## 3 Results
### Simulation of Micron-Sized Droplet  
The effect of wettability on the spreading of binder droplets was shown in Figure 7 at varying particle sizes. The particle sizes corresponding to Figure 7a-f were 200 \(\upmu\)m (\(\delta\) = 1:2), 100 \(\upmu\)m (\(\delta\) = 1:1), 50 \(\upmu\)m (\(\delta\) = 2:1), 25 \(\upmu\)m (\(\delta\) = 4:1), 10 \(\upmu\)m (\(\delta\) = 10:1) and 5 \(\upmu\)m (\(\delta\) = 20:1), respectively. The diameters of binder droplets were 100 \(\upmu\)m. The zero time of the curves was defined as the instant when the droplet contacts the powder bed. Figure 6: The characterization of 3D printed samples. (**a**) is the picture of powder bed taken during the printing process. (**b**) is the partial enlargement for the analysis of the binder print resolution according to graphic image analysis (**c**) is the histograms of Hounsfield Units with image analysis software. (**d**) are the cross-section SEM images of printed samples, where the small balls indicated by red arrows are bioceramic particles, and the irregular objects covering the ball indicated by yellow arrows are the cured binder. (**e**) is the mechanical properties and binder/volume ratio of samples printed in different resolution. (**f**) is the geometrical accuracy analysis of samples printed in different resolution. The blue solid line is the target value, and the red line is the trend line. Through an in-depth analysis of the results shown in Figure 7, the influence of the wettability on droplet spreading behavior can be summarized as follows:  
Under the condition of high wettability (contact angle = 30\({}^{\circ}\)), the unimodal curves were measured at different particle sizes, which means there are no obvious relaxation stages during the spreading of the droplets on high wettability powder surface (shown in Figure 7). When the contact angle was \(\geq\)60\({}^{\circ}\), the bimodal or even trimodal curves were measured, showing the occurrence of spreading and oscillation driven by impact. The larger the contact angle is at a fixed particle size, the smaller the peak value of the curve is. This phenomenon can be more intuitively reflected in Figure 8e. It means the wettability of the powder has a great influence on the binder droplets' spreading behavior; the lower wettability causes a smaller spreading diameter. This result applies to all particle size results. Figure 7: The curves of spreading ratio with time at contact angle of 30\({}^{\circ}\), 60\({}^{\circ}\), 90\({}^{\circ}\), 120\({}^{\circ}\), under different ratios of droplet diameter to powder particle diameter, respectively. The contact angles were used to define the wettability of powder materials, which covers a range of contact angles for the most common biocearamic. At a fixed particle size, the time duration of maximum spreading for the droplets was decreased with increasing contact angle. This observation was also valid for all particle sizes during simulations, which indicates that the impact kinetic energy is transformed and dissipated more quickly in a powder bed with lower wettable particles. The curves of spreading ratios against time at different contact angles indicated that the contact angle has almost no effect on spreading diameters in the first 10 us. However, when \(\delta\) = 1:2, the difference occurs at the time of 50 us and for \(\delta~{}\geq\) 1, the difference occurs earlier at about 30 us. The influence of powder particle sizes on the droplet spreading behavior can be summarized as follows:  
As shown in Figure 8a-d, under the same contact angle, the maximum spreading ratios have a strong relationship with the liquid-solid diameter ratio, especially the liquid-solid diameter ratio of \(\delta\) = 1:2, which shows a completely different trend. In other words, in 3D printing, the influence of particle size larger than the droplet size and the particle size smaller than or equal to the droplet size on droplet spreading behavior needs to be studied separately.