No abstract

We simulated the motions and interactions of circular squirmers under gravity in a two-dimensional channel at finite fluid inertia, aiming to provide a comprehensive analysis of the dynamic features of swimming microorganisms or engineered microswimmers. In addition to a squirmer-type factor (β), another control parameter (α) was introduced, representing ratio of the self-propelling strength to the sedimentation strength of squirmers. Simulations were performed at 0.4 ≤ α ≤ 1.2 and −5 ≤ β ≤ 5. We first considered the sedimentation of a single squirmer. Five patterns were revealed, depending on both α and β: steady downward falling, steady inclined falling or rising and small-scale or large-scale oscillating. Compared with a pusher (β < 0, gaining thrust from rear), a puller (β > 0, gaining thrust from front) is more likely to break down its symmetrical structure and subsequently lose stability, owing to the high-pressure regions on its lateral sides. Typically, a pusher settles faster than a puller, whereas a neutral squirmer (β = 0) settles in between. This is related to the ‘trailing negative flow’ behind a pusher and ‘leading negative flow’ before a puller. We then placed two squirmers in line with the gravity direction to study their interactions. Results show pullers attract each other and come into contact as a result of the low-pressure regions between them, whereas the opposite is observed for pushers. The interactions between two pullers are illustrated by their respective patterns. In contrast, pushers never come into contact and maintain distance from each other with increasing separation. We finally examined how a puller interacts with a pusher.

The swimming mode of two interacting squirmers under gravity in a narrow vertical channel is simulated numerically using the lattice Boltzmann method (LBM) in the range of self-propelling strength 0.1 ≤ α ≤ 1.1 and swimming type −5 ≤ β ≤ 5. The results showed that there exist five typical swimming patterns for individual squirmers, i.e., steady upward rising (SUR), oscillation across the channel (OAC), oscillation near the wall (ONW), steady upward rising with small-amplitude oscillation (SURO), and vertical motion along the sidewall (VMS). The parametric space (α, β) illustrated the interactions on each pattern. In particular, the range of oscillation angle for ONW is from 19.8° to 32.4° as α varies from 0.3 to 0.7. Moreover, the swimming modes of two interacting squirmers combine the two squirmers’ independent swimming patterns. On the other hand, the pullers (β < 0) attract with each other at the initial stage, resulting in a low-pressure region between them and making the two pullers gradually move closer and finally make contact, while the result for the pushers (β > 0) is the opposite. After the squirmers’ interaction, the squirmer orientation and pressure distribution determine subsequent squirmer swimming patterns. Two pushers separate quickly, while there will be a more extended interaction period before the two pullers are entirely separated.

In this work the liquid-vapor transition in flow boiling was numerically studied through a two-phase lattice Boltzmann method (LBM). The benchmark test of the nucleate boiling with a single nucleate site was adopted to validate our computational code. Then the focus moves to the behavior of the bubble growth and departure induced by a heated plate in shear flow. The effects of the shear flow were examined by varying its magnitude. In addition, the wettability of the heated plate was also taken into account. It has been shown that the shear flow has negligible effect on the period of bubble release for the neutral and hydrophilic surfaces, which, however, is not the case for the hydrophobic surface.

The liquid–vapor phase change and the boiling heat transfer induced by a microheater in a fluid are numerically studied through a two-phase lattice Boltzmann method. The fluid is subjected to simple shear. The effects of the gravity force, flow strength, and wall wettability are taken into account. A direct comparison between the cases of pool boiling and flow boiling is made in terms of the bubble release period, flow features, and the temperature of the microheater. In particular, it is shown that the flow motion has a negligible effect on the bubble release period for hydrophilic surfaces. By contrast, the bubble departure is considerably accelerated by the shear flow for hydrophobic surfaces which is associated with the formation of “bubble neck.”