Xiangyu Sun

Thermodynamic analysis of the effect of the hierarchical architecture of a superhydrophobic surface on a condensed drop state.

Condensed drops usually display a Wenzel state on a superhydrophobic surface (SHS) only with microrough architecture, while Cassie drops easily appear on a surface with micro-nano hierarchical roughness. The mechanism of this is not very clear. It is important to understand how the hierarchical structure affects the states of condensation drops so that a good SHS can be designed to achieve the highly efficient dropwise condensation. In this study, the interface free energy (IFE) of a local condensate, which comes from the growth and combination of numerous initial condensation nuclei, was calculated during its shape changes from the early flat shape to a Wenzel or Cassie state. The final state of a condensed drop was determined by whether the IFE continuously decreased or a minimum value existed. The calculation results indicate that the condensation drops on the surface only with microroughness display a Wenzel state because the IFE curve of a condensed drop first decreases and then increases, existing at a minimum value corresponding to a Wenzel drop. On a surface with proper hierarchical roughness, however, the interface energy curve of a condensed drop will continuously decline until reaching a Cassie state. Therefore, a condensed drop on a hierarchical roughness surface can spontaneously change into a Cassie state. Besides, the states and apparent contact angles of condensed drops on a SHS with different structural parameters published in the literature were calculated and compared with experimental observations. The results show that the calculated condensed drop states are well-coordinated with experimental clarifications. We can conclude that micro-nano hierarchical roughness is the key structural factor for sustaining condensed drops in a Cassie state on a SHS.

Neural network analysis of boiling heat transfer enhancement using additives

Abstract A model was developed to evaluate and predict boiling heat transfer enhancement using additives. The model is based on the molecular structures of the additives and uses artificial neural network technology. The effects of 30 additives tested by the authors and other researchers on the augmentation of boiling heat transfer were analyzed with the model. The results show that the evaluation of all 30 additives is consistent with the experimental data, which means that the training accuracy of the model is 100%. In addition, the boiling heat transfer enhancement with sodium oleate and 11 other additives was also predicted, with a prediction accuracy of over 90% since the calculated results for 10 of the 11 additives were in agreement with the experimental results.

Analysis on forces and movement of cultivated particles in a rotating wall vessel bioreactor

The forces acting on a microcarrier or a small piece of tissue and its movement in the rotating wall vessel (RWV) bioreactor were analyzed. The tracks of a particle in RWV reactor were calculated under different inner and outer cylinder wall rotating speeds, different particle sizes and different density difference between culture medium and the particle. The results show that cells or particles experience partial microgravity only in the upper area of RWV, which changes with angle θ, while in the lower part of RWV, they experience overweight. The trajectory of a moving particle in RWV reactor is an eccentric helix under ground-based condition. And the eccentric degree increases with the decrease of outer wall rotating speed, and with the increase of density difference and particle size. The proper match of rotating speeds of inner and outer cylinder walls is the key to prevent the particle colliding with the walls. For a relatively large piece of cultivated tissue, it can move around the inner wall only when the rotating speed of the outer cylinder is high. And the drag force acting on a particle inside RWV increases with the particle size and the density difference.

Formation process of mixed fouling of microbe and CaCO3 in water systems

Abstract The mixed fouling process of Pseudomonas fluorescens and CaCO 3 on different solid surfaces in a simulated cooling water system has been investigated. The mixed fouling behavior on different solid surfaces under the condition of different CaCO 3 saturation levels and bulk velocities has been examined, and the growth curves of mixed fouling have been obtained. The results show that the mixed fouling behavior on various material surfaces depends mainly on the affinity of bacteria to a material. The mixed fouling mass developed on polymer materials is much more than that on metal surfaces. With the increase of saturation level of CaCO 3 , however, this difference becomes minimal. The results also indicate that the mixed fouling mass decreases with the increase of CaCO 3 saturation degree and velocity. The induction period of mixed fouling declines with the rising of velocity. Moreover, the induction period of mixed fouling is longer than that of pure biofouling when the saturation level of CaCO 3 is less than 1 and greater than 0. The induction period decreases with CaCO 3 saturation degree when the level is more than or equal to 1. In addition, the sequence of adhesion and deposit of the bacteria and CaCO 3 on a glass surface was measured with micro-video technology. The results show that CaCO 3 first deposits on the solid surface, while the bacteria need longer time to adhere on the surface.

Growth modes of condensates on nano-textured surfaces and mechanism of partially wetted droplet formation

Condensed droplets on different nano-textured surfaces may appear in three distinct wetting states, the Cassie–Baxter state with composite wetting, Wenzel state with complete wetting, and the partially wetted (PW) state. To maintain the super-hydrophobicity of a textured surface, condensed drops on it are usually expected to be in a Cassie–Baxter or PW state. Therefore, it is of importance to clarify the relation between condensed droplet wetting states and the nano-pillar geometries of surfaces. In view of the fact that all condensed droplets in diverse wetting states originate from the nuclei and/or condensate spots growing along different pathways, we think that the distinct growth modes of a condensate correspond to different energy increasing rates (EIRs), and a condensed drop should grow along the route with the minimum EIR. In this paper, accordingly, the EIRs of a droplet on different textured surfaces were analyzed during its growth along three pathways. The results show that the smallest initial EIR of a condensate spot occurs in the increasing contact angel (CA) mode, so that it will grow with the CA enlarging and the base area initially remaining unchanged. Then the EIR of the increasing CA mode becomes much higher than that of the other two modes. The base area of the drop begins to enlarge while the CA remains unchanged. During this period, the increasing base area can be either in a wetted or composite state, resulting in a Wenzel or PW droplet forming, respectively. The growth mode and the wetting state of a condensed droplet are strongly related to the nano-structure of the surface. Additionally, the calculation results of this model are consistent with experimental observations in the literature for the wetting states of condensed drops on nano-textured surfaces, with an accuracy of 91.9%, which is higher than the accuracy of results calculated with previously reported formulas.