion and the development of the spatial inhomogeneity are dominated by particle inertia, therefore robust and insensitive to details of the surface wetting, capillarity and viscous drag.PACS numbers: 82.70. Kj, 45.70.Qj, 82.70.Dd, 47.57.Qk, 47.57.Gc Since the pioneering work by Worthington [1] the spreading of liquids droplets upon impact has remained an active research area [2,3]. At meters-per-second impact speeds, the spreading divides into two stages [4]: an initial, rapid spreading dominated by liquid inertia and, consequently, insensitive to surface wetting, capillary or liquid viscosity, followed by a slower evolution where the intricate interplay of these effects is important.Here we examine an analogous inertia-dominated spreading dynamics in dense suspension impact. We use a suspension of rigid, non-Brownian particles at high volume fraction (60% or above). This impact regime has received little study [5]. Previous studies on particleladen drop impact have mainly analyzed slow evolution in dilute and semi-dilute suspensions [6][7][8][9]. We find that impact at several meters per second produces a novel outcome ( Fig. 1): the suspension drop deforms into a splat comprised of a single layer of densely packed particles immersed in a thin liquid layer. As the splat expands, void-like regions appear and grow, causing the final splat to display a lace-like pattern of particle clusters separated by particle-free regions. Because particle inertia dominates the expansion and the instability, the monolayer splat dynamics is robust and only weakly modified by surface wetting, capillary and viscous drag. This insensitivity to material and surface properties is often the desired outcome in coating processes. This makes our results useful in ongoing efforts to assess the cohesive strengths of colloidal semiconductor quantum dots by measuring their maximal splat size after impact [11], as well as applications such as thermal spray coating of sintered powders [12] and additive manufacturing using inkjet printing [13,14]. These processes often involve concentrated suspensions. Moreover the impact speeds are often very large. As a result, despite the smaller particles used in these processes, the post-impact spreading dynamics belongs in the same particle-inertia dominated regime as our experiments. straight cylindrical syringe produces packing fractions of φ = 0.61 ± 0.02. As gravity pulls the suspension down, a pinch-off occurs below the cylinder opening [15,16]. In the dense suspension limit studied here, the plugs preserve the cylindrical shape of the syringe, resulting in a plug radius (R p ≈ 2.25 ± 0.05 mm), and have a height L ∼ 2R p . The substrate was a smooth, horizontal glass plate 1.6 ± 0.03 m below the syringe.arXiv:1307....

Dip-coating of suspensions

“…In this situation, the thickness of the liquid film can become comparable to the particle size and the displacement of a particle will be controlled by the drag force exerted by the fluid and the capillary force due to the air/liquid interface deformation. [41,42] Capillary effects are important when interfaces confine particles. [43] Recent studies have shown the influence of particles on the detachment of droplets from a jet of suspension, [44,45,46] which has important industrial applications, including printing.…”

An experimental study on particle effects in liquid sheets

“…Therefore, the modification of the spreading dynamics is an intrinsic local effect of the particles. Qualitatively, the presence of particles larger than the thickness of the sheet creates a deformation of the interface that limits its expansion and dynamics [11]. During the spreading and the fragmentation of the particleladen liquid sheet, droplets are generated from two different ways.…”

“…We did not notice a significant change in droplet size for Newtonian fluid with similar viscosities. Therefore, the influence of the particles goes beyond the increase in viscosity of the suspension, modifying the generation process of the droplets compared to a classical Newtonian liquid [11].…”

Pearling and arching instabilities of a granular suspension on a super-absorbing surface

“…[3][4][5] While signicant fundamental research has focused on the hydrodynamic instabilities of a homogeneous uid experiencing dripping, 6 splashing, 7,8 and dewetting, 9 the understanding of the coupling between the ow dynamics and solidication is still in its infancy. [3][4][5] While signicant fundamental research has focused on the hydrodynamic instabilities of a homogeneous uid experiencing dripping, 6 splashing, 7,8 and dewetting, 9 the understanding of the coupling between the ow dynamics and solidication is still in its infancy.…”