| Molecular dynamics (MD) simulation method is used to simulate the dynamics of nanotubes in liquid flow. This study was motivated by our experimental work on the fabrication and mechanical properties characterization of multi-walled carbon nanotube(MWNT)-high density polyethylene (HDPE) composites fabricated by a melt processing method. SEM and TEM pictures show that the nanotubes are dispersed in the polymer matrix in small aggregates, the size of which decrease with the shearing strength in the suspension. Three types of behaviors are addressed. First, a high aspect ratio nanotube is modeled as a flexible fiber and its dynamics in simple shear flow are simulated by coupling flexible fiber dynamics based on continuum mechanics, with drag forces on the nanotube obtained from MD simulations. Results show that curved nanotubes in simple shear flow become straight and aligned along the shearing direction. Calculations also show that the viscosity of a dilute nanotube suspension increases with nanotube aspect ratio (Ar) and volume fraction ( Vf). For Ar = 400 and V f=0.708%, the suspension viscosity is two orders higher than the suspending liquid, which is comparable to reported viscosity measurements. The results also show that suspension viscosity decreases as the initial curvature of the nanotube increases.; Second, Dynamics of a short nanotube (Ar < 4) in simple shear flow is analyzed using MD simulations. The results show that nanotubes rotate similar to macro scale fibers do in shear flow, but the time period is higher than the one calculated from Jeffery's theory. The difference increases with increasing shear rate and decreasing tube aspect ratio.; Third, MD simulations of nanotubes in suspension under sonication are conducted. The results show that nanotubes are more prone to breakage at a higher vibration frequency and amplitude, and increasing frequency is more effective in their attrition. It is also shown that a longer tube breaks more easily, which is consistent with experimental observations by other researchers. However, from our simulation, a larger diameter nanotube is stronger, which is opposite to experimental observations by other researchers. While the possible reason might be more defects in larger tubes, this discrepancy needs to be explored further. |
