Morphology of the impact of particle-laden drops onto solid substrates
The impact of particle-laden drops on substrates is relevant to many technologies, for example, to additive manufacturing, spraying of liquid friction modifiers, etc. These technologies can benefit from a better understanding of the influence of particles on drop impact phenomena.
To understand the influence of the particles, the impact was studied for millimeter droplets. Such droplets are mainly used in impact studies, so they are useful for comparative analysis. As carrier fluid, water was selected. The surface tension of water allows using substrates with different wettability, thereby allowing to cover a maximum number of possible phenomena seen for drop impact on substrates.
The impact velocity was chosen in the range from 1.7 to 3.7 m/s (150≤We≤750 and 7100≤Re≤16400). This allowed us to examine the effect of the particles on various possible number of phenomena (e.g. splashing, deposition, partial rebound, and jetting) occurring during drop impacts with hydrophilic (glass) and hydrophobic (polycarbonate) substrates (Fig. 1).
Figure 1. Outcomes of the drop impact onto substrates for water without and with solid particles.
The influence of particles on drop impact phenomena was studied in the case of round iso-dense microparticles with diameter of 200 and 500 μm. The particle diameters were chosen so that they lay in a range in which there are published data to allow comparison. Also, the selected particle sizes are easily distinguishable by high speed video recording systems with a wide field of view (~30x30 mm), which are used in impact studies of millimetric drops. The microparticles were hydrophobic to study the influence of their wettability on comparison with published data obtained for hydrophilic particles.
The addition of 200 μm and 500 μm particles to water drop changed its impact behavior on hydrophilic and hydrophobic substrates (Fig. 1). It was found that the particles suppressed the appearance of singular jetting and drop partial rebound on hydrophobic substrates. Also, on hydrophilic substrates the particles caused early splashing and on hydrophobic substrates the particles caused early splashing, receding breakup and rupture. The occurrences of these phenomena depended on the impact velocity, the particle diameter, and the volume fraction. The increase in drop impact velocity led to an increase in the probabilities for splashing and receding breakup. The increase in particle size caused the increase in the probabilities for splashing, receding breakup and reduction of the probability for partial rebound. The increase in particle volume fraction increased the likelihood of splashing and decreased the likelihood of partial rebound. So, the increase in mentioned parameters decreases the probability of drop deposition without its fragmentation.
The particles changed the spreading dynamics as well. The addition of the 200 μm particles to a water drop led to the fact that its spreading on a hydrophilic substrates happened in two phases: fast and slow. The fast spreading was caused by the action of inertial forces and the slow spreading was caused by the capillary forces, that is, surface wicking. The maximum spreading factor, caused by inertia, reduced linearly with the increase in particle concentration on hydrophilic and hydrophobic substrates. The explanation of this reduction was argued to be the result of energy dissipation through frictional losses between particles and the substrate.