## 4 Discussion  
Statistics revealed that the dome height-to-diameter ratio and disc coverage ratio were crucial parameters that influence the drag force of domed surfaces, while the dome base diameter had limited influence. The dome height-to-diameter ratio was arguably the most dominant parameter, with a percentage contribution of 58.5%. The dominance of the dome height-to-diameter ratio was mainly due to the close linkage between the capacity of domes to break the continuity of the interfacial water film and the extent of dome convexity [28; 29]. Moreover, the dome height-to-diameter ratio affected the extent of disc sinkage into soil paste, hence, soil resistance on the disc's front side. The disc coverage ratio also showed a significant contribution (29.4%) to the disc drag force. The results showed that the higher the disc coverage ratio, the more disc drag force. At a coverage ratio greater than 60%, the increase in drag force could be attributed to a blockage of soil movement between adjacent domes, causing the soil to accumulate in front of the disc along the disc stroke. Results revealed that the lowest drag force values were associated with the HDR range of 15% to 30%, while domed discs with a dome height-to-diameter ratio of 37.5% had increased drag force of up to 20% (Disc 9) compared to a flat disc. In contrast, [PERSON] and [PERSON] [23] reported that domed surfaces with a dome height-to-diameter ratio of 50% or less had decreased drag force. This contrast in results may be related to the different soil textures used in both experiments, since [PERSON] and [PERSON] [23] used heavy clay soil with 62%, whereas the current study used sility loam and sandy clay loam soils with 15% and 21% clay, respectively. According to [PERSON] et al. [10], the fine soil particles (clay and fine silt) have a higher capacity for water-holding due to their large surface area; thus, heavy clay soil is stickier than sility loam and sandy clay loam soils. The optimized domed disc (Disc 14) successfully reduced drag force compared to the flat disc under varying soil moisture contents ranging from 23% to 37%. The maximum recorded drag force reduction was 25% in the silty loam soil with 30% moisture content. The reduced drag force of Disc 14 could be attributed to the ability of domes to push the soil away from concave areas between adjacent domes while dragging, restricting interfacial water film continuity. Another reason for the drag force reduction by the optimized domed disc was air retention in the concave areas between adjacent domes, which provided a gas isolation layer, reducing soil-disc adhesion. Furthermore, the tangential movement of the  
Figure 7: Comparing the mean drag force of the flat surface with that of the optimized biomimetic surface in all treatments. Means with different letters are significantly different. optimized domed disc caused disturbances in the surrounding soil layer, resulting in a decrease in the soil-disc friction coefficient [30]. The drag force test in silty loam soil (S1) revealed that raising the soil moisture content below the liquid limit increased drag force. This increase in drag force associated with the increase in soil moisture content was probably due to the formation of a thin interfacial water film between the disc and the soil, increasing soil-disc adhesion. The soil-disc adhesion, therefore, acted as an extra load, resulting in increased drag force. On the other hand, the expected increase in initial disc subsidence due to rising soil moisture content could increase soil resistance on the disc's front side, hence drag force. In comparison, the soil-disc sliding resistance in sandy clay loam soil (S2) followed a distinct pattern. The soil-disc sliding resistance was initially low at the lowest moisture content (23%); as the moisture content increased to 30%, the soil-disc sliding resistance increased. Finally, as the moisture content increased even further, the soil-disc sliding resistance decreased.