## 1 Introduction  
In recent years, additive manufacturing (AM) [1] has been widely used in fabricating bone tissue engineering scaffolds. The biggest advantage of additive manufacturing is that it can produce patient-specific scaffolds with both a complex internal porous structure and a precise external architecture compared to the conventional scaffold fabrication methods [2, 3]. Among AM techniques, 3D powder-based printing (3 DPP) (the term \"3D printing\" mentioned in this paper is representative of 3D powder-based printing in particular) is the most promising technology to fabricate artificial bone implants [4, 5, 6], as 3D printing can be used to fabricate bone engineering scaffolds with biocearamic powders under low-temperature conditions [7, 8]. Thanks to their chemical and structural similarities to the inorganic phase of human bone [9, 10, 11], calcium phosphates, such as hydroxyapatite (HA), \(\beta\)-tricalcium phosphate (\(\beta\)-TCP), have been used in the 3D printing of artificial bone scaffolds [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. In the process of 3D printing bone scaffolds, the ceramic powder is deposited on the powder bed platform first, and the thin layer of powder is well compacted and smoothed by a special roller [16], and then the binder drops are sprayed onto the designated areas of the thin powder layer by the print head. This process aims to bind the powders in the selected region following the cross-sectional information generated by the CAD system. After the selective binding of the powder layer, the above processes are repeated until the end of the whole manufacturing process [23]. Previous studies [8, 23, 24, 25]have shown that the powder-droplet interaction plays a key role in the resolution and quality of 3D printing. The powder-droplet interaction involves three steps [11]: (1) the binder droplet impacts and spreads on the surface of powder bed; (2) the binder droplet penetrates the powder; and (3) wetting and setting occur to form nucleation. With regard to the fact that droplet spreading takes place in a very short time, and the time required for droplet penetration and nucleation is significantly longer [12; 26; 27]. Therefore, the process of powder-droplet interaction is often studied individually. With observation and analysis in relevant works [26; 27], it was found that the spreading process of millimeter-sized large droplets on micron-sized powder is measured on a microsecond scale, and the time scale for droplet penetration is of the order of a second. In the study [12], the droplet permeation process with different ratios of CaP:CaSO4 was studied. The results showed that the permeation time ranged from 0.73 s to 5.07 s, and the particle size of powder had a strong influence on the droplet permeation. The final resolution of the 3D printed products depends on how the droplet spreads on the powder bed [23]. Since the droplet spreading behavior on powders plays a crucial role in 3D printing [28], it will be challenging for precision manufacturing studies in the future. The common way to study droplet spreading behavior is to observe the 3 DP process empirically and by determining the final integrity of the printed specimen. In the study [27], the influence of TiNiHf particle size on binder drop spreading time was studied to observe the millimeter-scale drops spreading by a high-speed digital optical microscope. [PERSON] et al. [29; 30] studied the binder drop impact on powder during which the spreading, rebound and splashing were observed with high-speed photography. [PERSON] et al. [26] also studied the penetration performance of several kinds of droplets on different powder materials with a large number of observation experiments and proposed an absorption time prediction model depending on the Washburn equation [31]. With the common experimental methods, there are still limitations for developing and applying new materials used for 3D printing bone scaffolds.